(2R,1'S,2'R)- And (2S,1'S,2'R)-3-[2-mono(di,tri)fluoromethylcyclopropyl] alanines and their incorporation into hormaomycin analogues

Article abstract of DOI:10.3762/bjoc.10.302

Efficient and scalable syntheses of enantiomerically pure (2 R ,1' S ,2' R )- and (2 S ,1' S ,2' R )-3-[2-mono(di,tri)fluoromethylcyclopropyl] alanines 9a - c , as well as allo-D-threonine ( 4 ) and ( 2 S,3R)-β-methylphenylalanine (3), using the Belokon' approach with (S )- and (R )-2-[( N-benzylprolyl)amino]benzophenone [(S)- and (R)-10 ] as reusable chiral auxiliaries have been developed. Three new fluoromethyl analogues of the naturally occurring octadepsipeptide hormaomycin ( 1 ) with (fluoromethylcyclopropyl) alanine moieties have been synthesized and subjected to preliminary tests of their antibiotic activity.

Full text of DOI:10.3762/bjoc.10.302

(2R,1'S,2'R)- and (2S,1'S,2'R)-3-[2-Mono(di,tri)fluoromethyl-
cyclopropyl]alanines and their incorporation into
hormaomycin analogues
Armin de Meijere*1, Sergei I. Kozhushkov1, Dmitrii S. Yufit2, Christian Grosse3,
Marcel Kaiser4,5 and Vitaly A. Raev*1,6
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2014, 10, 2844–2857.
1Institut für Organische und Biomolekulare Chemie der
Georg-August-Universität Göttingen, Tammannstrasse 2, 37077
Göttingen, Germany, 2Department of Chemistry, University of
Durham, South Rd., Durham DH1 3L, UK, 3Institut für Anorganische
Chemie der Georg-August-Universität Göttingen, Tammannstrasse 4,
37077 Göttingen, Germany, 4Swiss Tropical and Public Health
Institute, Parasite Chemotherapy, Socinstrasse 57, CH-4002 Basel,
Switzerland, 5University of Basel, Petersplatz, 1 CH-4003 Basel
Switzerland and 6Institut für Organische Chemie der TU
Carolo-Wilhelmina zu Braunschweig, Hagenring 30, 38106
Braunschweig, Germany
doi:10.3762/bjoc.10.302
Received: 29 August 2014
Accepted: 18 November 2014
Published: 03 December 2014
This article is part 163 of the series "Cyclopropyl Building Blocks for
Organic Synthesis". For parts 162 and 161 see [1] and [2], respectively.
Dedicated to Professor Max Malacria on the occasion of his 65th birthday.
Associate Editor: N. Sewald
Email:
© 2014 de Meijere et al; licensee Beilstein-Institut.
License and terms: see end of document.
Armin de Meijere* - ameijer1@uni-goettingen.de; Vitaly A. Raev* -
v.raev@tu-braunschweig.de
* Corresponding author
Keywords:
amino acids; biosynthesis; building blocks; cyclopropanes; natural
products; synthetic methods
Abstract
Efficient and scalable syntheses of enantiomerically pure (2R,1'S,2'R)- and (2S,1'S,2'R)-3-[2-mono(di,tri)fluoromethyl-
cyclopropyl]alanines 9a–c, as well as allo-D-threonine (4) and (2S,3R)-β-methylphenylalanine (3), using the Belokon' approach
with (S)- and (R)-2-[(N-benzylprolyl)amino]benzophenone [(S)- and (R)-10] as reusable chiral auxiliaries have been developed.
Three new fluoromethyl analogues of the naturally occurring octadepsipeptide hormaomycin (1) with (fluoromethyl-
cyclopropyl)alanine moieties have been synthesized and subjected to preliminary tests of their antibiotic activity.
Introduction
The intermolecular signal metabolite hormaomycin (1, Germany and structurally identified by Zeeck et al. in
Figure 1) was first isolated from a Streptomyces griseoflavus Göttingen, Germany in 1989–1990 [3,4]. Once the absolute
(strain W-384) fermentation broth by Zähner et al. in Tübingen, configuration of all the previously unassigned stereogenic
2844

Beilstein J. Org. Chem. 2014, 10, 2844–2857.
centers in the octapeptidolactone had been established [5,6] and Results and Discussion
feasible enantioselective syntheses of all the amino acid As prerequisites for the hormaomycin analogues 8a–c, the
building blocks had been developed [7,8], a total synthesis of 1 correctly configured two diastereomers each of the
could be embarked on and eventually completed [9]. Several [2-tri(di,mono)fluoromethylcyclopropyl]alanines 9a–c had to be
precursor-induced biosyntheses [10,11] and chemical syntheses synthesized in enantiomerically pure form. In view of the
[11,12] have provided close to 20 hormaomycin analogues that successful employment of the enantiomeric nickel(II)
have contributed to an understanding of the biosynthetic path- complexes (R)-10 and (S)-10 [18-20] in the synthesis of
ways, the conformational behavior in solution and the struc- (2R,1R',2R')- and (2S,1R',2R')-3-(2-nitrocyclopropyl)alanine (5)
ture–activity relationship.
for the native hormaomycin (1) [7,9], a completely analogous
route was chosen towards (2R,1'S,2'R)- and (2S,1'S,2'R)-9a–c.
After the initial observation that hormaomycin (1) has a marked This required the racemic trans-(2-fluoromethylcyclopropyl)
influence on the secondary metabolite production of various iodides 11a–c as alkylating agents for the enolates of (R)-10 and
streptomycetes including the producing strain itself, and an (S)-10 (Figure 2).
exceptionally selective inhibitory effect on coryneform bacteria
[3,4], an interesting antimalarial activity was discovered [13]. It The initially intended preparations of the three precursors 14a–c
was this property that led us to consider the synthesis of even to the iodides 11a–c all starting from the known dimethyl trans-
more analogues of 1. Bearing in mind that tri(di,mono)fluo- cyclopropane-1,2-dicarboxylate (12) [21] through the
romethyl substituents often enhance the activity of pharmaco- monomethyl ester 13 [22], the hydroxymethyl 15 [22] and the
logically relevant compounds [14-17], we embarked on the formyl derivative 16 [23,24] as outlined in Scheme 1, were only
project to synthesize analogues 8 of hormaomycin (1), in which partially successful.
the (2-nitrocyclopropyl)alanine moieties would be replaced by
[2-tri(di,mono)fluoromethylcyclopropyl]alanine residues As expected, the hydroxymethyl (15) and formyl derivative 16
(Figure 1).
underwent smooth conversion into the monofluoro- and difluo-
Figure 1: Structure and absolute configuration of hormaomycin (1), its fluoromethyl-substituted analogues 8a–c and 3-(2-
fluoromethylcyclopropyl]alanine building blocks 9a–c. 2: (S)-Ile; 3: (2S,3R)-(β-Me)Phe; 4: (2R)-α-Thr; 5: (1'R,2'R)-(3-Ncp)Ala; 6: (2S,4R)-4-(Z)-(4-
PE)Pro; 7: Chpca; 9: (1'S,2'R)-(3-FMcp)Ala.
2845

