Tetrahydrofuran amino acids: Secondary structure in tetrameric and octameric carbopeptoids derived from a D-allo 5-(aminomethyl)-tetrahydrofuran-2-carboxylic acid

Article abstract of DOI:10.1039/b002996n

The synthesis of a D-allo-5-(azidomethyl)tetrahydrofuran-2-carboxylate as an amino acid precursor (in which the carboxylic acid is cis to the azidomethyl substituent and trans to the diol moiety) is reported from D-ribose. The oligomerisation of this mono

Full text of DOI:10.1039/b002996n

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Tetrahydrofuran amino acids: Secondary structure in tetrameric
and octameric carbopeptoids derived from a D-allo 5-(aminomethyl)-
tetrahydrofuran-2-carboxylic acid
1
Natasha L. Hungerford, Timothy D. W. Claridge, Mark P. Watterson, Robin T. Aplin,
Andres Moreno and George W. J. Fleet*
The Dyson Perrins Laboratory, University of Oxford, Oxford Centre for Molecular Sciences,
South Parks Road, Oxford, UK OX1 3QY
Received (in Cambridge, UK) 13th April 2000, Accepted 14th August 2000
First published as an Advance Article on the web 19th October 2000
The synthesis of a -allo-5-(azidomethyl)tetrahydrofuran-2-carboxylate as an amino acid precursor (in which
the carboxylic acid is cis to the azidomethyl substituent and trans to the diol moiety) is reported from -ribose. The
oligomerisation of this monomer to dimeric, tetrameric and octameric carbopeptoids is described. NMR studies
into the solution structures of cyclohexylidene-protected oligomers and of a deprotected tetramer with eight
free hydroxy groups are described.
Introduction
Peptidomimetics constitute rigid templates with the ability to
exhibit conformational influences on the peptide backbone;
the preceding paper describes the synthesis of oligomers of an
all-cis-THF amino acid monomer in which there is essentially
no evidence for significant secondary structure arising from
the THF template.1 Dipeptide analogues can be generated by
substitution of the amide bond with an isosteric unit. Such
replacement often increases the metabolic stability of peptide-
based drugs. If the isosteric unit is stereochemically constrained
then the conformational possibilities are limited compared
with that of the regular dipeptide. Carbohydrate-derived tetra-
hydrofuran amino acids (e.g., 1) have been employed as con-
formationally restricted dipeptide isosteres. Structure 1 can be
likened to a conventional dipeptide 2 in which the ether linkage
replaces the amide bond and rigidly links the two amino acids.2
For example, a tetrahydrofuran sugar amino acid was incorpor-
ated into Leu-enkephalin as a Gly-Gly substitute.3 When linked
together into oligomers these compounds constitute a subset
of ‘carbopeptoids’.4 Different substitution patterns around
the tetrahydrofuran ring and changes in backbone stereo-
chemistries lead to oligomers which display different secondary
structures in solution. For instance, different diastereomers of 1
form β-turn5 and left-handed helical6 solution structures. Thus
the set of different stereoisomers of the 5-(aminomethyl)tetra-
hydrofurancarboxylic acid 1 scaffold show promise as dipeptide
isostere building blocks that are predisposed towards different
secondary structures2 and perhaps will lead one day to an
increased understanding of foldamers.7
The ability to readily deprotect or derivatise the polyhydroxy
function of carbohydrate-derived oligomers opens the possi-
bility of creating peptide mimics possessing hydrophobic or
hydrophilic properties. This is of interest, considering protein
secondary structures are determined by such interactions.8
This paper describes the efficient synthesis of a stereoisomer
of tetrahydrofuran amino acid 1, initially as a protected form of
the azido ester 3 (with -allo configuration), and its conversion
to homooligomers,6 including the protected dimer 4, tetramer 5
and octamer 6, with the latter two exhibiting evidence of
secondary structure in solution. The -allo stereochemistry of 3
with the 3,4-cis-diol unit permits efficient protection of these
systems with ketal protecting groups, providing readily manage-
able oligomeric materials, while deprotection of the ketals
affords water-soluble compounds such as the dimer 7 and
tetramer 8. The structure of the deprotected tetramer (with
eight free hydroxy groups) is also examined in solution.
Results and discussion
Tetrahydrofuran-2-carboxylates are generally and readily
accessible via acid- or base-catalysed ring rearrangements of
2-O-trifluoromethanesulfonate derivatives of carbohydrate
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This journal is © The Royal Society of Chemistry 2000
DOI: 10.1039/b002996n

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lactones.9,10 The key step in this conversion to tetrahydrofuran
C-glycosides involves treatment of the 2-O-triflate derivative of
the lactone with acidic or basic methanol, conditions which
result in methanolysis of the lactone and intramolecular SN2
displacement of the triflate by 5-OH. In the synthesis of the
-talo-THF azido ester 10 by a base-catalysed ring contraction
of the 2-O-triflate of the azido lactone 9, the -allo tetrahydro-
furan amino acid 11 (enantiomeric to 3) was obtained11 as a
minor product (13%) (Scheme 1); inversion of configuration
occurred at both C-2 and C-5 in the transformation of 9 to 11.
The minor product 11 was subsequently protected and homo-
oligomerised to the dimer and tetramer in which initial studies
indicated that such materials might have secondary structure
arising from intramolecular hydrogen bonding.6
was therefore treated with 1 equivalent of tosyl chloride in
pyridine to give the corresponding tosyl ester at C-6 13 in 61%
yield (Scheme 3), together with a small amount of the ditosyl
compound 14. Treatment of the monotosyl derivative 13 with
sodium azide in dimethylformamide (DMF) at 70 ЊC produced
only a small amount of the required azide 15 and gave the 2,6-
anhydro-lactone 16 as the major product by base-catalysed
elimination of tosic acid. The spectroscopic data for 1515 and
1616 obtained were in accord with their reported values.
It was thus clear that it was necessary to generate the THF
ring prior to introduction of the azide. However, it was con-
sidered that in the current synthesis in which azide was to be
introduced at C-6, a protection/deprotection sequence at 6-OH
could be avoided if the 6-O-tosyl ester was employed as a tem-
porary protecting group during the ring contraction, sub-
sequently being displaced by azide. Thus, treatment of C-6
tosyl ester 13 with trifluoromethanesulfonic anhydride (Tf2O)
and pyridine in dichloromethane (DCM) resulted in the isol-
ation of 2-O-triflate 17 (Scheme 4), which was immediately
treated with potassium carbonate in methanol to yield -allo-
tetrahydrofuran 19, with the C-6 tosyl ester intact. A small
amount of the -altro product 20 was also formed. Subsequent
reaction of 19 with sodium azide in DMF resulted in displace-
ment of the tosyl ester to afford the azido ester 18 in 92% yield
(32% overall yield in 4 steps from the lactone diol 12).
A shorter sequence to the target azido ester 18 could be
achieved if the 2,6-di-O-tosyl lactone 14 formed the THF tosyl
ester 19 on treatment with basic methanol; this would avoid
both the need for a selective tosylation of the 6-OH in 12 as well
as the use of the relatively expensive triflic anhydride. Treatment
of the diol 12 with an excess of tosic anhydride gave the ditosyl
compound 14 in 89% yield. When the ditosyl compound 14 was
treated with potassium carbonate in methanol, the required
-allo tetrahydrofuran tosyl ester 19 was obtained as the major
product in 56% yield, together with the -altro epimer 20 in 6%
Synthesis of protected building blocks of the monomer 3
A direct and unambiguous synthesis of an allo-configured
equivalent of the amino acid 3 was required for further studies
of these homooligomers and this paper reports the synthesis of
a protected form of 3 (P) = (H), the enantiomer of 11, from
-ribose. The alternative strategies for the formation of 3 are
shown in Scheme 2 in which the azide may be introduced prior –
or subsequent – to the generation of the THF ring; in both
cases the THF ring is formed by triflate displacement by the
C-5 oxygen with inversion of configuration at C-2 of a suitable
protected δ-lactone with -altro stereochemistry. The common
starting material for both routes is the cyclohexylidene sugar
lactone 12 accessible via the Kiliani ascension on 2,3-O-cyclo-
hexylidene--ribose;12 the cyclohexylidene ketal is more
accessible than the corresponding acetonide.13,14
The most attractive route to a protected equivalent of the
-allo ester 3 involved introduction of azide prior to THF ring
formation; this would avoid the need to protect the 6-OH and
hence remove a protection/deprotection sequence. The diol 12
Scheme 1 Reagents and conditions: (i) Tf2O, pyridine, ethyl acetate, Ϫ10 ЊC; (ii) MeOH.
Scheme 2
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Scheme 3 Reagents, conditions and yields: (i) 1 eq. pTsCl, py, 16 h (61%) (ii) NaN3, DMF, 16 h, 70 ЊC.
Scheme 4 Reagents, conditions and yields: (i) 1 eq. pTsCl, py, 16 h (61%) (ii) Tf2O, py, Ϫ40 to Ϫ20 ЊC, DCM (85%) (iii) K2CO3, MeOH (67%)
(2 steps) (iv) NaN3, DMF, 85 ЊC, 2 h (92%) (v) excess of pTs2O, py, 16 h (89%) (vii) K2CO3, MeOH, 1 h (56%).
Scheme 5 Reagents, conditions and yields: (i) aq. NaOH, 1,4-dioxane, 1 h; then Amberlite IR-120 (Hϩ) (ii) H2, Pd black, MeOH, 1 hour (iii) Hϩ,
PriOH, 80 ЊC, 3 h (83%) (iv) H2 Pd black, iPrOH, 30 min.
yield. Again subsequent azide displacement of 6-O-tosyl group
in 19 yielded 18 in 92% yield (and in 3 steps from protected
sugar lactone 12 in 46% overall yield).
Hydrolysis of the azido ester 18 with aq. sodium hydroxide
gave the azido acid 21 as the N-protected monomeric acid com-
ponent for oligomer formation (Scheme 5). Hydrogenation of
methyl ester 18 gave the bicyclic lactam 22 in 64% yield. It
was necessary to exchange the methyl for the more hindered
isopropyl ester to produce an amine sufficiently stable to act as
the C-protected component for oligomerisation studies. Accord-
ingly, the methyl ester 18 was treated with a solution of HCl in
propan-2-ol to give the isopropyl ester 24 in 83% yield; the diol
23 which arose from acid-catalysed loss of the cyclohexylidene
protecting group was isolated in 11% yield. Hydrogenation of
this azido isopropyl ester 24 in propan-2-ol in the presence of
palladium black gave the corresponding amine 25, which on
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storage or on concentration of the solvent spontaneously
formed the lactam 22. However, the crude amine 25 in solution
was used directly in the coupling reactions.
followed by 2,2-DMP in acetone in the presence of CSA, gave
the acetonide 30 with identical physical properties to those
previously reported.17
Structural proof of the -allo stereochemistry for azido ester
18 was obtained by conversion to the isopropylidene lactam 27
(Scheme 6) for comparison with the reported enantiomeric
Synthesis of protected and deprotected oligomers of 3
Homooligomerisation of the allono THF azido ester 18
was conducted via an iterative solution-phase sequence.18 The
-allo acid 21 and amine 25 were coupled (Scheme 8) with 1-[3-
(dimethylamino)propyl]-3-ethylcarbodiimide
hydrochloride
(EDCI)19 and 1-hydroxybenzotriazole (HOBt)20 in the presence
of diisopropylethylamine (DIPEA) in dichloromethane to give
the dimer 4 in 73% yield, together with the lactam 22 (derived
from the cyclisation of 25) in 14% yield.
Repetition of this procedure for the dimer 4 (azide reduction
to give the amine 32 and ester hydrolysis to afford the acid
31 with subsequent coupling in the presence of EDCI, HOBt
and DIPEA in dichloromethane) allowed the isolation of the
tetramer 5 in 65% yield. An analogous procedure (involving
reduction of the azide moiety in tetramer 5 to give amine 34
and hydrolysis to give the acid 33 followed by the coupling
protocol) afforded the octamer 6 in 52% yield.