Beilstein J. Org. Chem. 2014, 10, 2844–2857.
Figure 2: Structures of the Belokon'-type glycine complexes (BGC) (R)- and (S)-10.
Scheme 1: Intended routes to methyl trans-2-(fluormethyl)cyclopropanecarboxylates 14a–c.
romethyl derivatives 14c and 14b, respectively upon treatment achieve acceptable yields. The reduction of the ketoester 20 to
with a solution of Deoxo-Fluor in toluene at ambient tempera- the hydroxyester 21 under the previously described conditions
ture. Unfortunately, conducting the reaction with carboxylic (H2/PtO2) proceeded very slowly, and in several runs (2 h, 24 h,
acid 13 under the same conditions only provided the acid fluo- 72 h) the yield of 21 was never better than 60% with up to 25%
ride and not the trifluormethylated compound 14a.
recovered ketoester 20. However, with powdered sodium boro-
hydride in diethyl ether, the conversion of 20 was quantitative,
and the yield of 21 was excellent (98%). In the final step, the
attempted 1,3-dehydrotosylation of 22 with potassium tert-
butoxide, when carried out in dimethyl sulfoxide as reported
[26,27], compound 23 as an intermolecular condensation prod-
uct of the expected ethyl trans-(2-trifluoromethyl)cyclo-
Therefore, an alternative approach to trans-(2-trifluoro-
methyl)cyclopropanecarboxylic acid 23 by way of the Claisen
condensation product of ethyl trifluoroacetate (17) with diethyl
succinate (18) [25] was used. The conditions for two of the
known further steps [26,27] (Scheme 2) had to be modified to
2846

Beilstein J. Org. Chem. 2014, 10, 2844–2857.
Scheme 2: Synthesis of trans-(2-trifluoromethyl)cyclopropanecarboxylic acid (24).
propanecarboxylate with DMSO was obtained in 74% yield. treatment with the iodine/triphenylphosphine reagent in the
Among several other base/solvent combinations tested – presence of imidazole, the racemic trans-(2-fluoromethylcyclo-
NaOEt/EtOH, NaOMe/MeOH, t-BuOK/t-BuOH, NaH/THF, propyl)methanols 25a–c were smoothly converted to the corres-
t-BuOK/THF under reflux – the last one gave the best results ponding iodides 11a–c in very good yields (Scheme 3).
with up to 47% yield of the trans-(2-trifluoromethyl)cyclo-
propanecarboxylic acid (24).
Alkylation of the glycine equivalent enolates derived from (S)-
and (R)-2-[(N-benzylprolyl)amino]benzophenone [(S)- and (R)-
The conversion of the carboxylic acid 24 and esters 14b,c to the 10] as reusable chiral auxiliaries with the racemic iodides
corresponding cyclopropylmethyl alcohols was attempted 11a–c, employing the protocol of Larionov and de Meijere [7],
according to the standard protocol by adding the respective sub- in each case led to a mixture of diastereomeric products, which
strate to a twofold excess of LiAlH4 in diethyl ether under were separated by column chromatography. Unfortunately, the
reflux. (2-Trifluoromethylcyclopropyl)methanol (25a) thus was diastereomers could not be separated by fractional crystalliza-
obtained in excellent yield (88%), but the difluoromethyl- (25b) tion as easily as was previously reported for the corresponding
and especially monofluoromethylcyclopropylmethanol 25c, res- 3-(trans-2-nitrocyclopropyl)alanine derivatives [7]. The
pectively, were obtained from the corresponding methyl cyclo- absolute configuration of the arbitrarily selected nickel(II)
propanecarboxylates 14b and 14c, respectively, in very poor complexes (2S,1'R,2'S)-26a, (2S,1'R,2'S)-26b and
yields (3% and 4%, respectively). In the case of monofluoro (2R,1'R,2'S)-26b were determined by a single crystal X-ray
derivative 14c the main product (38%) was trans-(2-methylcy- structure analysis (see Figure 3 and Supporting Information
clopropyl)methanol. In the case of the difluoro compound 14b, File 1) [28].
a mixture of the mono- (25c) and difluoromethylcyclopropyl-
methanol 25b along with the non-fluorinated alcohol was The isolated nickel complexes 26a–c were decomposed by
obtained in a ratio of approximately 1:1:1.
treatment with refluxing aqueous methanolic hydrogen chloride
to give, after ion-exchange chromatography, the corresponding
To avoid this overreduction, inverse addition of 1.1 equiv of (2S,1'S,2'R)- [see Scheme 3, derived from (S)-10] and
LiAlH4 in diethyl ether solution (ca. 1 M) to a solution of the (2R,1'S,2'R)-3-(2'-fluoromethylcyclopropyl)alanines [derived
acid 24 or the respective ester 14b,c in diethyl ether (ca. 1 M) from (R)-10] in good to excellent yields. The chiral auxiliary
was practiced. This way, the desired alcohols 25a–c were was recovered as the hydrochloride of 2-[(N-benzyl-
obtained in good yields (88, 82 and 76%, respectively). Upon prolyl)amino]benzophenone (~95%).
2847