The 13C isotope distribution provides strong evidence for the
constitutional formula of compounds containing a high num-
ber of carbon atoms (e.g. C51 for tetramer 5 and C99 for octamer
6); 13C isotope constitutes 1.1% of carbon and so for a com-
pound containing over 100 carbon atoms the base peak of the
mass spectrum is shifted one Dalton higher. The experimental
isotope distribution (positive-ion electrospray) observed
matched the calculated distribution for each oligomeric
material. All the cyclohexylidene oligomers were readily soluble
in organic solvents and could be purified by conventional flash
chromatography
Removal of the protecting groups in these oligomers should
permit solubility in aqueous solutions and an examination of
any secondary structures formed in such an environment. The
cyclohexylidene protecting groups could be efficiently removed
from the dimer 4 and tetramer 5 without any significant
decomposition of the materials by reaction with trifluoroacetic
acid in a mixed solvent system of chloroform, propan-2-ol and
water to give the deprotected dimer 7 and the deprotected
tetramer 8 in yields of 81 and 100%, respectively (Scheme 9).
Scheme 6 Reagents, conditions and yields: (i) 40% aq. TFA, 1 h (ii) 2,2-
DMP, acetone, CSA, 2 h; (iii) Pd black, H2, MeOH, 1 h (68%) (over
3 steps).
lactam 28.6,11 Removal of the protecting groups in 18 with aq.
trifluoroacetic acid (TFA) followed by treatment of the crude
product with 2,2-dimethoxypropane (2,2-DMP) in acetone in
the presence of ( )-camphorsulfonic acid (CSA) gave the
isopropylidene methyl ester 26. Hydrogenation of the azide 26
in methanol in the presence of palladium black afforded the
lactam 27 in 68% overall yield and with spectroscopic data (1H
and 13C NMR) identical with its enantiomer 28,11 but with the
opposite sign for the specific rotation.
Proof of structure of the minor product 20 obtained in the
ring-rearrangement procedure was provided by conversion to
the known isopropylidene protected 6-azido altronate 30
(Scheme 7); 30 has been prepared in 8 steps from -allono-1,4-
NMR and IR studies on protected and deprotected oligomers of 3
The solution secondary structural characteristics of the pro-
tected tetramer 5 and octamer 6, and unprotected tetramer 8
were conducted by NMR spectroscopy, and compared with
similar studies on the per-acetylated tetramer (with a trans
secondary diol unit 35) and with the acetonide 36, enantiomeric
with respect to the sugar component of the tetramer 5.
A comparison of the CDCl3 spectra of the protected
tetramer 5 and octamer 6 is shown in Fig. 1. In the proton
spectra of both the tetramer 5 and the octamer 6, good proton
dispersion for the sugar-ring protons was observed despite the
repeating monomer units. In addition for both oligomers, one
low-frequency amide proton (δ ≈ 7.2) was observed with the
others resonating between δ 8.0–8.8, a chemical-shift range in
CDCl3 characteristic of amide protons involved in hydrogen
bonding.21 The low-frequency proton in each case was observed
to be the N-terminal NH residue (NHB)22 which is not able to
H-bond to the carbonyl of a preceding residue. Additional
evidence for such H-bond formation is evident in the CDCl3
solution IR spectra in which the tetramer 5 displays a sharp
band at 3415 cmϪ1 characteristic of the N–H stretch of a free,
solvated amide together with a broader band at 3320 cmϪ1
characteristic of an H-bonded amide proton.23 Similarly, the
octamer 6 displays bands at 3415 cmϪ1 (sharp) and 3299 cmϪ1
(broad) for which the intensity of the lower-frequency band
relative to the higher is proportionately greater than that
observed for the tetramer 5 (Fig. 2), consistent with the amide-
Scheme 7 Reagents, conditions and yields: (i) NaN3, DMF, 90 ЊC, 2 h
(70%) (ii) 40% aq. TFA, 24 h; (iii) 2,2-DMP, acetone, CSA, 16 h (31%)
(over 2 steps).
lactone.17 Treatment of the tosyl ester 20 with sodium azide
in DMF at 90 ЊC for 2 h afforded the azide 29 in 70% yield.
Treatment of the cyclohexylidene azide 29 with aq. TFA,
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Scheme 8 Reagents and conditions: (i) aq. NaOH, 1,4-dioxane; then Amberlite IR-120 (Hϩ) (ii) H2, Pd black, PriOH; (iii) EDCI, HOBt, DIPEA,
DCM, 0 ЊC to RT, 16 h.
Scheme 9 Reagents and conditions: (i) TFA-CHCl3 (1:1), PriOH, water.
1
shift distribution observed in the H NMR spectra. Confirm-
assignment of NMR signals was achieved with 2D NMR
experiments, including COSY, TOCSY, HMQC and HMBC
experiments. From the analysis of rotating-frame NOE data,
and through comparison with tetramer 36 for which extensive
NOE studies (ROESY and Tr-ROESY24) were conducted pre-
viously, a repeating ‘β-turn’ type structure is again suggested.
Such a secondary structure also resembles that previously
reported for the per-acetylated tetramer 35.5,25 Significant
NOE interactions were observed from NHi to H2i Ϫ 1 and from
NHi to H6i Ϫ 1 (stereo-specifically) for NHC and NHD in both
tetramer 36 and tetramer 5. Such NOE enhancements suggest
H-bond interactions between NHi and COi Ϫ 2 as shown in
Fig. 4 for 5.
1
ation that the lowest-frequency H NMR amide resonances
were due to those amide protons exposed to the solvent was
provided by CDCl3–dimethyl sulfoxide (DMSO) solvent
titrations (Fig. 3). For both the tetramer 5 and octamer 6 the
N-terminal amide NHB demonstrated the greatest sensitivity by
far to addition of DMSO, with shifts to high frequency arising
from H-bonding interactions with the added solvent. In con-
trast, all other amide resonances showed markedly less alter-
ation, indicating these to be relatively inaccessible to the solvent
and involved in intramolecular hydrogen bonds. It was also
apparent that for each of tetramer 5 and octamer 6, NHB was
observed as a doublet of doublets (dd), compared with the
remaining NH signals in the oligomer which were all apparent
triplets (a-t).
This suggests that isopropylidene rather than cyclohexyl-
idene moieties (as different ketal protecting groups) had very
little effect on the gross conformation in solution.26 This is
consistent with the conformation described, as the secondary
hydroxy-protecting groups would lie on the outside and away
Very similar NMR spectra were obtained for the cyclo-
hexylidene-protected tetramer 5 and the enantiomeric iso-
propylidene-protected tetramer 36. For tetramer 5 in CDCl3,
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Fig. 1 1H NMR spectra of allonate tetramer 5 and allonate octamer 6
(CDCl3; 298 K).
Fig. 2 IR spectra of tetramer 5 and octamer 6 showing the amide
N–H stretch region (2 mM in CHCl3).
Fig. 3 Solvent titration plots (DMSO additions to initial 7 mM CDCl3
solutions) for the amide NH protons of tetramer 5 and octamer 6.
A similar secondary structure is suggested for the octamer 6
1
based on the similarity of the NMR spectra. The H chemical
shifts (as identified in the 2D TOCSY experiment) suggested
that the central sugar residues of the repeating-turn structure
experience similar environments and hence similar chemical
shifts. Signals from rings A, B and H tended to be separated
from the same protons in these central rings, which were
observed to overlap. Despite this, distinct resonances were
observed for each of NHB through to NHH. Again NHB (δ 7.12)
was observed at lowest frequency, typical of no hydrogen-
bonding interactions, whilst the H-bonded C-terminal NHH
(δ 8.26) was observed at lower frequency than NHC–G (which
could not be assigned sequentially due to sugar-resonance over-
lap). This observation is consistent with a structure in which the
from the hydrogen bonds of the turn structure. Interestingly,
a similar repeating structure to this, referred to as a ‘β-bend
ribbon spiral’, has been observed crystallographically for
oligomeric peptides of alternating -proline and α-amino-
isobutyric acid units,27,28 and in solution for a fungal peptide
also containing this dipeptide motif.29 These peptides contain
a repeating 10-membered hydrogen-bonded ring structure
highly reminiscent of that proposed by us for tetramers 35, 36
and 5.
C᎐O of residue G (which forms the amide bond with NH )
᎐
H
is not able to participate in H-bonding, meaning the NHH
amide proton is therefore less deshielded than those of residues
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sufficient to produce stable conformations in the absence of
more conventional peptide N–H-to-C᎐O hydrogen bonds. Pre-
᎐
vious NOE and molecular-modelling studies on tetramer 355
have shown carbonyl H-bonds alone to satisfy the NOE con-
straints, so although the formation of NH–Oether interactions
(and possibly simultaneous HN interactions with the THF ring
oxygen within the ‘β-turn’ in particular) cannot be ruled out
completely, these do not appear to be responsible for the
observed secondary structure on tetramer 5, octamer 6 and
related oligomers.
In the case of the deprotected tetramer 8, preliminary NMR
studies were also conducted into its solution conformation.
NMR spectra in d5-pyridine enabled a comparison between the
protected and deprotected tetramers 5 and 8, respectively. In
both cases, amide and sugar-ring proton dispersion was again
observed, despite the repeating monomer units. For protected
tetramer 5, amide protons were observed at δ 8.96, 9.02, and
9.24, whereas for the deprotected tetramer 8, NH signals were
observed at δ 8.68, 9.06, and 9.26. It is interesting to note that
for protected tetramer 5 in d5-pyridine, the pattern of amide
protons had changed, with two at lower-frequency and one
slightly more separated and at higher frequency. In this case all
NH signals were observed as apparent triplets. By comparison,
the deprotected tetramer 8 in d5-pyridine again showed the
same pattern as for protected tetramer 5 and octamer 6 in
CDCl3, with the low-frequency NH (δ 8.68) observed as a doub-
let of doublets, and the higher-frequency signals all observed as
apparent triplets.
Fig. 4 The H-bonding pattern suggested for tetramer 5 from the
observed strong NOEs.
The spectrum of deprotected tetramer 8 was additionally
recorded in CD3OH to assess whether secondary structure was
observed in this more polar protic solvent. Again, sufficient
dispersion enabled assignment of all sugar-ring protons and the
amide protons were observed at δ 8.07 (NHB), 8.36 (NHD) and
8.53 (NHC) in a similarly disperse pattern to that observed for
protected tetramer 5 in CDCl3. Again, the lower-frequency NH
was observed as a doublet of doublets compared with the
others which were apparent triplets. The 1H and 13C NMR were
assigned with the aid of DQFCOSY, TOCSY, HSQC and
HMBC 2D NMR experiments. The CD3OH NOESY experi-
ment for deprotected 8 exhibited for NHB, NHC and NHD,
cross-peaks between NHi and H2i Ϫ 1, NHi and H3i Ϫ 1, NHi and
H4i Ϫ 1 and between NHi and H6i Ϫ 1 (overlapping signals pre-
vent this last observation for NHD to H-6C). As for protected 5
in CDCl3, this evidence suggests a ‘β-turn’-type solution struc-
ture. Any difference to 5 could be attributed to the differing
conformation in this solvent or be due to the lack of extra
constraint without the cyclohexylidene groups. It is also likely
that methanol could disrupt the stable conformer through
solvent–solute H-bond interactions. Nevertheless a repeating
pattern of NOEs is observed from NHi to protons in the
adjacent residue and good dispersion of protons resonances is
seen. Further evidence is needed to establish whether a repeat-
ing structure along the molecule 8 remains in CD3OH.
1
Fig. 5 The H–15N HSQC spectra of the tetramer 5 and octamer 6
(CDCl3; 298 K).
C–G in which both the NH and C᎐O of each amide bond do
᎐
participate.21 A similar, although less marked, pattern was also
observed for NHD in tetramer 5 which resonated at lower fre-
quency than NHC. These trends are also reflected by the 15N
amide chemical shifts observed in the 1H–15N HSQC spectra of
tetramer 5 and octamer 6 (Fig. 5). In both cases, the lowest-
frequency 15N resonance observed corresponded to the
C-terminal residue in which the NH is involved in hydrogen
bonding but the carbonyl of the amide bond is not [NHD
(δ 107.2) and NHH (δ 107.4) respectively]. The next-lowest-
frequency 15N resonance at δ 108.0 for tetramer 5 and δ 108.8
for octamer 6 both corresponded to NHB signals, associated
with involvement of C᎐O in hydrogen bonding, but no partici-
pation of NHB in further hydrogen-bonding interactions.
Simultaneous involvement of both the NH and the C᎐O in
᎐
᎐
intramolecular hydrogen bonding resulted in higher-frequency
resonances for 1H and 15N signals. Furthermore, the greater 1H
and 15N shifts observed for the central residues of the octamer 6
relative to the shifts of the tetramer 5 amide groups suggest a
greater polarisation of the amide bonds, and a net strengthen-
ing of H-bonds in the longer structure. The lower-frequency
hydrogen bonded amide NH IR stretch of the octamer 6
relative to the tetramer 5 (3299 vs. 3320 cmϪ1) is also consistent
with this.
A
1H NMR spectrum of deprotected 8 was additionally
recorded in a solvent system consisting of 10% D2O in H2O. In
this aqueous system, three distinct NH signals were observed at
δ 8.18, 8.25 and 8.40, again suggesting that the structure
exhibits a conformational preference in this environment,
although further studies are again required to fully assess this
preference.
1
This simple rationalisation of the observed patterns of H
and 15N resonance shifts on the basis of amide NH and/or CO
involvement argues for the amide protons intramolecularly
hydrogen bonding to the backbone carbonyl moieties as in
Fig. 4 rather than with side-chain or backbone ether oxygens.
Such main-chain-to-side-chain interactions have been observed
crystallographically in conformationally restricted peptides and
shown to destabilise helix formation.30 Whilst such interactions
may provide a competitive destabilising influence on otherwise
well defined conformations, it is unlikely that these would be
Summary
Homooligomers of -allo-configured amino acid monomer
units are readily synthesised and purified, with the ketal protect-
ing group providing manageable oligomeric materials. Pre-
liminary evidence indicates that the tetramer and octamer, as
short oligopeptides, exhibit secondary structure in solution,
consistent with a ‘β-turn’-like structure. Deprotection can
afford materials that are soluble in aqueous environments. Evi-
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dence is accumulating that different diastereomeric 5-(amino-
methyl)-THF-carboxylate building blocks may have structural
features that are predisposed to induce different types – or a
lack – of secondary structures leading to a family of foldamers.
Further NMR studies and CD spectra should provide further
evidence of defined structures and help establish tetrahydro-
furan amino acids as peptidomimetic building blocks with pre-
dictable predisposition to the formation of defined secondary
structures.
mixture was washed successively with saturated aq. CuSO4
(50 mL), water (30 mL) and saturated aq. sodium bicarbonate
(30 mL). The organic solution was then dried (MgSO4), and
concentrated by rotary evaporation. Flash chromatography
(ethyl acetate–hexane 1:2) gave the ditosyl compound 14 (0.049
g, 7%) as a foam, followed by the monotosylester 13 (0.29 g,
61%), mp 45–47 ЊC; [α]D24 ϩ75.4 (c 1.14 in CHCl3) [HRMS m/z
ϩ
(CIϩ): Found: 430.1531 (M ϩ NH4 ). C19H28NO8S requires
m/z, 430.1536]; νmax (thin film) 3469 (OH), 1769 (C᎐O) cmϪ1;
᎐
δH (CDCl3; 500 MHz) 1.40–1.78 (10H, m, cyclohexylidene),
2.50 (3H, s, CH3Ar), 4.23–4.36 (4H, m, H-3, -4, -5, -6), 4.43 (1H,
d, J 6.5, H-2), 4.48 (1H, d, J 10.4, HЈ-6), 7.42 (2H, d, J 8.2,
ArH), 7.87 (2H, d, J 8.2, ArH); δC (CDCl3; 126 MHz) 22.1 (q,
CH3Ar), 23.8, 24.2, 25.3, 34.4, 37.1 (5 × t, cyclohexylidene),
67.6 (t, C-6), 70.3, 71.2, 75.9, 77.0 (4 × d, C-2, -3, -4, -5), 114.2
(s, O–C–O), 128.6 (d, Ar CH), 130.5 (d, Ar CH), 132.7 (s, Ar C),
145.8 (s, Ar C), 171.9 (s, C-1); m/z (APCIϩ) 413 (M ϩ Hϩ,
100%).
Experimental
1H NMR (δH) spectra were recorded on a Bruker DPX 400
spectrometer (at 400 MHz), Bruker DPX 200 or Varian Gemini
200 (200 MHz), Bruker AMX 500 or AM 500 (500 MHz) spec-
trometer at ambient probe temperatures (≈298 K). Coupling
constants (J) were measured in Hz and are averaged. Carbon
nuclear magnetic resonance (δC) spectra were recorded on a
Bruker DPX 200 (at 50 MHz), on a Bruker DPX 400 spec-
trometer (at 100 MHz) or a Bruker AMX 500 or AM500 (125
MHz) spectrometer; multiplicities were assigned using a DEPT
sequence. All chemical shifts are quoted on the δ-scale using
residual solvent as internal standard, except for the CDCl3–
DMSO solvent titrations which were referenced to internal
SiMe4 (TMS) which was assumed to be invariant to solvent
changes. TOCSY spectra were collected using the MLEV-17
mixing sequence with mixing times of up to 100 ms. HSQC
Continued chromatography with ethyl acetate–hexane 1:1
gave residual starting material 12 (0.043 g, 14% recovery).
2,6-Anhydro-3,4-O-cyclohexylidene-D-altrono-1,5-lactone 16
and 6-azido-3,4-O-cyclohexylidene-6-deoxy-D-altrono-1,5-
lactone 15
The monotosyl derivative 13 (0.050 g, 0.12 mmol) was dissolved
in dry DMF (1 mL), and sodium azide (0.017 g, 0.25 mmol) was
added. The mixture was stirred at 70 ЊC for 16 h followed by 1 h
at 90 ЊC. TLC analysis (ethyl acetate–hexane 1:1) showed two
non-UV-active spots (Rf 0.5 and 0.4) while the starting material
was UV-active (Rf 0.4). Water (5 drops) was added and the
DMF was removed in vacuo. Brine (10 mL) was added to the
residue, which was then extracted with ethyl acetate (4 × 10
mL). The combined organic extracts were dried over MgSO4
and concentrated. Flash chromatography (ethyl acetate–hexane
1:2) enabled separation of the two products, giving the bicyclic
lactone 1616 (0.010 g, 35%) and the azide 1515 (0.002 g, 6%),
both with physical properties identical with those reported
previously.15,16
spectra were acquired with gradient selection.31 For H–15N
1
HSQC spectra, 15N was referenced against external liquid NH3.
ROESY spectra were acquired with either a continuous-wave
spin-lock or with the 180x Ϫ 180Ϫx composite spin-lock
sequence to suppress TOCSY transfers (Tr-ROESY). Mixing
times were 300 ms.24 IR spectra were recorded on a Perkin-
Elmer 1750 IR FT spectrophotometer. Solution IR spectra were
recorded in a 1 mm cell at a concentration of 2 mM in CHCl3.
Mass spectra were recorded on VG Micromass 20–250, ZAB
1F, Micromass Platform 1 or Trio-1 GCMS (DB-5 column)
spectrometers using chemical ionisation (CI, NH3), atmos-
pheric pressure chemical ionisation (APCI) or electrospray
techniques (ES) as stated. Optical rotations were measured on
a Perkin-Elmer 241 polarimeter with a path length of 1 dm.
[α]D-Values are given in 10Ϫ1 deg cm2 gϪ1. Concentrations are
given in g 100 mlϪ1. Mps were recorded on a Kofler block and
are uncorrected. Microanalyses were performed by the micro-
analysis service of the Inorganic Chemistry Laboratory, Oxford.
TLC was carried out on plastic or aluminium sheets coated
with 60F254 silica or on glass plates coated with silica blend 41.
Plates were developed using a spray of 0.2% w/v cerium()
sulfate and 5% ammonium molybdate in 2 M sulfuric acid.
Flash chromatography was carried out using Sorbsil C60 40/60
silica. Hydrogenations were conducted under an atmosphere of
hydrogen gas maintained by an inflated balloon. Solvents and
commercially available reagents were dried and purified before
use according to standard procedures. A solution of KH2PO4
(85 g) and NaOH (14.5 g) in distilled water (950 ml) was used as
a pH 7 buffer solution. The cyclohexylidene δ-lactone 12 was
prepared as previously described.14,16
3,4-O-Cyclohexylidene-6-O-(p-toluenesulfonyl)-2-O-trifluoro-
methanesulfonyl-D-altrono-1,5-lactone 17
Dry pyridine (0.067 mL, 0.066 g, 0.83 mmol) was added to a
solution of the tosyl ester 13 (0.149 g, 0.36 mmol) dissolved in
dry DCM (2 mL). Upon cooling of the mixture to Ϫ40 ЊC,
trifluoromethanesulfonic anhydride (0.079 mL, 0.133 g, 0.47
mmol) was added dropwise via syringe. Stirring was continued
for 20 min between Ϫ40 and Ϫ20 ЊC. TLC analysis (ethyl
acetate–hexane 1:1) revealed a single product (Rf 0.7) and no
residual starting material (Rf 0.4). The reaction mixture was
diluted with DCM, washed with water with drops 2 M HCl
added (total aqueous volume 10 mL) and then with pH 7 buffer,
and then dried (MgSO4) and concentrated. Flash chrom-
atography (ethyl acetate–hexane 1:4) gave the triflate 17 (0.168
g, 85%), an unstable foam which was used directly in the next
reaction; νmax (thin film) 1790 (C᎐O) cmϪ1; δ (CDCl3; 400
᎐
H
MHz) 1.38–1.73 (10H, m, cyclohexylidene), 2.46 (3H, s,
CH3Ar), 4.26–4.36 (2H, m), 4.39–4.44 (2H, m), 4.55 (1H, a-t,
J 7.6, H-3), 5.19 (1H, d, J 7.6, H-2), 7.38 (2H, d, J 8.2, ArH),
7.81 (2H, d, J 8.2, ArH); δC (CDCl3; 100.6 MHz) 21.6
(q, CH3Ar), 23.3, 23.7, 24.7, 34.0, 36.5 (5 × t, cyclohexylidene),
66.7 (t, C-6), 70.0, 73.7, 75.7, 81.3 (4 × d, C-2, -3, -4, -5), 114.9
(s, O–C–O), 118.3 [q, J(13C–19F) 320, CF3], 128.1 (d, Ar CH),
130.0 (d, Ar CH), 132.0 (s, Ar C), 145.6 (s, Ar C), 162.9 (s, C-1);
m/z (APCIϩ) 567 (M ϩ Naϩ, 20%), 545 (M ϩ Hϩ, 12), 411
(20), 297 (85), 295 (80), 217 (100), 122 (85).