Beilstein J. Org. Chem. 2014, 10, 2844–2857.
Scheme 3: Preparation of racemic trans-2-(fluoromethyl)cyclopropylmethyl iodides 11a–c and their conversion to (2S,1'S,2'R)- and (2R,1'S,2'R)-3-
(trans-2'-fluoromethylcyclopropyl)alanines 9a–c (only (2S)-enantiomers are shown in the Scheme). Compounds (S)-10 and (R)-10 are shown in
Figure 2. For details see Table 1.
Table 1: Alkylation of the enolates of the Belokon'-type glycine equivalents (S)- and (R)-10 with the racemic trans-2-(fluoromethyl)cyclopropylmethyl
iodides rac-11a–c (see Scheme 3). Yields in % based on converted (S)- and (R)-10.
from (S)-10
from (R)-10
Iodide
2S,1'S,2'R
2S,1'R,2'S
2R,1'S,2'R
2R,1'R,2'S
rac-11a
rac-11b
rac-11c
46
45
44
49
48
45
44
47
47
42
45
42
(R)-allo-Threonine (4) is commercially available, but extremely 5 g) than the target amino acid, the conversion requires five
expensive (from 77.80 € for 250 mg to 60.80 € for 25 mg). steps, and the overall yield is not better than 72%.
Therefore a simple and inexpensive access to (R)-allo-threo-
nine was desirable.
The Belokon' protocols are among the best to access enan-
tiomerically pure non-proteogenic amino acids. Nickel(II) or
The synthesis of 4 from (R)-threonine as a chiral precursor was copper(II) complexes of Schiff bases derived from glycine and
performed according to the known protocol reported by Tanner (S)- or (R)-2-N-(N'-benzylprolyl)aminobenzophenone (BPB)
et al [29]. Although (R)-threonine is less expensive (21–37 € for [30,31], aminoacetophenone (BPA) [32] or aminobenzaldehyde
2848

Beilstein J. Org. Chem. 2014, 10, 2844–2857.
Figure 3: Structure and absolute configurations of the nickel(II) complexes (2S,1'R,2'S)-26a, (2S,1'R,2'S)-26b and (2R,1'R,2'S)-26b in the crystals.
Less important hydrogen atoms are omitted for clarity.
(BPH) [33] can be used as chiral nucleophilic glycine equiva- the (R)-threonine complex 29 (inverse configuration relative to
lents in reactions with alkyl halides or carbonyl compounds. that of the proline moiety of (S)-10 due to epimerization on
The most versatile one is the nickel(II) aminobenzophenone C-2), but when a weaker base such as triethylamine was
derivative.
employed, a mixture of (R)-threonine 29 and (S)-allo-threonine
31 complexes [35] was obtained.
It is interesting that nickel(II) complexes of Schiff bases derived
from 2-bromoglycine and (S)-BPB can be used as electrophilic The hypothesis, that the reaction of the Belokon' glycine com-
glycine equivalents [34]. Alkylations of the nickel(II) plex (BGC) 10 with aldehydes under strongly basic conditions
complexes of Schiff bases derived from glycine and (S)- or (R)- proceeds in two steps and is thermodynamically controlled, was
BPB with alkyl halides virtually yield single stereoisomers, in corroborated by experimental tests [36]. The initially formed
which the configuration of the newly formed stereogenic center main product (R,R,R)-28 in the aldol reaction of acetaldehyde
at C-2 of the amino acid moiety is the same as that in the with 10 had the same configuration at C-2 as the proline unit in
proline moiety of the chiral auxiliary in the starting material. 10. The absolute configuration of this nickel(II) complex was
determined by a single crystal X-ray structure analysis (see
In reactions of the enolate of this chiral glycine equivalent with Figure 4 and Supporting Information File 1) [28]. However, the
aldehydes the situation is more complicated. The reaction of product ratio changed in time from 95:5 after 30 s through
(S)-10 with acetaldehyde under strongly basic conditions led to 70:18 after 10 min to 5:95 after 24 h at ambient temperature.
2849

Beilstein J. Org. Chem. 2014, 10, 2844–2857.
late moiety and thus making the proton at C-2 accessible to base
attack (Scheme 4). In order to obtain (R)-allo-threonine (4), it is
necessary to carry out the aldol reaction of (R)-10 with an
excess of acetaldehyde under strongly basic conditions at low
temperature and to quench the reaction after a short time to
avoid epimerization of 28.
This modified protocol indeed gave the (R)-allo-threonine (4) in
relatively poor yield [7.5% for the Ni complex, 91% (7%
overall) for the amino acid], but with high enantiomeric purity
in two steps. Bearing in mind that the starting materials are
inexpensive and the chiral auxiliary is reusable (≥95%
recovery), this protocol represents one of the best routes to the
rather expensive (R)-allo-threonine (4).
It is also possible to obtain (R)-allo-threonine starting from (R)-
10 and acetaldehyde under thermodynamic control (Et3N as the
base, (S)-threonine:(R)-allo-threonine = 1:7), but it is necessary
to leave the reaction mixture for two months for the reaction to
go to completion [37].
Figure 4: Structure and absolute configuration of nickel(II) complex
(R,R,R)-28 in the crystal. Hydrogen atoms are omitted for clarity.
This epimerization comes along with a possible rearrangement
in the Ni complex. The newly formed hydroxide group of the (2S,3R)-3-Methylphenylalanine (L-β-methylphenylalanine,
product 28 can coordinate the Ni atom liberating the carboxy- (β-Me)Phe, MeF, 3) also is a constituent of the peptidolactone
Scheme 4: Mechanism of epimerization of the threonine nickel(II) complex 29.
2850