3,4-O-Cyclohexylidene-6-O-(p-tolylsulfonyl)-D-altrono-1,5-
lactone 13
Powdered dry 3 Å sieves (0.3 g) and the protected lactone 12
(0.30 g, 1.16 mmol) were stirred in dry pyridine (3 mL) under a
N2 atmosphere for 30 min. The reaction mixture was then
cooled to 0 ЊC and toluene-p-sulfonyl chloride (p-TsCl) (0.289
g, 1.5 mmol) was added. Stirring was continued overnight at
room temperature, when TLC analysis (ethyl acetate–hexane
1:1) indicated a major product (Rf 0.4) and a minor component
(Rf 0.6) and little residual starting material (Rf 0.2). Water
(1 mL) was added and the pyridine was removed in vacuo by co-
evaporation with toluene. DCM (80 mL) was added and the
3,4-O-Cyclohexylidene-2,6-bis-O-(p-tolylsulfonyl)-D-altrono-
1,5-lactone 14
Powdered dry 3 Å sieves (1.0 g) and the lactone 12 (2.74 g,
J. Chem. Soc., Perkin Trans. 1, 2000, 3666–3679
3673

View Article Online
10.6 mmol) were stirred in dry pyridine (50 mL) under a N2
atmosphere for 30 min. The reaction mixture was then cooled
to 0 ЊC and toluene-p-sulfonic anhydride (8.09 g, 24.8 mmol)
was added. Stirring was continued overnight at room tempera-
ture. TLC analysis (ethyl acetate–hexane 1:2) indicated a major
product (Rf 0.3) and a minor component (Rf 0.2) and no
residual starting material (Rf 0.08). This mixture was filtered
through Celite and then concentrated in vacuo. The residue
obtained was dissolved in DCM (150 mL) and washed with
buffer (80 mL). The buffer wash was further extracted with
DCM (3 × 50 mL) and the combined DCM phases were dried
over MgSO4 and concentrated. Flash chromatography (ethyl
acetate–hexane 1:2) gave 3,4-O-cyclohexylidene-2,6-bis-O-(p-
tolylsulfonyl)-D-altrono-1,5-lactone 14 (5.34 g, 89%) as a foaming
white solid, mp 63–64 ЊC [HRMS m/z (CIϩ) Found: 567.1350
(M ϩ Hϩ). C26H31O10S2 requires m/z, 567.1359] (Found: C,
55.23; H, 5.48. C26H30O10S2 requires C, 55.11; H, 5.34%); [α]D24
afterwards washed with DCM. Concentration of the filtrate
gave a dark red crude product, which was purified by flash
chromatography (ethyl acetate–hexane 1:4) to yield the
allonate 19 (2.25 g, 56%) (see data above).
Further elution afforded the altronate 20 (0.22 g, 6%); [α]D24
Ϫ7.3 (c 2.55 in CHCl3) [HRMS m/z (CIϩ) Found: 444.1697
ϩ
(M ϩ NH4 ). C20H30NO8S requires m/z, 444.1692]; νmax (thin
film) 1765 (C᎐O, ester) cmϪ1; δ (CDCl3; 400 MHz) 1.29–1.43
᎐
H
(2H, m, cyclohexylidene), 1.45–1.60 (6H, m, cyclohexylidene),
1.61–1.73 (2H, m, cyclohexylidene), 2.45 (3H, s, CH3Ar), 3.78
(3H, s, OCH3), 4.09 (1H, dd, J 10.7, 3.1, H-6), 4.14 (1H, dd,
J 10.7, 3.0, HЈ-6), 4.45 (1H, m, H-5), 4.62 (1H, d, J 5.1, H-2),
4.76 (1H, dd, J 6.0, 1.3, H-4), 5.03 (1H, a-t, H-3), 7.36 (2H, d,
J 8.3, ArH), 7.77 (2H, d, J 8.3, ArH); δC (CDCl3; 100.6 MHz)
21.7 (q, CH3Ar), 23.7, 23.8, 24.9, 34.5, 35.7 (5 × t, CH2,
cyclohexylidene), 52.1 (q, OCH3), 70.6 (t, C-6), 81.4 (d, C-3 or
-4), 81.5 (d, C-4 or -3), 82.1 (d, C-2), 82.4 (d, C-5), 114.5 (s,
O–C–O), 127.9 (d, Ar CH), 130.1 (d, Ar CH), 132.0 (s, Ar C),
ϩ13.5 (c 1.35 in CHCl3); νmax (thin film) 1786 (C᎐O, lactone)
᎐
cmϪ1; δH (CDCl3; 400 MHz) 1.30–1.70 (10H, m, cyclohexyl-
idene), 2.45 (3H, s, CH3Ar), 2.46 (3H, s, CH3Ar), 4.22–4.31 and
4.35–4.40 (3H and 2H, m, H-3, -4, -5, H2-6), 5.15 (1H, d, J 7.2,
H-2), 7.36 (4H, m, ArH), 7.81 (2H, d, J 8.2, ArH), 7.87 (2H, d,
J 8.2 Hz, ArH); δC (CDCl3; 100.6 MHz) 21.7, 21.7 (2 × q,
2 × CH3Ar), 23.3, 23.7, 24.7, 34.0, 36.5 (5 × t, cyclohexylidene,
5 × CH2), 66.9 (t, C-6), 70.0, 74.5, 75.3, 76.3 (4 × d, C-2, -3, -4,
-5), 114.1 (s, O–C–O), 128.1, 128.3, 129.7, 130.0 (4 × d, 4 × Ar
CH), 132.1, 133.2 (2 × s, 2 × Ar C), 145.3, 145.4 (2 × s, 2 × Ar
145.4 (s, Ar C), 168.3 (s, O–C᎐O); m/z (APCIϩ) 427 (M ϩ Hϩ,
᎐
100%).
Methyl 2,5-anhydro-6-azido-3,4-O-cyclohexylidene-6-deoxy-D-
allonate 18
The THF tosyl ester 19 (2.25 g, 5.28 mmol) was dissolved in
DMF (45 mL), sodium azide (0.721 g, 11.1 mmol) was added in
one portion, and the mixture was stirred at 85 ЊC for 2 h. TLC
(ethyl acetate–hexane 1:1) indicated complete conversion of
the starting material (Rf 0.6) to a single product (Rf 0.7). Water
(10 drops) was added and the mixture was allowed to cool and
was concentrated in vacuo (co-evaporation with toluene). Buffer
(pH 7; 50 mL) was added and the mixture was extracted with
ethyl acetate (3 × 50 mL). The combined organic extracts were
dried over MgSO4 and concentrated. Flash chromatography
(ethyl acetate–hexane 1:5) afforded the azide 18 (1.44 g, 92%)
as a colourless oil, [α]D24 ϩ23.9 (c 1.15 in CHCl3) [HRMS
m/z (CIϩ) Found: 297.1330 (Mϩ). C13H19N3O5 requires M,
297.1325]; νmax (thin film) 2096 (N ), 1747 (C᎐O, ester) cmϪ1;
C), 164.2 (s, O–C᎐O); m/z (APCIϩ) 567 (M ϩ Hϩ, 100%).
᎐
Methyl 2,5-anhydro-3,4-O-cyclohexylidene-6-O-(p-tolyl-
sulfonyl)-D-allonate 19 and methyl 2,5-anhydro-3,4-O-cyclo-
hexylidene-6-O-(p-tolylsulfonyl)-D-altronate 20
Method 1 (from the triflate 17). Potassium carbonate (0.030 g,
0.22 mmol) was added to a stirred solution of the triflate 17
(0.119 g, 0.22 mmol) in MeOH (4 mL). After 1 h, TLC analysis
(ethyl acetate–hexane 1:1) indicated complete conversion of
the starting material (Rf 0.73) to a major product (Rf 0.65). The
reaction mixture was concentrated in vacuo, the residue was re-
dissolved in DCM, and the solution was filtered through Celite,
which was then washed with DCM. Concentration in vacuo gave
a residue, which was purified by flash chromatography (ethyl
acetate–hexane 1:4) to yield the allonate 19 (0.062 g, 67%) as a
colourless oil, [α]D24 Ϫ11.5 (c 1.13 in CHCl3) [HRMS m/z (CIϩ)
Found: 426.1344 (Mϩ). C20H26O8S requires M, 426.1348]
(Found: C, 56.24; H, 6.49. C20H26O8S requires C, 56.32; H,
᎐
3
δH (CDCl3; 500 MHz) 1.40–1.52 (2H, m, cyclohexylidene),
1.55–1.75 (6H, m, cyclohexylidene), 1.76–1.85 (2H, m, cyclo-
hexylidene), 3.48 (1H, dd, J 12.9, 4.6, H-6), 3.59 (1H, dd, J 12.9,
5.9, HЈ-6), 3.85 (3H, s, OCH3), 4.36 (1H, m, H-5), 4.62–4.64
(1H, m, H-4), 4.63 (1H, d, J 2.8, H-2), 4.97 (1H, dd, J 6.2, 2.9,
H-3); δC (CDCl3; 126 MHz) 23.6, 23.9, 24.9, 34.7, 36.8 (5 × t,
CH2, cyclohexylidene), 52.5 (q, OCH3), 52.5 (t, C-6), 82.1 (d,
C-2 or -4), 83.3 (d, C-3), 84.0 (d, C-4 or -2), 84.9 (d, C-5), 115.0
(s, O–C–O), 170.8 (s, O–C᎐O); m/z (APCIϩ) 270 (M ϩ Hϩ Ϫ
6.14%); νmax (thin film) 1752 (C᎐O, ester) cmϪ1; δ (CDCl ; 400
᎐
᎐
H
3
N2, 60%), 154 (100).
MHz) 1.30–1.48 (2H, m, cyclohexylidene), 1.50–1.69 (6H, m,
cyclohexylidene), 1.70–1.78 (2H, m, cyclohexylidene), 2.46 (3H,
s, CH3Ar), 3.77 (3H, s, OCH3), 4.09 (1H, dd, J 10.5, 5.0, H-6),
4.14 (1H, dd, J 10.5, 4.2, HЈ-6), 4.37 (1H, m, H-5), 4.54 (1H, d,
J 2.7, H-2), 4.63 (1H, dd, J 6.1, 1.9, H-4), 4.95 (1H, dd, J 6.1,
2.7, H-3), 7.36 (2H, d, J 8.3, ArH), 7.80 (2H, d, J 8.3, ArH);
δC (CDCl3; 100.6 MHz) 21.6 (q, CH3Ar), 23.6, 23.9, 24.9, 34.5,
36.6 (5 × t, CH2, cyclohexylidene), 52.5 (q, CH3O), 69.4 (t,
C-6), 81.7 (d, C-4), 83.2, 83.4 (2 × d, C-3, -5), 84.5 (d, C-2),
114.7 (s, O–C–O), 128.0 (d, Ar CH), 129.9 (d, Ar CH), 132.3 (s,
6-Amino-2,5-anhydro-3,4-O-cyclohexylidene-6-deoxy-D-
allonolactam 22
A solution of the azidomethyl ester 18 (0.039 g, 0.13 mmol) in
methanol (0.5 mL) was stirred under an atmosphere of hydro-
gen in the presence of palladium black (3 mg, 10%-wt). After
1 h, TLC (ethyl acetate–hexane 1:5) indicated an absence of
starting material (Rf 0.3) and a major product (Rf 0.0). Further
analysis by TLC (ethyl acetate–hexane 5:1) revealed a product
spot (Rf 0.2). Filtration through Celite and flash chromato-
graphy purification (ethyl acetate–hexane 3:1) gave the bicyclic
lactam 22 as a white solid (0.020 g, 64%), mp 213–215 ЊC; [α]D24
ϩ23.3 (c 0.48 in CHCl3) [HRMS m/z (CIϩ) Found: 240.1235
(M ϩ Hϩ). C12H18NO4 requires m/z, 240.1236]; νmax (KBr disc).
3435, 3224 (NH), 1696 (C᎐O, amide) cmϪ1; δ (CDCl3; 400
Ar C), 145.1 (s, Ar C), 170.6 (s, O–C᎐O); m/z (APCIϩ) 427
᎐
(M ϩ Hϩ, 100%), 449 (M ϩ Naϩ, 50), 255 (50).