Beilstein J. Org. Chem. 2014, 10, 2844–2857.
hormaomycin (1) and is contained in the molecule twice. Thus All these approaches ought to be applicable to prepare unsubsti-
it is required for the synthesis of hormaomycin and the tuted β-methylphenylalanine, but most if not all of them have
analogues envisaged here. In addition, a versatile protocol for severe drawbacks. Among the chiral auxiliary approaches to
the preparation of other β-alkylarylalanines for incorporation β-branched arylalanines, including all four stereoisomers of
into hormaomycin analogues as well as into other peptides β-methylphenylalanine, the one employing the "Evans amide"
would be desirable as the incorporation of conformationally method with a 4-benzyl- or 4-phenyl-2-oxazolidinone moiety,
constrained α-amino acids such as 3 into peptides is frequently has been used most frequently. Along this route, which requires
used to study structure–activity relationships [38-40].
eight procedural steps (including a transmetallation), the
(2S,3R)-3-methyl-3-phenylalanine required for hormaomycin
Several methods have been developed for the preparation of and its analogues, has previously been utilized [9,54,55].
analogues of β-methylphenylalanine in enantiopure form. These
include classical resolution [41], enzymatic resolution in In view of the good performance of the Belokoń protocol for
conjunction with HPLC [42], or HPLC separation of derived various electrophilic reagents it appeared attractive to apply it
peptides [43], preparative HPLC separation on a chiral phase for the synthesis of β-methylphenylalanines as well (Scheme 5).
column [44], asymmetric synthesis from chiral precursors Towards this, the (S)-configured glycine nickel(II) complex (S)-
[45,46] including the stereoselective alkylation of aromatic 10 was alkylated with 1-phenylethyl iodide and some analogues
compounds with triflates of threonine stereoisomers [47], the with substituents in the aryl moiety, all in racemic form. The
chiral auxiliary approach [48-51] and enantioselective hydro- diastereomeric product Ni(II) complexes obtained in each case,
genation over a chiral catalyst [52,53].
could be separated by column chromatography. The pure
Scheme 5: A new general approach to (2S,3R)-β-methylarylalanines 3 by alkylation of the glycine nickel(II) complex (S)-10 with 1-arylethyl iodides
35. For details see Table 2.
2851

Beilstein J. Org. Chem. 2014, 10, 2844–2857.
diastereomers with (2S,3R) configuration were decomposed (R)-allo-threonine (allo-Thr, 4) and (2S, 3R)-β-methylphenyl-
with an aqueous methanolic HCl solution to furnish the target alanine [(β-Me)Phe, 3] had been prepared. The assembly of the
amino acids, which were purified by ion-exchange chromatog- hormaomycin analogues 8a–c with 3-(2'-fluoromethylcyclo-
raphy, in good yields (Table 2). (2S,3R)-β-Methylphenyl- propyl)alanine residues was initiated, employing the same
alanine was thus prepared from acetophenone in only four steps sequence as developed by Zlatopolskiy for the synthesis of
in an overall yield of 30%.
hormaomycin 1 [9] and its aza-analogue [12]. To start with, the
dicyclopropylmethyl (DCPM) ester of N-Fmoc-protected Ile 37,
was condensed with N-Z-protected (βMe)Phe-OH 39. After
removal of the Z group from the N-terminus of the resulting
dipeptide 42 by catalytic hydrogenation, the product was
coupled with N-Fmoc-protected (2R,1'R,2'R)-[3-(mono-, di- or
tri-)fluoromethylcyclopropyl]alanines 41a–c to yield tripep-
tides 47a–c, which, in turn, after deprotection with Et2NH/THF,
were coupled with N-Fmoc-protected (βMe)Phe-OH 46 to give
N,C-protected tetrapeptides 49a–c.
The N-Boc-protected (4-Pe)Pro-OH 43 and N,C-protected allo-
Thr 40 were condensed under 4-pyrrolidinopyridine catalysis to
give the ester 45, which, after deallylation under palladium
catalysis, was coupled with the tetrapeptides employing the
HATU reagent in the presence of HOAt to give the corres-
ponding hexadepsipeptides 51a–c.
From the latter, the DCPM and Boc groups were cleaved off
from both termini, leaving the MeZ group intact as proved by
an ESIMS spectrum, and the cyclizing peptide condensation
was achieved under high dilution conditions with the HATU
reagent. The cyclohexadepsipeptides 52a–c were obtained in
54, 60 and 53%, respectively, yield over 8 steps, after HPLC
purification (Scheme 6).
Figure 5: Structure and absolute configuration of nickel(II) complex
(2S,3S)-32 in the crystal. Hydrogen atoms are omitted for clarity.
A similar protocol for the synthesis of 3-alkylphenylalanines
was independently developed by Soloshonok et al. [56]. These The assemblies of the corresponding hormaomycin analogues
authors used sodium hydroxide for the deprotonation of 10 and were completed after removal of the N-MeZ groups from the
1-phenylalkyl bromides for the alkylation of the enolate.
cyclic intermediates 52a–c, subsequent coupling with the
corresponding N-Teoc-protected (2S,1'R,2'R)-[3-(mono-, di- or
Once sufficient quantities of (2S,1'S,2'R)-9a–c and (2R,1'S,2'R)- tri-)fluoromethylcyclopropyl]alanines 53a–c and, after removal
9a–c (FmcpA), as well as the N-Boc-protected (2S,4R)-4-(Z)- of the Teoc groups, the intermediates 56a–c were in turn
propenylproline [(4-Pe)Pro, 6] [8], the O-MOM-protected coupled with the 1-O-MOM-protected 5-chloro-1-hydroxypyr-
5-chloro-1-hydroxypyrrole-2-carboxylic acid (Chpca, 7) [8], role-2-carboxylic acid 54. Eventually, the MOM group was
Table 2: Substituted β-methylphenylalanines by alkylation of the glycine nickel(II) complex (S)-10 with 1-arylethyl iodides (yields based on converted
10, d.e. ≥98%). See Scheme 5. The yields of the liberated amino acids 3 based on the respective Ni complexes 32 are in parentheses.
X
Product
Yield (%)
Product
Yield (%)
H
(2S,3S)-32a
(2S,3S)-32-o-Cl
(2S,3S)-32-m-Cl
(2S,3S)-32-p-Cl
(2S,3S)-32-p-F
35
38
37
42
46
(2S,3R)-32
38 (59)
42
o-Cl
m-Cl
p-Cl
p-F
(2S,3R)-32-o-Cl
(2S,3R)-32-m-Cl
(2S,3R)-32-p-Cl
(2S,3R)-32-p-F
42 (96)
40 (89)
43
aThe absolute configurations of the arbitrarily selected nickel(II) complexes (2S,3S)-32, (2S,3R)-32-m-Cl and (2S,3R)-32-p-F were determined by
single crystal X-ray structure analyses (see Figure 5 and CCDC-deposited material) [28].
2852