Method 2 (from the ditosylate compound 14). Potassium
carbonate (1.30 g, 9.4 mmol) was added to a stirred solution of
the ditosyl compound 14 (5.34 g, 9.4 mmol) in MeOH (150
mL). The solution immediately turned from colourless to
orange and stirring was continued for 1 h. TLC analysis (ethyl
acetate–hexane 1:4) then indicated complete conversion of the
starting material (Rf 0.1) to a major product (Rf 0.15) and a
minor product at slightly lower Rf (0.1). The reaction mixture
was concentrated in vacuo, the residue was re-dissolved in
DCM, and the solution was filtered through Celite, which was
᎐
H
MHz) 1.36–1.45 (2H, m, cyclohexylidene), 1.52–1.68 (6H, m,
cyclohexylidene), 1.72–1.79 (2H, m, cyclohexylidene), 3.12
(1H, dd, J 11.9, 2.5, H-6), 3.66 (1H, dd, J 11.9, 4.9, HЈ-6),
4.43–4.47 (2H, m, H-2, -5), 4.71 (1H, d, J 5.8, H-3 or -4), 4.82
(1H, d, J 5.8, H-4 or -3), 6.33 (1H, br s, NH); δC (CDCl3;
100.6 MHz) 23.6, 23.9, 25.0, 34.3, 35.6 (5 × t, CH2, cyclohexyl-
idene), 43.4 (t, C-6), 77.7, 81.9, 83.1, 83.8 (4 × d, C-2, -3, -4, -5),
3674
J. Chem. Soc., Perkin Trans. 1, 2000, 3666–3679

View Article Online
114.0 (s, O–C–O), 168.7 (s, NH–C᎐O); m/z (APCIϩ) 240
acetate–hexane 1:5) to give the isopropylidene methyl ester 26,
᎐
(M ϩ Hϩ, 100%).
[α]D24 ϩ28.0 (c 0.45 in CHCl3) [HRMS m/z (CIϩ) Found:
258.1080 (M ϩ Hϩ). C10H16N3O5 requires m/z, 258.1090]; νmax
Isopropyl 2,5-anhydro-6-azido-3,4-O-cyclohexylidene-6-deoxy-
D-allonate 24 and isopropyl 2,5-anhydro-6-azido-6-deoxy-D-
allonate 23
(thin film) 2106 (N ), 1755 (C᎐O, ester) cmϪ1; δ (CDCl3; 500
᎐
3
H
MHz) 1.36 (3H, s, CH3C), 1.56 (3H, s, CH3C), 3.45 (1H, dd,
J 12.9, 4.7, H-6), 3.55 (1H, dd J 12.9, 5.7, HЈ-6), 3.81 (3H, s,
CH3O), 4.31 (1H, m, H-5), 4.58 (1H, d, J 3.2, H-2), 4.60 (1H,
dd, J 6.4, 2.7, H-4), 4.94 (1H, dd, J 6.3, 3.2, H-3); δC (CDCl3; 50
MHz) 25.2 (q, CH3C), 27.0 (q, CH3C), 52.5 (t, C-6), 52.6 (q,
OCH3), 82.4, 83.6, 83.8, 84.7 (4 × d, C-2, -3, -4, -5), 114.3 (s,
O–C–O), 170.7 (s, O–C᎐O); m/z (APCIϩ) 230 (M ϩ Hϩ Ϫ N ,
The methyl ester 18 (0.120 g, 0.404 mmol) was stirred in a 5%
v/v solution of HCl in propan-2-ol (78 µL of acetyl chloride
was added to 1.5 mL of propan-2-ol), and stirred at 80 ЊC for
3 h. TLC (ethyl acetate–hexane 1:5) showed a new product (Rf
0.4) and very little residual starting material (Rf 0.3) with a
small amount of base-line product (Rf 0.0). Solid sodium
bicarbonate (100 mg, excess) was added, and the reaction mix-
ture was stirred for 20 min and then filtered through Celite.
Concentration in vacuo and purification of the residue by flash
chromatography (ethyl acetate–hexane 1:10) afforded the oily
isopropyl ester 24 (0.109 g, 83%) as the major product, [α]D24
ϩ16.0 (c 0.23 in CHCl3) [HRMS m/z (CIϩ) Found: 298.1663
(M ϩ Hϩ Ϫ N2). C15H24NO5 requires m/z, 298.1654] (Found: C,
55.70; H, 7.04; N, 12.47. C15H23N3O5 requires C, 55.37; H, 7.13;
N, 12.91%); νmax (thin film) 2103 (N ), 1745 (C᎐O, ester) cmϪ1;
᎐
2
100%).
6-Amino-2,5-anhydro-6-deoxy-3,4-O-isopropylidene-D-
allonolactam 27. Methyl 2,5-anhydro-6-azido-6-deoxy-3,4-O-
isopropylidene--allonate 26 (0.040 g, 0.16 mmol) was dis-
solved in MeOH (0.5 mL) under a nitrogen atmosphere. Pd
black (4 mg, 10%-wt) was added and the reaction mixture was
stirred under a hydrogen atmosphere. After 1 h, TLC (ethyl
acetate–hexane 5:1) showed the absence of starting material
and a new product (at Rf 0.08). Filtration through Celite, con-
centration, and flash chromatography purification (100% ethyl
acetate) yielded the isopropylidene lactam 27 (0.021 g, 68%) as
a white solid, mp 193–195 ЊC (lit.11 -enantiomer 28, 192 ЊC);
[α]D24 ϩ34.4 (c 0.43 in CHCl3) {lit.,11 -enantiomer 28 [α]D23 Ϫ39.2
(c, 0.75 in CHCl3), [α]D22 Ϫ25.7 (c, 0.75 in EtOH)} [HRMS
m/z (CIϩ) Found: 200.0926 (M ϩ Hϩ). C9H14NO4 requires
᎐
3
δH (CDCl3; 400 MHz) 1.30 [6H, a-d, J 6.3, CH(CH3)2], 1.32–
1.48, (2H, m, cyclohexylidene), 1.53–1.71 (6H, m, cyclohexyl-
idene), 1.75–1.80 (2H, m, cyclohexylidene), 3.39 (1H, dd,
J 12.9, 4.8, H-6), 3.56 (1H, dd, J 12.9, 6.3, HЈ-6), 4.32 (1H, m,
H-5), 4.56 (1H, d, J 2.6, H-2), 4.58 (1H, dd, J 6.2, 2.6, H-4), 4.90
(1H, dd, J 6.2, 2.6, H-3), 5.10 [1H, sept, J 6.3 Hz, CH(CH3)2];
δC (CDCl3; 100.6 MHz) 21.69, 21.71 [2 × q, CH(CH3)2],
23.6, 23.9, 24.9, 34.8, 36.7 (5 × t, CH2, cyclohexylidene), 52.5 (t,
C-6), 69.4 [d, OCH(CH3)2], 82.2, 83.5, 84.4, 85.2 (4 × d, C-2, -3,
m/z, 200.0923]; νmax (KBr disc) 3433, 3215 (NH), 1698 (C᎐O,
᎐
amide) cmϪ1; δH (CDCl3; 400 MHz) 1.34 (3H, s, CH3), 1.52 (3H,
s, CH3), 3.13 (1H, dd, J 11.9, 2.7, H-6), 3.67 (1H, dd, J 11.9, 4.9,
HЈ-6), 4.44–4.47 (2H, m, H-2, -5), 4.72 (1H, d, J 5.8, H-3 or -4),
4.84 (1H, d, J 5.8, H-4 or H-3), 6.23 (1H, br s, NH); δC (CDCl3;
100.6 MHz) 24.8, 26.0 [2 × q, C(CH3)2], 43.5 (t, C-6), 77.6, 81.9,
83.6, 84.2 (4 × d, C-2, -3, -4, -5), 113.3 (s, O–C–O), 168.6 (s,
-4, -5), 114.8 (O–C–O), 170.1 (O–C᎐O); m/z (APCIϩ): 298
᎐
(M ϩ Hϩ Ϫ N2).
Also eluted was with the isopropyl diol 23 (0.011 g, 11%)
isolated as the minor product, mp 78–79 ЊC; [α]D24 ϩ72.4 (c 0.45
in MeOH) [HRMS m/z (CIϩ) Found: 246.1086 (M ϩ Hϩ).
C9H16N3O5 requires m/z, 246.1090] (Found: C, 44.14; H, 6.32;
N, 16.32. C9H15N3O5 requires: C, 44.08; H, 6.17; N, 17.13%);
νmax (thin film) 3415 (OH), 2099 (N ), 1731 (C᎐O, ester) cmϪ1;
NH–C᎐O); m/z (APCIϩ) 200 (M ϩ Hϩ, 100%). The 1H and 13
C
᎐
NMR spectra of 27 were therefore identical with those of the
enantiomeric lactam 28.11
᎐
3
Confirmation of structure of the altronate product 20
δH (CD3OD; 500 MHz) 1.26 (3H, d, J 6.3, CHCH3), 1.27 (3H,
d, J 6.3, CHCH3), 3.43 (1H, dd, J 13.2, 6.1, H-6), 3.49 (1H, dd,
J 13.2, 3.2, HЈ-6), 3.93 (1H, dd, J 6.9, 5.0, H-4), 4.01 (1H, m,
H-5), 4.16 (1H, dd, J 4.9, 3.1, H-3), 4.31 (1H, d, J 3.0, H-2),
5.04 [1H, sept, J 6.3, CH(CH3)2]; δC (CD3OD; 126 MHz) 21.88,
21.92 [2 × q, CH(CH3)2], 53.7 (t, C-6), 70.3, 73.7, 75.7, 83.5,
Methyl 2,5-anhydro-6-azido-3,4-O-cyclohexylidene-6-deoxy-
D-altronate 29. The minor methyl ester product 20 (0.181 g, 0.42
mmol) was dissolved in DMF (4 mL), sodium azide (0.058 g,
0.89 mmol) was added in one portion, and the mixture was
stirred at 90 ЊC for 2 h. TLC (ethyl acetate–hexane 1:1) indi-
cated complete conversion of the starting material (Rf 0.5) to a
single product (Rf 0.6). Water (10 drops) was added and the
mixture was concentrated in vacuo (co-evaporation with tolu-
ene). Buffer (pH 7; 20 mL) was added and the mixture was
extracted with ethyl acetate (3 × 20 mL). The combined extracts
were dried over magnesium sulfate and concentrated. Flash
chromatography (ethyl acetate–hexane 1:4) afforded the azide
29 (0.088 g, 70%) as a colourless oil, [α]D25 ϩ22.1 (c 0.67 in
CHCl3) [HRMS m/z (CIϩ) Found: 298.1397 (M ϩ Hϩ).
C13H20N3O5 requires m/z 298.1403] (Found: C, 52.25; H, 6.84;
N, 13.92. C13H19N3O5 requires C, 52.52; H, 6.44; N, 14.13%);
84.3 [5 × d, C-2, -3, -4, -5, OCH(CH ) ], 172.0 (s, O–C᎐O);
᎐
3
2
m/z (APCIϪ) 280 (M ϩ ClϪ, 100%).
A small amount of the starting methyl ester 18 (0.008 g, 6%)
was also recovered.
Proof of structure: conversion from cyclohexylidene to isopropyl-
idene protecting groups
Methyl 2,5-anhydro-6-azido-6-deoxy-3,4-O-isopropylidene-D-
allonate 26 from 18. The cyclohexylidene-protected methyl ester
18 (0.076 g, 0.26 mmol) was stirred in a mixture of trifluoro-
acetic acid and water (2:3; 8 mL) at room temperature. After 1
h, TLC (ethyl acetate–hexane 1:1) suggested complete conver-
sion to a single product (Rf 0.1) (no starting material at Rf 0.6).
The solvent was removed under reduced pressure and the resi-
due was triturated with toluene; the crude residue together with
( )-camphorsulfonic acid (0.003 g, 1.25 × 10Ϫ5 mol) and 2,2-
dimethoxypropane (0.03 g, 0.3 mmol) was dissolved in acetone
(1 mL) and the solution was stirred at room temperature for 2 h.
TLC (ethyl acetate–hexane 1:1) indicated a new product (Rf
0.5). Solid sodium bicarbonate was added and the mixture was
stirred for a further 30 min and then filtered through a small
pad of Celite, which was then washed with acetone. The filtrate
was concentrated to give crude product (0.065 g, 100%) which
was used directly in the following reaction. A portion of
the crude mixture was purified by flash chromatography (ethyl
νmax (thin film) 2104 (N ), 1763 (C᎐O, ester) cmϪ1; δ (CDCl3;
᎐
3
H
400 MHz) 1.29–1.43 (2H, m, cyclohexylidene), 1.45–1.62 (6H,
m, cyclohexylidene), 1.63–1.72 (2H, m, cyclohexylidene), 3.47
(1H, dd, J 13.0, 4.2, H-6), 3.56 (1H, dd, J 13.0, 3.9, HЈ-6), 3.80
(3H, s, OCH3), 4.46 (1H, m, H-5), 4.71 (1H, dd, J 6.2, 1.9, H-4),
4.77 (1H, d, J 5.3, H-2), 5.06 (1H, a-t, H-3); δC (CDCl3; 100.6
MHz) 23.7, 23.9, 24.9, 34.7, 35.9 (5 × t, cyclohexylidene), 52.1 (q,
OCH3), 52.8 (t, C-6), 81.5, 81.6, 82.1, 83.5 (4 × d, C-2, -3, -4, -5),
114.8 (s, O–C–O), 168.7 (s, O–C᎐O); m/z (APCIϩ) 320 (M ϩ
᎐
Naϩ, 5%), 298 (M ϩ Hϩ, 10), 270 (M ϩ Hϩ Ϫ N2, 30), 154 (100).
Methyl 2,5-anhydro-6-azido-6-deoxy-3,4-O-isopropylidene-D-
altronate 30. The cyclohexylidene protecting group in 29 was
exchanged for the isopropylidene ketal in 30. The cyclohexyl-
idene ketal 29 (0.073 g, 0.25 mmol) was stirred in a mixture of
J. Chem. Soc., Perkin Trans. 1, 2000, 3666–3679
3675

View Article Online
trifluoroacetic acid and water (2:3; 8 mL) at room temperature.
After 24 h, TLC (ethyl acetate–hexane 1:1) suggested complete
conversion to a single product (Rf 0.1) (no starting material at
Rf 0.6). The solvent was removed under reduced pressure, and
the residue was co-evaporated with toluene; the crude diol
intermediate (0.053 g, 0.25 mmol) was not isolated further and
immediately treated with ( )-camphorsulfonic acid (0.003 g,
1.3 × 10–5 mol) and 2,2-dimethoxypropane (0.033 g, 0.3 mmol)
dissolved in acetone (1 mL) and the solution was stirred at
room temperature for 16 h. TLC (ethyl acetate–hexane 1:1)
indicated a new product (Rf 0.5), but some remaining starting
material (Rf 0.1). Solid sodium bicarbonate was added and the
mixture was stirred for a further 30 min and then filtered
through a small pad of Celite, with washing of the pad with
acetone. The filtrate was concentrated, and purified by flash
chromatography (ethyl acetate–hexane 1:5) to give the aceto-
nide 30 (0.021 g, 31%) and recovered intermediate diol, methyl
2,5-anhydro-6-azido-6-deoxy--altronate. Title product showed
[HRMS m/z (CIϩ) Found: 258.1097 (M ϩ Hϩ). C10H16N3O5
requires m/z, 258.1090]; [α]D24 ϩ17.0 (c 1.06 in CHCl3) {lit.,17 [α]D25
atmosphere; then the filtered solution of the crude amine 25
(0.335 mmol) was added. The reaction mixture was allowed to
warm to room temperature overnight. TLC (ethyl acetate–
hexane 5:1) indicated the formation of a major product (Rf 0.8)
together with a small amount of residual lactam 22 (Rf 0.2). The
reaction mixture was diluted with dichloromethane (30 mL)
and washed with 0.5 M HCl (1 × 20 mL). The aqueous layer
was further extracted with dichloromethane (2 × 10 mL). The
combined dichloromethane phases were washed with pH 7
buffer, dried over MgSO4, and concentrated in vacuo. The
resultant residue was purified by flash chromatography (ethyl
acetate–hexane 1:4) to yield the dimer 4 (0.138 g, 73%), [α]D24
ϩ25.4 (c 1.14 in CHCl3) [HRMS m/z (CIϩ) Found: 565.2886
(M ϩ Hϩ). C27H41N4O9 requires m/z, 565.2874] (Found: C,
57.22; H, 7.37; N, 8.89. C27H40N4O9 requires C, 57.43; H, 7.14;
N, 9.92%); νmax (thin film) 3407 (NH), 2106 (N ), 1739 (C᎐O,
᎐
3
ester), 1681 (C᎐O, amide I), 1530 (amide II) cmϪ1; δ (CDCl3;
᎐
H
500 MHz) 1.32 (3H, d, J 6.3, CHCH3), 1.32 (3H, d, J 6.3,
CHCH3), 1.37–1.86 (20H, m, cyclohexylidene), 3.09 (1H, ddd,
J 13.8, 9.2, 4.5, H-6B), 3.59–3.67 (2H, m, CH2N3), 3.76 (1H,
ddd, J 13.8, 7.6, 4.7, HЈ-6B), 4.26 (1H, a-q, J ≈ 4.5, H-5A), 4.35
(1H, ddd, J 9.1, 4.5, 1.8, H-5B), 4.52–4.58 (4H, m, H-2A, -2B,
-4A, -4B), 4.88 (1H, dd, J 6.1, 2.3, H-3A or -3B), 4.93 (1H, dd,
J 6.3, 2.9, H-3B or H-3A), 5.11 [1H, sept, J 6.3, CH(CH3)2], 7.42
(1H, dd, J 7.2, 4.5, NH); δC (CDCl3; 126 MHz) 21.6, 21.7 [2 × q,
CH(CH3)2], 23.5, 23.6, 23.9, 23.9, 24.9, 24.9, 34.7, 34.7, 36.6,
37.0 (10 × t, CH2, cyclohexylidene), 40.3 (t, CH2NH), 52.4
(t, CH2N3), 69.5 [d, OCH(CH3)2], 81.1, 82.3, 83.3, 83.9, 84.0,
84.0, 84.5, 84.7 (8 × d, C-2A, -2B, -3A, -3B, -4A, -4B, -5A, -5B),
ϩ17.9 (c 1.00 in CHCl3)}; νmax (thin film) 2101 (N ), 1760 (C᎐O,
᎐
3
ester) cmϪ1; δH (CDCl3; 200 MHz) 1.33 (3H, s, CH3), 1.46 (3H,
s, CH3), 3.46 (1H, dd, J 13.0, 4.2, H-6), 3.57 (1H, dd, J 13.0, 4.0,
HЈ-6), 3.80 (3H, s, OCH3), 4.46 (1H, m, H-5), 4.71 (1H, dd,
J 6.2, 2.0, H-4), 4.79 (1H, d, J 5.3, H-2), 5.06 (1H, a-t, H-3);
δC (CDCl3; 100.6 MHz) 25.0, 26.2 [2 × q, C(CH3)2], 52.1
(q, OCH3), 52.8 (t, C-6), 81.3 (d, C-2), 81.9 (d, C-3), 82.4 (d,
C-4), 83.3 (d, C-5), 114.1 (s, O–C–O), 168.6 (s, O–C᎐O); m/z
᎐
(APCIϩ) 258 (M ϩ Hϩ, 5%), 230 (M ϩ Hϩ Ϫ N2, 20), 154
1
(100). The H and 13C NMR spectra of our product 30 were
114.5 (s, O–C–O), 115.0 (s, O–C–O), 169.8 (s, C᎐O), 171.1
᎐
therefore identical with those of an authentic sample.17
(s, C᎐O); m/z (APCIϩ) 565 (M ϩ Hϩ, 40%).
᎐
Continued elution of the flash column (ethyl acetate–hexane
3:1) gave the lactam 22 (0.011 g, 14%).
Protected and deprotected oligomers
The protected dimer, isopropyl 2,5-anhydro-6-(2,5-anhydro-6-
azido-3,4-O-cyclohexylidene-6-deoxy-D-allononamido)-3,4-O-
cyclohexylidene-6-deoxy-D-allonate 4. Aq. sodium hydroxide
(1 M; 0.36 mL, 0.36 mmol) was added to a stirred solution of
methyl 2,5-anhydro-6-azido-3,4-O-cyclohexylidene-6-deoxy--
allonate 18 (0.098 g, 0.33 mmol) in 1,4-dioxane (1.5 mL)–water
(0.25 mL). The reaction mixture was stirred for 1 h at room
temperature. TLC (ethyl acetate–hexane 1:5) indicated com-
plete conversion of the starting material (Rf 0.3) to a major
product (Rf 0.0). The solvent was removed in vacuo (co-
evaporation with toluene), the residue was dissolved in water
(1 mL), and the solution was stirred with Amberlite IR-120(Hϩ)
resin for 10 min, during which time a white precipitate formed.
DCM (2 mL) was added and this mixture was filtered directly
through a sinter to remove the resin. Concentration in vacuo
gave the crude carboxylic acid 21 (0.096 g) which was used
directly in the coupling reaction.
A solution of the isopropyl azide 24 (0.109 g, 0.335 mmol) in
propan-2-ol (0.5 mL) was stirred under an atmosphere of
hydrogen in the presence of palladium black (11 mg, 10%-wt).
After 30 min, TLC (ethyl acetate–hexane 1:5) indicated an
absence of starting material (Rf 0.4) and the presence of a
major product (Rf 0.0). Further analysis by TLC (ethyl acetate–
hexane 5:1) revealed the presence of two products: the major
component (Rf 0.1) was the amine 25 and the minor component
(Rf 0.2) was the lactam 22. To avoid further formation of the
lactam 22 from the amine 25, the reaction mixture was filtered
through a small pad of tissue paper directly into the coupling-
reaction vessel and the residue was washed with dichloro-
methane.
Deprotected dimer, isopropyl 2,5-anhydro-6-(2,5-anhydro-6-
azido-6-deoxy-D-allonamido)-6-deoxy-D-allonate 7. The cyclo-
hexylidene-protected dimer 4 (0.030 g, 5.3 × 10Ϫ5 mol) was
dissolved in a mixture of CHCl3–TFA (1:1, 2 mL) to which
water (2 drops) and propan-2-ol (2 drops) were also added.
After stirring of the mixture for 30 min, TLC analysis (11%
MeOH in CHCl3) showed complete consumption of starting
material (Rf 0.8) and two product spots (Rf 0.3 and 0.1). Con-
centration of the reaction mixture in vacuo (with toluene co-
evaporation) and mass spectral analysis indicated the products
to be the monodeprotected species with some of the fully
deprotected azido isopropyl ester 7. The crude product mixture
was subjected to the same reaction conditions for 5 h. This
procedure was continued until mass spectral analysis indicated
complete conversion to the fully deprotected product 7. Con-
centration in vacuo (with toluene co-evaporation) yielded a
residue which was purified by flash chromatography (10%
MeOH in CHCl3) to give the deprotected dimer 7 (0.017 g,
81%), [α]D24 ϩ28.4 (c 0.29 in MeOH) [HRMS m/z (CIϩ) Found:
405.1612 (M ϩ Hϩ). C15H25N4O9 requires m/z, 405.1622]; νmax
(KBr disc) 3500–3200 (NH, OH), 2106 (N ), 1724 (C᎐O,
᎐
3
ester), 1674, 1652 (C᎐O, amide I), 1534 (C᎐O, amide II)
᎐
᎐
cmϪ1; δH (CD3OD; 500 MHz) 1.30 (3H, d, J 6.3, CHCH3),
1.31 (3H, d, J 6.3, CHCH3), 3.51 (1H, dd, J 14.0, 6.0, H-6B),
3.59 (1H, dd, J 14.0, 4.1, HЈ-6B), 3.68 (1H, dd, J 13.2, 3.7,
H-6A), 3.71 (1H, dd, J 13.2, 4.8, HЈ-6A), 3.89 (1H, dd, J 7.0,
5.0, H-4A or 4B), 3.90 (1H, dd, J 7.5, 5.0, H-4B or -4A), 4.01 (2H,
m, H-5A, -5B), 4.16 (1H, dd, J 5.0, 2.8, H-3A or -3B), 4.17
(1H, dd, J 5.2, 2.9, H-3B or -3A), 4.29 (1H, d, J 2.8, H-2A),
4.33 (1H, d, J 2.8, H-2B), 5.08 [1H, sept, J 6.3, CH(CH3)2];
δC (CD3OD; 125.8 MHz) 21.9, 22.0 [2 × q, CH(CH3)2], 41.8 (t,
C-6B), 53.1 (t, C-6A), 70.5 [d, OCH(CH3)2], 73.0, 73.6 (2 × d,
2 × C-4), 75.8, 76.1 (2 × d, 2 × C-3), 82.5, 82.5 (2 × d, 2 ×
1-[3-(Dimethylamino)propyl]-3-ethylcarbodiimide
hydro-
chloride (0.095 g, 0.50 mmol) was added to a stirred solution of
the crude carboxylic acid 21 (0.33 mmol), followed by 1-hydroxy-
benzotriazole (0.068 g, 0.5 mmol) and diisopropylethylamine
(87 µL, 0.065 g, 0.5 mmol) in dichloromethane (1 mL) at 0 ЊC.