Beilstein J. Org. Chem. 2014, 10, 2844–2857.
Scheme 6: Synthesis of the cyclohexadepsipeptides 52a–c for the hormaomycin analogues 8a–c with 3-(2'-fluoromethylcyclopropyl)alanine residues.
a: trifluoromethyl-, b: difluoromethyl-, c: monofluoromethylcyclopropylalanine. Reagents and conditions: i) oxalyl chloride, pyridine/dicyclopropyl-
methanol, DMAP, CH2Cl2, 0→20 °C, 20 h; ii) ZOSu, NaHCO3, acetone/water, 2 h; iii) 50% Et2NH/THF, 20 °C, 1 h; iv) EDC, HOAt, 2,4,6-collidine,
CH2Cl2, 4→20 °C, 14 h; v) FmocOSu, NaHCO3, acetone/water, 3 h; vi) H2, Pd/C, EtOAc, 20 °C, 2 h; vii) MeZOSu, NaHCO3, water/dioxane, 20 °C,
3 h; viii) All-Br, K2CO3, MeCN, 85 °C, 3 h, 60 °C, 16 h; ix) EDC, 4-pyrrolidinopyridine, CH2Cl2, 4→20 °C, 16 h; x) [Pd(PPh3)4], N-methylaniline, DME,
20 °C, 1 h; xi) HATU, HOAt, DIEA, 2,4,6-collidine, CH2Cl2, 4→20 °C, 15 h; xii) 2 M HCl in EtOAc, 20 °C, 20 min; xii) HATU, HOAt, DIEA, 2,4,6-colli-
dine, CH2Cl2, 4→20 °C, 22 h. Boc = tert-butyloxycarbonyl, DCPM = dicyclopropylmethyl, Fmoc = 9-fluorenylmethyloxycarbonyl, DIEA = N,N-diiso-
propylethylamine, DMAP = 4-dimethylaminopyridine, EDC = N'-(3-dimethylaminopropyl)-N-ethylcarbodiimide hydrochloride, HATU = O-(7-azabenzo-
triazole-1-yl)-N,N,N',N'-tetramethyluronium hexafluorophosphate, HOAt = 7-aza-1-hydroxybenzotriazole, MeZ = (4-methylbenzyl)oxycarbonyl, Z =
benzyloxycarbonyl.
cleaved off by treatment with MgBr2∙Et2O and EtSH in skiy et al. [9,12], were reproduced in order to provide large
dichloromethane to give, after HPLC purification, the target enough quantities for biological tests of their antimalarial activi-
compounds 8a–c in 72, 82 and 84% yield, respectively ties. Results of the in vitro tests of the antiparasitic activities of
(Scheme 7).
hormaomycin (1) [57], its analogues 8a, 52a, 58 and 59, as well
as the aza-analogues 60 and 61 are presented in Table 3.
Since the MeZ-protected cyclohexadepsipeptide core of the
native hormaomycin was found to have a significant antipara- All the newly prepared and tested hormaomycin analogues and
sitic activity, N-acetylated 58 and N-trifluoroacetylated 59 the native hormaomycin (1) [57] showed good and selective
derivatives were prepared by coupling the deprotected cyclic antimalarial activities. Compounds 8a and 1 additionally
intermediate 52a with acetic and trifluoroacetic acid (Figure 6). showed an activity against L. donovani. However, the activities
of the (trifluoromethylcyclopropyl)alanine-containing
Some antiparasitic activities of hormaomycin,
hormaomycin analogue 8a against L. donovani and P. falci-
its all-peptide analogue and the new
analogues
parum were 37 and 45% lower than those of hormaomycin (1)
itself. Further exploration of structure–activity relationships of
the hormaomycins would be required to prepare new antipara-
sitic lead compounds.
The syntheses of hormaomycin (1) itself and of its all-peptide
aza-analogues 60 and 61 (Figure 6) as developed by Zlatopol-
2853

Beilstein J. Org. Chem. 2014, 10, 2844–2857.
Conclusion
At first sight, the oligopeptide assembly leading to
hormaomycin does not appear to be a very complicated
problem. "State of the art" peptide coupling methodology
[59,60] allows one to prepare almost any peptides, that do not
contain extremely sterically congested fragments such as α,α-
dialkyl-substituted amino acids, N-alkyl amino acids or even the
more challenging N-aryl amino acids. With a proper choice of
the coupling reagent, solvent and other experimental conditions,
the oligopeptides in this study were obtained in high yields and
with high optical purities. As almost all amino acids, which
comprise hormaomycin (1) itself and its anticipated analogues,
are β-branched with the exception of 3-(2'-nitrocyclo-
propyl)alanine and the 3-(2'-fluoromethylcyclo-propyl)alanines,
HATU as well as the combination of EDC and 7-aza-1-
hydroxybenzotriazole (HOAt) [61] were used for each conden-
sation step to ensure high yields. The most unusual fragment in
hormaomycin (1) and its analogues is the ester bond between
the secondary (4-Pe)Pro moiety and the hydroxy group of allo-
Thr. Among several methods described in the literature for the
creation of such bonds, the dialkylaminopyridine-promoted
carbodiimide-mediated esterification was successfully
employed here [62].
Scheme 7: Synthesis of hormaomycin analogues with a: trifluo-
romethyl-, b: difluoromethyl-, c: monofluoromethylcyclopropylalanine
residues. Reagents and conditions: i) anisole, TFA, 20 °C, 2 h;
ii) TeocOSu, NaHCO3, N,N-dimethylaminopropylamine, water/acetone,
20 °C, 2 h; iii) HATU, HOAt, DIEA, 2,4,6-collidine, CH2Cl2, 20 °C, 15 h;
iv) TFA, 20 °C, 1 h; v) HATU, DIEA, 2,4,6-collidine, CH2Cl2, 20 °C, 4 h;
vi) MgBr2·Et2O, EtSH, CH2Cl2, 20 °C, 3.5 h. DIEA = N,N-diisopropyl-
ethylamine, HATU = O-(7-azabenzotriazole-1-yl)-N,N,N',N'-tetra-
methyluronium hexafluorophosphate, HOAt = 7-aza-1-hydroxybenzotri-
azole, MeZ = (4-methylbenzyl)oxycarbonyl, MOM = methoxymethyl,
Teoc = (2-trimethylsilylethyl)oxycarbonyl.
Figure 6: Two derivatives 58 and 59 of cyclohexadepsipeptide 52a containing the (trifluoromethylcyclopropyl)alanine moiety, the aza-analogue 60 of
hormamycin (1) itself as well as a simplified analogue 61 for biological testing.
2854

Beilstein J. Org. Chem. 2014, 10, 2844–2857.
Table 3: In vitro activities of some hormaomycin-derived compounds [58] and established reference drugs against L. donovani (axenic amastigotes),
P. falciparum and L6 cells (IC50, concentration in µg/mL).a
Compound
Leishmania donovani strain MHOM-ET-67/L82
Plasmodium falciparum strain K1
L6 cells
Miltefosine
0.143
–
–
–
–
Chloroquine
0.089
–
Podophyllotoxin
–
0.006
17.20
>90
>90
40
1
52a
58
8a
59
60
61
0.15
2.13
1.73
0.205
2.37
4.8
0.129
0.042
0.151
0.183
0.265
0.023
0.061
>90
?
–
?
aIC50 values reported are the averages of two independent assays which varied less than ±50%.
References
As far as the biological activities against L. donovani and
1. Arndt, M.; Hilt, G.; Khlebnikov, A. F.; Kozhushkov, S. I.; de Meijere, A.
Eur. J. Org. Chem. 2012, 3112–3121. doi:10.1002/ejoc.201200105
2. Khlebnikov, A. F.; Kozhushkov, S. I.; Yufit, D. S.; Schill, H.;
Reggelin, M.; Spohr, V.; de Meijere, A. Eur. J. Org. Chem. 2012,
1530–1545. doi:10.1002/ejoc.201101715
P. falciparum are concerned, a certain degree of lowering was
observed, but by far no complete loss. Thus, further modifica-
tions would be desirable to eventually arrive at a new lead com-
pound.
3. Andres, N.; Wolf, H.; Zähner, H.; Rössner, E.; Zeeck, A.; König, W. A.;
Sinnwell, V. Helv. Chim. Acta 1989, 72, 426–437.
Experimental
doi:10.1002/hlca.19890720303
For detailed experimental procedures of all the described
4. Rössner, E.; Zeeck, A.; König, W. A. Angew. Chem. 1990, 102, 84–85.
doi:10.1002/ange.19901020122
syntheses see the Supporting Information File 1.
Angew. Chem., Int. Ed., Engl. 1990, 29, 64–65.
In vitro antiprotozoal activity assays: The in vitro activities
against the protozoan parasites L. donovani and P. falciparum
as well as cytotoxicity assessments against L6 cells were deter-
mined as reported elsewhere [58]. The following strains, para-
site forms and positive controls were used: L. donovani,
MHOM/ET/67/L82, axenic amastigote forms, miltefosine, IC50
of 0.143 μg/mL; P. falciparum, K1 (chloroquine and
pyrimethamine resistant), erythrocytic stages, chloroquine, IC50
of 0.089 μg/mL and L6 cells, rat skeletal myoblasts, podophyl-
lotoxin, IC50 of 0.006 μg/mL.
doi:10.1002/anie.199000641
5. Zindel, J.; Zeeck, A.; König, W. A.; de Meijere, A. Tetrahedron Lett.
1993, 34, 1917–1920. doi:10.1016/S0040-4039(00)91962-2
6. Zlatopolskiy, B. D.; Loscha, K.; Alvermann, P.; Kozhushkov, S. I.;
Nikolaev, S. V.; Zeeck, A.; de Meijere, A. Chem. – Eur. J. 2004, 10,
4708–4717. doi:10.1002/chem.200400406
7. Larionov, O. V.; Savel'eva, T. F.; Kochetkov, K. A.; Ikonnokov, N. S.;
Kozhushkov, S. I.; Yufit, D. S.; Howard, J. A. K.; Khrustalev, V. N.;
Belokon', Y. N.; de Meijere, A. Eur. J. Org. Chem. 2003, 869–877.
doi:10.1002/ejoc.200390131
8. Zlatopolskiy, B. D.; Kroll, H.-P.; Melotto, E.; de Meijere, A.
Eur. J. Org. Chem. 2004, 4492–4502. doi:10.1002/ejoc.200400480
9. Zlatopolskiy, B. D.; de Meijere, A. Chem. – Eur. J. 2004, 10,
4718–4727. doi:10.1002/chem.200400249
Supporting Information
10.Kozhushkov, S. I.; Zlatopolskiy, B. D.; Brandl, M.; Alvermann, P.;
Razdom, M.; Geers, B.; de Meijere, A.; Zeeck, A. Eur. J. Org. Chem.
2005, 854–863. doi:10.1002/ejoc.200400608
Supporting Information File 1
Experimental procedures and analytical data.
[http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-10-302-S1.pdf]
11.Zlatopolskiy, B. D.; Razdom, M.; Zeeck, A.; de Meijere, A.
Eur. J. Org. Chem. 2006, 1525–1534. doi:10.1002/ejoc.200500856
12.Reinscheid, U. M.; Zlatopolskiy, B. D.; Griesinger, C.; Zeeck, A.;
de Meijere, A. Chem. – Eur. J. 2005, 11, 2929–2945.
doi:10.1002/chem.200400977
13.Otoguro, K.; Ui, H.; Ishiyama, A.; Arai, N.; Kobayashi, M.;
Takahashi, Y.; Masuma, R.; Shiomi, K.; Yamada, H.; Omura, S.
J. Antibiot. 2003, 56, 322–324. doi:10.7164/antibiotics.56.322
Acknowledgements
This work was supported by the Land Niedersachsen. We thank
M. Cal and S. Schneeberger (Swiss TPH) for parasite assay
results.
2855