The reaction mixture was stirred for 30 min under a nitrogen
C-5), 84.0 (d, C-2 ), 85.5 (d, C-2 ), 172.8 (s, O–C᎐O), 173.1 (s,
᎐
B
A
NH–C᎐O); m/z (APCIϩ) 405 (M ϩ Hϩ, 100%), 363 (M ϩ Hϩ
᎐
Ϫ Pri, 10).
3676
J. Chem. Soc., Perkin Trans. 1, 2000, 3666–3679

View Article Online
Table 1 δH-Values for tetramer 5 (CDCl3) from 2D NMR experiments
(COSY, TOCSY, HMQC, HMBC)
The protected tetramer, isopropyl 2,5-anhydro-6-[2,5-anhydro-
6-azido-3,4-O-cyclohexylidene-6-deoxy-D-allonamido-(N→6)-
2,5-anhydro-3,4-O-cyclohexylidene-6-deoxy-D-allonamido-
(N→6)-2,5-anhydro-3,4-O-cyclohexylidene-6-deoxy-D-
Ring
NH
H-2a
H-3
H-4
H-5
H-6/HЈ-6
allonamido-(N→6)-2,5-anhydro-3,4-O-cyclohexylidene-6-deoxy-
D-allonamido]-3,4-O-cyclohexylidene-6-deoxy-D-allonate 5. Aq.
sodium hydroxide (1 M; 0.13 mL, 0.13 mmol) was added to a
stirred solution of the protected dimer 4 (0.065 g, 0.12 mmol) in
1,4-dioxane (0.6 mL)–water (0.1 mL). TLC (ethyl acetate–
hexane 1:1) indicated complete conversion of the starting
material (Rf 0.5) to a major product (Rf 0.0). The solvent was
removed in vacuo (co-evaporation with toluene), the residue was
dissolved in 1,4-dioxane (0.5 mL)–water (1 mL), and the solu-
tion was stirred with Amberlite IR-120 (Hϩ) resin for 15 min.
The resin was removed by filtration and the filtrate was concen-
trated to give crude dimer acid 31.
A
B
[4.50,
4.51, 4.51,
4.53(B)]
4.93
4.88
4.55
4.40
4.28
4.11
3.42/3.64
3.33/3.74
7.21
C
D
8.10
8.04
4.87
4.84
4.48
4.60
4.19
4.29
3.54/3.54
3.35/3.48
a H-2 Assignments other than HB not confirmed to a particular ring
system
57), 1065.54 (M ϩ Naϩ, 100), 1063.03 ([2M ϩ H ϩ K]2ϩ,
12), 1045.49 (M ϩ Hϩ, 8), 1044.55 (M ϩ Hϩ, 28), 1043.57
(M ϩ Hϩ, 49), (isotope distribution).
A solution of dimer 4 (0.065 g, 0.12 mmol) in propan-2-ol
(0.3 mL) was stirred with palladium black (6 mg, 10%-wt)
under a hydrogen atmosphere. After 4 h, TLC (ethyl acetate–
hexane 1:1) indicated conversion of the starting material (Rf
0.5) to the amine 32 as the major product (Rf 0.0). The reaction
mixture was filtered through Celite (eluted with PriOH) and
concentrated to give crude dimer amine 32.
Deprotected tetramer, isopropyl 2,5-anhydro-6-[2,5-anhydro-
6-azido-6-deoxy-D-allonamido-(N→6)-2,5-anhydro-6-deoxy-D-
allonamido-(N→6)-2,5-anhydro-6-deoxy-D-allonamido]-6-
deoxy-D-allonate 8. Cyclohexylidene-protected tetramer
5
(0.015 g, 1.4 × 10Ϫ5 mol) was dissolved in a mixture of CHCl3–
TFA (1:1; 1 mL) to which water (1 drop) and propan-2-ol
(1 drop) were also added. After stirring of the mixture for 3 h,
concentration of the reaction in vacuo (with toluene co-
evaporation) enable mass spectral analysis. A mixture of partly
deprotected species was observed. The crude product mixture
was subjected to the same reaction conditions repeatedly until
mass spectral analysis indicated almost complete conversion to
the fully deprotected product 8. The final residue obtained was
purified by flash chromatography (60% CHCl3, 30% MeOH, 5%
water, 3% AcOH) to give the deprotected tetramer 8 (0.010 g,
100%), [α]D25 ϩ47.1 (c 0.48 in MeOH); νmax (thin film) 3500–3200
1-[3-(Dimethylamino)propyl]-3-ethylcarbodiimide
hydro-
chloride (0.055 g, 0.18 mmol) was added to a stirred solution of
the crude dimer acid 31 (0.12 mmol), 1-hydroxybenzotriazole
(0.024 g, 0.18 mmol) and diisopropylethylamine (31 µL, 0.023
g, 0.18 mmol) in dichloromethane (0.6 mL) at 0 ЊC. The mixture
was stirred for 30 min under a nitrogen atmosphere, and then a
solution of the crude dimer amine 32 (0.12 mmol) in DCM (0.8
mL) was added. The reaction mixture was allowed to warm to
room temperature overnight. TLC (ethyl acetate–hexane 1:1)
indicated the formation of a major product (Rf 0.2). The reac-
tion mixture was diluted with DCM (40 mL) and washed with
0.5 M HCl (1 × 20 mL). The aqueous layer was further
extracted with DCM (2 × 20 mL). The combined DCM
phases were washed with pH 7 buffer, dried over MgSO4, and
concentrated in vacuo. The resultant residue was purified by
flash chromatography (ethyl acetate–hexane 1:1) to yield the
tetramer 5 (0.081 g, 65%), [α]D25 ϩ43.2 (c 1.59 in CHCl3); νmax
(NH, OH), 2101 (N ), 1724 (C᎐O, ester), 1654 (C᎐O, amide I),
᎐
᎐
3
1546 (C᎐O, amide II) cmϪ1; δ (CD OH; 500 MHz) 1.25 (3H, d,
᎐
H
3
J 6.3, CHCH3), 1.26 (3H, d, J 6.3, CHCH3), 3.38 (1H, ddd,
J 13.8, 4.8, 3.3, HЈ-6B), 3.44 (1H, a-dt, J 13.8, 5.4, HЈ-6D), 3.56
(2H, m, H2-6C), 3.59 (1H, m, H-6D), 3.61 (1H, m, HЈ-6A), 3.68
(1H, m, H-6B), 3.70 (1H, m, H-6A), 3.81 (1H, dd, J 7.1, 4.7,
H-4B), 3.86 (1H, dd, J 7.0, 5.0, H-4C), 3.89 (1H, dd, J 7.7, 4.9,
H-4A), 3.91 (1H, dd, J 6.7, 5.0, H-4D), 3.97 (1H, m, H-5B), 3.99
(1H, m, H-5C), 4.00 (1H, m, H-5D), 4.02 (1H, m, H-5A), 4.13
(1H, dd, J 5.0, 3.3, H-3D), 4.19 (1H, m, H-3C), 4.20 (1H, m,
H-3A), 4.21 (1H, m, H-3B), 4.24 (1H, d, J 2.7, H-2B), 4.25 (1H,
d, J 2.9, H-2C), 4.27 (1H, d, J 3.3, H-2D), 4.29 (1H, d, J 2.7,
H-2A), 5.03 [1H, sept, J 6.3, OCH(CH3)2], 8.07 (1H, dd, J 7.8,
4.9, NHB), 8.36 (1H, a-t, J 5.9, NHD), 8.53 (1H, a-t, J 6.2,
NHC); δH (d5-pyridine; 500 MHz)32 8.68 (1H, dd, J 8.0, 4.6,
NH), 9.06 (1H, a-t, J 5.9, NH), 9.26 (1H, a-t, J 6.2, NH);
δH (10% D2O–H2O; 500 MHz)32 8.18 (br s, NH), 8.25 (br s,
NH), 8.40 (br t, J 5.5, NH); δC (CD3OD; 125.8 MHz) 20.7, 20.7
[2 × q, CH(CH3)2], 40.9 (t, C-6D), 41.5 (t, C-6C), 41.8 (t, C-6B),
52.0 (t, C-6A), 68.9 [OCH(CH3)2], 71.3 (d, C-4A), 72.8 (d, C-4D),
73.0 (d, C-4B), 73.0† (d, C-4C), 74.6 (d, C-3D), 74.9† (d, C-3A),
74.9† (d, C-3B), 74.9†(d, C-3C), 81.2† (d, C-5A), 81.2†; (d, C-5C),
81.2† (d, C-5D), 81.7 (d, C-5B), 82.6 (d, C-2D), 83.9† (d, C-2B),
83.9† (d, C-2C), 84.2 (d, C-2A), 171.2 (s, C-1D), 172.1 (s, C-1C),
172.7† (s, C-1B), 172.7† (s, C-1A); m/z (ESϩ) 761.48 (M ϩ Kϩ,
7%), 747.44 (M ϩ Naϩ, 9), 746.44 (M ϩ Naϩ, 30) 745.49 (M ϩ
Naϩ, 100), 742.48 ([2M ϩ H ϩ K]2ϩ, 4) 723.40 (M ϩ Hϩ, 7)
(isotope distribution).
(thin film) 3307 (NH), 2105 (N ), 1740 (C᎐O, ester), 1667 (C᎐O,
᎐
᎐
3
amide I), 1539 (C᎐O, amide II) cmϪ1; δ (CDCl3; 400 MHz)
᎐
H
1.28 [6H, a-d, J 6.2, CH(CH3)2], 1.30–1.80 (40H, m, cyclo-
hexylidene 20 × CH2), 3.33 (1H, m, HЈ-6B), 3.35 (1H, m,
HЈ-6D), 3.42 (1H, dd, J 13.3, 6.2, HЈ-6A), 3.48 (1H, dd, J 13.7,
6.6, H-6D), 3.54 (2H, m, H2-6C), 3.64 (1H, dd, J 13.2, 3.6,
H-6A), 3.74 (1H, ddd, J 14.0, 9.6, 8.7, H-6B), 4.11 (1H, a-dt,
J 9.6, 3.6, H-5B), 4.19 (1H, a-dt, J 8.2, 4.2, H-5C), 4.28 (1H,
a-dt, J, 6.3, 4.1, H-5A), 4.29 (1H, a-dt, J 6.9, 2.4, H-5D), 4.40
(1H, dd, J, 5.7, 4.9, H-4B), 4.48 (1H, dd, J 6.2, 4.2, H-4C), 4.50
(1H, d, J 2.6, H-2), 4.51 (2H, 2 × d, J ≈ 2.2, 2 × H-2), 4.53
(1H, d, J 2.0, H-2), 4.55 (1H, dd J 6.4, 3.7, H-4A), 4.60 (1H, dd,
J 6.2, 2.4, H-4D), 4.84 (1H, dd, J 6.2, 2.6, H-3D), 4.87 (1H,
dd J 6.4, 2.7, H-3C), 4.88 (1H, dd, J 5.7, 2.4, H-3B), 4.93 (1H,
dd, J 6.3, 2.8, H-3A), 5.07 [1H, sept, OCH(CH3)2], 7.21 (1H,
dd, J 8.1, 4.8, NHB), 8.04 (1H, a-t, J 5.9, NHD), 8.10 (1H,
a-t, J 6.1, NHC); δH (d5-pyridine; 500 MHz)32 8.96 (1H,
a-t, J 6.2, NH), 9.02 (1H, a-t, J 5.9, NH) and 9.24 (1H, a-t,
J 6.1, NH); δC (CDCl3; 100.6 MHz) 21.7, 21.7 [2 × q,
CH(CH3)2], 23.5, 23.6, 23.9, 34.6, 34.7, 34.8, 36.7, 36.9, 37.2
(9 × t, cyclohexylidene, 20 × CH2), 40.6, 41.7, 42.0 (3 × t,
3 × CH2NH), 51.8 (t, CH2N3), 69.5 [d, OCH(CH3)2], 80.7,
81.1, 81.6, 82.6, 83.3, 83.5, 83.8, 83.8, 84.0, 84.1, 84.4, 84.5,
85.3, 85.8 (14 × d, 16 × CHO), 114.6, 114.7, 115.0, 115.1 (4 × s,
The octamer, isopropyl 2,5-anhydro-6-[2,5-anhydro-6-azido-
3,4-O-cyclohexylidene-6-deoxy-D-allonamido-(N→6)-2,5-
anhydro-3,4-O-cyclohexylidene-6-deoxy-D-allonamido-(N→6)-
2,5-anhydro-3,4-O-cyclohexylidene-6-deoxy-D-allonamido-
(N→6)-2,5-anhydro-3,4-O-cyclohexylidene-6-deoxy-D-
4 × O–C–O), 170.6, 170.7, 170.7, 171.3 (4 × s, 4 × C᎐O);
᎐
(15N) (CDCl3; 50.7 MHz) 107.2 (NHD), 108.0 (NHB), 108.8
(NHC); m/z (APCIϩ) 1043 (M ϩ Hϩ), 1065 (M ϩ Naϩ);
m/z (ESϩ) 1084.45 (M ϩ Kϩ, 4%), 1083.47 (M ϩ Kϩ, 15),
1082.45 (M ϩ Kϩ, 31), 1081.50 (M ϩ Kϩ, 52), 1068.47
(M ϩ Naϩ, 4), 1067.46 (M ϩ Naϩ, 20), 1066.50 (M ϩ Naϩ,
allonamido-(N→6)-2,5-anhydro-3,4-O-cyclohexylidene-6-deoxy-
† Values obtained from broad/overlapping HSQC/HMBC crosspeaks.
J. Chem. Soc., Perkin Trans. 1, 2000, 3666–3679
3677

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Table 3 δH-Values for octamer 6 (CDCl3) from 2D NMR experi-
ments (COSY, TOCSY, HMQC, HMBC)
Table 2 δH-Values for tetramer 8 (CD3OH) from 2D NMR experi-
ments (DQFCOSY, TOCSY, HSQC, HMBC)
Ring
NH
H-2a
H-3
H-4
H-5
H-6/HЈ-6
Ring
NH
H-2
H-3
H-4
H-5
H-6/HЈ-6
A
B
C
D
4.29
4.24
4.25
4.27
4.20
4.21
4.19
4.13
3.89
3.81
3.86
3.91
4.02
3.97
3.99
4.00
3.61/3.70
3.38/3.68
3.56/3.56
3.44/3.59
A
B
4.49
4.56
4.93
4.95
4.95
4.91
4.97
4.90
4.94
4.57
4.39
4.41
4.43
4.42
4.47
4.42
4.30
4.12
4.19
4.18
4.18
4.16
4.21
3.40/3.67
3.27/3.76
3.42/3.56
3.48/3.53
3.49/3.49
3.47/3.56
3.52/3.56
8.07
8.53
8.36
7.12
8.80
8.72
8.64
8.56
8.52
Ca
Da
Ea
Fa
Ga
(4.50,
4.51, 4.52,
4.52,
a Assigned via TOCSY crosspeaks from H-2 to H-4.
4.53)b
D-allonamido-(N→6)-2,5-anhydro-3,4-O-cyclohexylidene-6-
deoxy-D-allonamido-(N→6)-2,5-anhydro-3,4-O-cyclohexyl-
idene-6-deoxy-D-allonamido]-3,4-O-cyclohexylidene-6-deoxy-D-
allonate 6. Aq. sodium hydroxide (1 M; 36 µL, 3.6 × 10Ϫ5 mol)
was added to a stirred solution of tetramer 5 (0.034 g, 3.3 ×
10Ϫ5 mol) in 1,4-dioxane (0.3 mL)–water (0.1 mL). After 1 h,
TLC (ethyl acetate–hexane 1:1) indicated complete conversion
of the starting material (Rf 0.2) to a major product (Rf 0.0).
The solvent was removed in vacuo (co-evaporated with toluene),
the residue was dissolved in 1,4-dioxane (0.5 mL)–water (0.5
mL), and the solution was stirred with Amberlite IR-120 (Hϩ)
resin for 10 min. The resin was removed by filtration and the
filtrate was concentrated to give crude tetramer acid 33 (0.033 g,
100%).
H
8.26
4.48
4.84
4.65
4.28
3.42/3.42
a Rings C, D, E, F and G are randomly assigned and therefore inter-
changeable. However, the connectivities within the ring systems were
established by a TOCSY NMR experiment. b H-2 connectivities to the
remainder of the ring system were not determined for rings C, D, E, F
or G due to signal overlap.
7 × CH2NH), 52.2 (t, CH2N3), 69.5 [d, OCH(CH3)2], 81.0
(C-4A), 81.2, 81.6, 81.8 (C-4), 83.0 (C-4H), 83.8 (C-3H), 83.5,
84.2, 84.3, 84.6 (C-3), 84.3 (C-2B), 84.5, 84.7 (C-2), 84.2 (C-5A),
84.6 (C-5H), 86.3, 86.9 (C-5), 87.0 (C-5B); (15N) (CDCl3; 50.7
MHz) 107.4 (NHH), 108.8 (NHB), 110.1 (NH), 111.9 (NH),
112.0 (NH), 112.0 (NH), 112.5 (NH); m/z (ESϩ) 1021.85
([M ϩ H ϩ K]2ϩ, 6%), 1021.35 ([M ϩ H ϩ K]2ϩ, 15), 1020.84
([M ϩ H ϩ K]2ϩ, 31), 1020.36 ([M ϩ H ϩ K]2ϩ, 76), 1019.82
([M ϩ H ϩ K]2ϩ, 100), 1019.34 ([M ϩ H ϩ K]2ϩ, 87), 781.69
([M ϩ H ϩ K]2ϩ-dimer, 5), 781.24 ([M ϩ H ϩ K]2ϩ-dimer, 13),
780.73 ([M ϩ H ϩ K]2ϩ-dimer, 24), 780.24 ([M ϩ H ϩ K]2ϩ-
dimer, 31), 693.62 ([M ϩ H ϩ K2]3ϩ, 11), 692.95 ([M ϩ H ϩ
K2]3ϩ, 32), 692.74 ([M ϩ H ϩ K2]3ϩ, 27) (isotope distribution);
m/z (ESϩ, 10 mM NH4OAc) 1002.96 ([M ϩ 2H]2ϩ, 5%),
1002.43 ([M ϩ 2H]2ϩ, 12), 1001.96 ([M ϩ 2H]2ϩ, 32), 1001.43
([M ϩ 2H]2ϩ, 66), 1000.96 ([M ϩ 2H]2ϩ, 100), 1000.44 ([M ϩ
2H]2ϩ, 81), (isotope distribution).
A solution of tetramer 5 (0.038 g, 3.6 × 10Ϫ5 mol) in propan-
2-ol (0.2 mL) was stirred with palladium black (4 mg, 10%-wt)
under a hydrogen atmosphere. After 4 h, TLC (ethyl acetate–
hexane 1:1) indicated a mixture of starting material (Rf 0.2)
and product (Rf 0.0). Additional palladium black (6 mg) was
added at this time and again after 6 h. The reaction mixture was
then stirred overnight, when TLC analysis showed an absence
of starting material. The reaction mixture was filtered through
Celite and the product was eluted with propan-2-ol. Concen-
tration in vacuo gave crude tetramer amine 34 (0.032 g, 86%).
1-[3-(Dimethylamino)propyl]-3-ethylcarbodiimide
hydro-
chloride (9.5 mg, 4.9 × 10Ϫ5 mol) was added to a stirred
solution of the crude tetramer acid 33 (3.3 × 10Ϫ5 mol),
1-hydroxybenzotriazole (6.6 mg, 4.9 × 10Ϫ5 mol) and diiso-
propylethylamine (8.5 µL, 6.3 mg, 4.9 × 10Ϫ5 mol) in dichloro-
methane (0.2 mL) at 0 ЊC. The mixture was stirred for 30 min at
0 ЊC under a nitrogen atmosphere, and then a solution of the
crude tetramer amine 34 (3.6 × 10Ϫ5 mol) in dichloromethane
(0.2 mL) was added. The reaction mixture was allowed to warm
to room temperature and was stirred overnight. TLC (ethyl
acetate–hexane 3:2) indicated the formation of a major
product (Rf 0.2). The reaction mixture was diluted with
dichloromethane (50 mL) and washed with 0.5 M HCl (1 × 20
mL). The aqueous layer was further extracted with dichloro-
methane (3 × 15 mL). The combined dichloromethane phases
were washed with pH 7 buffer, dried over MgSO4, and con-
centrated in vacuo. The resultant residue was purified by flash
chromatography (ethyl acetate–hexane 3:2) to yield the
octamer 6 (0.034 g, 52%), [α]D24 ϩ65.9 (c 0.79 in CHCl3); νmax
Acknowledgements
This work was supported by BBSRC and EPSRC. We are grate-
ful to Dr J. H. Jones (University of Oxford) for helpful
discussions.
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᎐
᎐
3
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᎐
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1.78 (80H, m, 40 × CH2), 3.24–3.29 (1H, m, H-6B), 3.37–3.57
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(1H, m, HЈ-6B), 4.09–4.22 (6H, m, 6 × H-5), 4.25–4.32 (2H, m,
H-5A, -5H), 4.36–4.43 (5H, m, 5 × H-4), 4.45–4.49 (3H, m,
1 × H-4, 2 × H-2), 4.50–4.53 (5H, m, 5 × H-2), 4.56 (1H, br s,
H-2B), 4.57 (1H, dd, J 6.4, 3.6, H-4A), 4.65 (1H, dd, J 6.3, 2.6,
H-4H), 4.84 (1H, dd, J 6.3, 2.8, H-3H), 4.88–4.96 (8H, m, 8 ×
H-3), 5.06 [1H, sept, J 6.3, CH(CH3)2], 7.12 (1H, dd, J 8.8, 4.6,
NHB), 8.26 (1H, a-t, J 6.0, NHH), 8.52 (1H, a-t, J 6.4, NH), 8.56
(1H, a-t, J 6.2, NH), 8.64 (1H, a-t, J 6.4, NH), 8.72 (1H, a-t,
J 6.4, NH), 8.80 (1H, a-t, J 6.4, NH); δC (CDCl3; 100.6 MHz)33
21.9, 21.9 [2 × q, CH(CH3)2], 23.8, 24.2, 25.2, 29.9, 35.1, 37.5
(6 × t, cyclohexylidene, 40 × CH2), 40.7, 42.3, 42.7 (3 × t,
3678
J. Chem. Soc., Perkin Trans. 1, 2000, 3666–3679

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J. Chem. Soc., Perkin Trans. 1, 2000, 3666–3679
3679