Beilstein J. Org. Chem. 2014, 10, 2844–2857.
14.O'Hagan, D. Beilstein J. Org. Chem. 2008, 4, No. 11.
doi:10.3762/bjoc.4.11
33.Belokon', Y. N.; Maleyev, V. I.; Vitt, S. V.; Ryzhov, M. G.;
Kondrashov, Y. D.; Golubev, S. N.; Vauchskii, Y. P.; Kazika, A. I.;
Novikova, M. I.; Krasutskii, P. A.; Yurchenko, A. G.; Dubchak, I. L.;
Shklover, V. E.; Struchkov, Y. T.; Bakhmutov, V. I.; Belikov, V. M.
J. Chem. Soc., Dalton Trans. 1985, 17–26. doi:10.1039/dt9850000017
34.Belokon', Y. N.; Popkov, A. N.; Chernoglazova, N. I.;
Saporovskaya, M. B.; Bakhmutov, V. I.; Belikov, V. M.
J. Chem. Soc., Chem. Commun. 1988, 1336–1338.
doi:10.1039/C39880001336
15.O'Hagan, D. Beilstein J. Org. Chem. 2010, 6, No. 36.
doi:10.3762/bjoc.6.36
16.Al-Maharik, N.; O'Hagan, D. Aldrichimica Acta 2011, 44, 65–75.
17.Gouverneur, V.; Müller, K., Eds. Fluorine in Pharmaceutical and
Medicinal Chemistry; World Scientific Imperial College Press:
Singapore, 2012. doi:10.1142/p746
18.Belokon', Y. N.; Kochetkov, K. A.; Ikonnikov, N. S.; Strelkova, T. V.;
Harutyunyan, S. R.; Saghiyan, A. S. Tetrahedron: Asymmetry 2001,
12, 481–485. doi:10.1016/S0957-4166(01)00071-4
19.Saghiyan, A. S.; Geolchanyan, A. V.; Djamgaryan, S. M.;
Vardapetyan, S. M.; Tararov, V. I.; Kuz'mina, N. A.; Ikonnikov, N. S.;
Belokon', Y. N.; North, M. Russ. Chem. Bull. 2000, 49, 1460–1463.
doi:10.1007/BF02495097
35.Belokon', Y. N.; Bulychev, A. G.; Vitt, S. V.; Struchkov, Y. T.;
Batsanov, A. S.; Timofeeva, T. V.; Tsyryapkin, V. A.; Ryzhov, M. G.;
Lysova, L. A.; Bakhmutov, V. I.; Belikov, V. M. J. Am. Chem. Soc.
1985, 107, 4252–4259. doi:10.1021/ja00300a030
36.Soloshonok, V. A.; Avilov, D. V.; Kukhar, V. P.; Tararov, V. I.;
Savel'eva, T. F.; Churkina, T. D.; Ikonnikov, N. S.; Kochetkov, K. A.;
Orlova, S. A.; Pysarevsky, A. P.; Struchkov, Y. T.; Raevsky, N. I.;
Belokon', Y. N. Tetrahedron: Asymmetry 1995, 6, 1741–1756.
doi:10.1016/0957-4166(95)00220-J
20.Belokon', Y. N. Janssen Chim. Acta 1992, 10, 4–12.
21.Csuk, R.; von Scholz, Y. Tetrahedron 1994, 50, 10431–10442.
doi:10.1016/S0040-4020(01)89583-1
22.Baldwin, J. E.; Chang, G. E. C. Tetrahedron 1982, 38, 825–835.
doi:10.1016/0040-4020(82)80163-4
37.Belokon', Y. N.; Sagyan, A. S.; Djamgaryan, S. A.; Bakhmutov, V. I.;
Vitt, S. V.; Batsanov, A. S.; Struchkov, Y. T.; Belikov, V. M.
J. Chem. Soc., Perkin Trans. 1 1990, 2301–2310.
doi:10.1039/p19900002301
23.Le Corre, M.; Hercouet, A.; Bessieres, B. Tetrahedron: Asymmetry
1995, 6, 683–684. doi:10.1016/0957-4166(95)00060-3
The aldehyde 16 has previously been prepared by oxidation of 15 with
pyridinium chlorochromate.
38.Balaram, P. Curr. Opin. Struct. Biol. 1992, 2, 845–851.
doi:10.1016/0959-440X(92)90110-S
24.Braña, M. F.; Guisado, C.; Alguacil, L. F.; Garrido, E.;
Pérez-Garcıa, C.; Ruiz-Gayo, M. Bioorg. Med. Chem. Lett. 2002, 12,
3561–3563. doi:10.1016/S0960-894X(02)00793-X
Swern oxidation of 15 to yield the aldehyde 16 has previously been
mentioned but without experimental details.
39.Hruby, V. J.; Al-Obeidi, F. A.; Kazmierski, W. Biochem. J. 1990, 268,
249–262.
40.Hruby, V. J. Acc. Chem. Res. 2001, 34, 389–397.
doi:10.1021/ar990063q
41.Kataoka, Y.; Seto, Y.; Yamamoto, M.; Yamada, T.; Kuwata, S.;
Watanabe, H. Bull. Chem. Soc. Jpn. 1976, 49, 1081–1084.
doi:10.1246/bcsj.49.1081
25.Brown, P.; Burdon, J.; Smith, T. J.; Tatlow, J. C. Tetrahedron 1960, 10,
164–170. doi:10.1016/S0040-4020(01)97803-2
26.Hanack, M.; Meyer, H. Justus Liebigs Ann. Chem. 1968, 720, 81–97.
doi:10.1002/jlac.19687200107
42.Alías, M.; López, M. P.; Cativiela, C. Tetrahedron 2004, 60, 885–891.
doi:10.1016/j.tet.2003.11.044
27.Ratier, M.; Pereyre, M.; Davies, A. G.; Sutcliffe, R.
J. Chem. Soc., Perkin Trans. 2 1984, 1907–1915.
doi:10.1039/p29840001907
43.Pastó, M.; Moyano, A.; Pericàs, M. A.; Riera, A. J. Org. Chem. 1997,
62, 8425–8431. doi:10.1021/jo971178f
44.Huang, Z.; He, Y.-B.; Raynor, K.; Tallent, M.; Reisine, T.; Goodman, M.
J. Am. Chem. Soc. 1992, 114, 9390–9401. doi:10.1021/ja00050a019
45.Tourwé, D.; Mannekens, E.; Nguyen Thi Diem, T.; Verheyden, P.;
Jaspers, H.; Tóth, G.; Péter, A.; Kertész, I.; Török, G.; Chung, N. N.;
Schiller, P. W. J. Med. Chem. 1998, 41, 5167–5176.
doi:10.1021/jm981011u
28.CCDC-991863 [(2S,1'R,2'S)-26a], -991864 [2S,1'R,2'S)-26b], -991865
[(2R,1'R,2'S)-26b], -991862 [(R,R,R)-28], -991866 [(2S,3S)-32],
-1018305 [(2S,3R)-32-m-Cl] and -1018304 [(2S,3R)-32-p-F] contain the
supplementary crystallographic data for this paper. These data can be
obtained free of charge via
http://www.ccdc.cam.ac.uk/conts/retrieving.html (or from the
Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge
CB21EZ, UK; fax: (+44)1223-336-033; or deposit@ccdc.cam.ac.uk).
29.Andersson, P. G.; Guijarro, D.; Tanner, D. J. Org. Chem. 1997, 62,
7364–7375. doi:10.1021/jo970918h
46.Davis, F. A.; Liang, C.-H.; Liu, H. J. Org. Chem. 1997, 62, 3796–3797.
doi:10.1021/jo9702610
47.Effenberger, F.; Weber, T. Angew. Chem. 1987, 99, 146–147.
doi:10.1002/ange.19870990217
Angew. Chem., Int. Ed. Engl. 1987, 26, 142–143.
doi:10.1002/anie.198701421
30.Belokon', Y. N.; Bakhmutov, V. I.; Chernoglazova, N. I.;
Kochetkov, K. A.; Vitt, S. V.; Garbalinskaya, N. S.; Belikov, V. M.
J. Chem. Soc., Perkin Trans. 1 1988, 305–312.
48.Dharanipragada, R.; Nicolas, E.; Toth, G.; Hruby, V. J.
Tetrahedron Lett. 1989, 30, 6841–6844.
doi:10.1039/p19880000305
doi:10.1016/S0040-4039(01)93366-0
31.Belokon', Y. N.; Bulychev, A. G.; Pavlov, V. A.; Fedorova, E. B.;
Tsyryapkin, V. A.; Bakhmutov, V. A.; Belikov, V. M.
J. Chem. Soc., Perkin Trans. 1 1988, 2075–2083.
doi:10.1039/P19880002075
49.Li, G.; Jarosinski, M. A.; Hruby, V. J. Tetrahedron Lett. 1993, 34,
2561–2564. doi:10.1016/S0040-4039(00)77625-8
50.Oppolzer, W.; Tamura, O.; Deerberg, J. Helv. Chim. Acta 1992, 75,
1965–1978. doi:10.1002/hlca.19920750622
32.Belokon', Y. N.; Zel'tzer, I. E.; Ryzhov, M. G.; Saporovskaya, M. B.;
Bakhmutov, V. I.; Belikov, V. M. J. Chem. Soc., Chem. Commun. 1982,
180–181. doi:10.1039/C39820000180
51.Shapiro, G.; Buechler, D.; Marzi, M.; Schmidt, K.; Gomez-Lor, B.
J. Org. Chem. 1995, 60, 4978–4979. doi:10.1021/jo00121a010
52.Burk, M. J.; Gross, M. F.; Martinez, J. P. J. Am. Chem. Soc. 1995, 117,
9375–9376. doi:10.1021/ja00141a039
2856

Beilstein J. Org. Chem. 2014, 10, 2844–2857.
53.Burk, M. J.; Bedingfield, K. M.; Kiesman, W. F.; Allen, J. G.
Tetrahedron Lett. 1999, 40, 3093–3096.
doi:10.1016/S0040-4039(99)00437-2
54.Dharanipragada, R.; VanHulle, K.; Bannister, A.; Bear, S.; Kennedy, L.;
Hruby, V. J. Tetrahedron 1992, 48, 4733–4748.
doi:10.1016/S0040-4020(01)81570-2
55.Li, G.; Patel, D.; Hruby, V. J. J. Chem. Soc., Perkin Trans. 1 1994,
3057–3059. doi:10.1039/p19940003057
56.Soloshonok, V. A.; Boettiger, T. U.; Bolene, S. B. Synthesis 2008,
2594–2602. doi:10.1055/s-2008-1067172
57.A sample of the native hormaomycin (1) was kindly provided by Prof.
Axel Zeeck, Göttingen, see [3,4].
58.Orhan, I.; Şener, B.; Kaiser, M.; Brun, R.; Tasdemir, D. Mar. Drugs
2010, 8, 47–58. doi:10.3390/md8010047
59.Albeiicio, F.; Chinchilla, R.; Dodsworth, D. J.; Nájera, C.
Org. Prep. Proced. Int. 2001, 33, 203–303.
doi:10.1080/00304940109356592
See for a review on peptide coupling agents.
60.Elmore, D. T.; Sovago, I.; Farkas, E.; Penke, B.; Toth, G.; Varadi, G.;
Zarandi, M. Amino Acids, Peptides And Proteins. Davies, J. S., Ed.;
RSC Publishing: U.K., 2007; Vol. 36, pp 3–346.
doi:10.1039/9781847558459
See for a review on peptide coupling agents; covers literature
published during 2003.
61.Carpino, L. A.; El-Faham, A. Tetrahedron 1999, 55, 6813–6830.
doi:10.1016/S0040-4020(99)00344-0
62.Kobertz, W. R.; Essigmann, J. M. J. Am. Chem. Soc. 1996, 118,
7101–7107. doi:10.1021/ja960511e
License and Terms
This is an Open Access article under the terms of the
Creative Commons Attribution License
(http://creativecommons.org/licenses/by/2.0), which
permits unrestricted use, distribution, and reproduction in
any medium, provided the original work is properly cited.
The license is subject to the Beilstein Journal of Organic
Chemistry terms and conditions:
(http://www.beilstein-journals.org/bjoc)
The definitive version of this article is the electronic one
which can be found at:
doi:10.3762/bjoc.10.302
2857