Dipeptide analogues containing 4-ethoxy-3-pyrrolin-2-ones

Article abstract of DOI:10.1021/ol060500i

Pyrrolidine-2,4-diones (1) are naturally occurring analogues of amino acids. We herein present a facile synthesis of N-acylated, O-alkylated pyrrolin-2-ones (2) in high yield and excellent enantiopurity. Molecular mechanics calculations suggest that the r

Full text of DOI:10.1021/ol060500i

ORGANIC
LETTERS
2006
Vol. 8, No. 10
2103-2106
Dipeptide Analogues Containing
4-Ethoxy-3-pyrrolin-2-ones
Masood Hosseini,†,‡ Henriette Kringelum,‡ Anthony Murray,† and
Janne E. Tønder*,‡
Medicinal Chemistry Research, NoVo Nordisk A/S, NoVo Nordisk Park, DK-2760
MåløV, and Department of Chemistry, Technical UniVersity of Denmark, Building 201,
KemitorVet, DK-2800 Kgs. Lyngby, Denmark
jet@kemi.dtu.dk
Received February 28, 2006
ABSTRACT
Pyrrolidine-2,4-diones (1) are naturally occurring analogues of amino acids. We herein present a facile synthesis of N-acylated, O-alkylated
pyrrolin-2-ones (2) in high yield and excellent enantiopurity. Molecular mechanics calculations suggest that the resulting dipeptide analogues
adopt a linear, extended conformation.
N-Acylated pyrrolidine-2,4-diones (also known as N-acylated
tetramic acids) are present in a range of natural products,1
many of these displaying interesting biological activities, e.g.,
antibiotic, antiviral, or cytotoxic activities. The structure of
the pyrrolidine-2,4-diones is closely related to that of amino
acids, and the biosynthetic pathway has indeed been sug-
gested to occur via an intramolecular condensation of
N-acetyl amino acid methyl esters.2
Likewise, pyAla-OMe is present in mirabamide E6 and
dysideapyrrolidone7, pyVal-OMe is present in dysidin and
mirabimide A-D,8 pySer-OMe is present in the malynga-
mides Q and R,9 pyPhe-OMe is present in dolastatin 15,10
In nature, only the O-methylated form of N-acylated
pyrrolidine-2,4-diones is seen (Figure 1), hence the pyrro-
lidinone analogue of glycine methyl ester (in the following
named pyGly-OMe) is present in the natural products
malyngamide A,3 althiomycin,4 and the pukeleimides.5
† Medicinal Chemistry Research, Novo Nordisk A/S.
‡ Department of Chemistry, Technical University of Denmark.
(1) For a review on tetramic acids, see: Royles, B. J. L. Chem. ReV.
1995, 95, 1981.
(2) Pettit, G. R.; Kamano, Y.; Dufresne, C.; Cerny, R. L.; Herald, C. L.;
Schmidt, J. M. J. Org. Chem. 1989, 54, 6005.
(3) Cardellina, J. H., II.; Marner, F.-J.; Moore, R. E. J. Am. Chem. Soc.
1979, 101, 240.
(4) Yamaguchi, H.; Nakayama, Y.; Takeda, K.; Tawara, K. J. Antibiot.
Ser. A 1957, 10, 195.
(5) (a) Simmons, C. J.; Marner, F.-J.; Cardellina, J. H., II.; Moore, R.
E.; Seff, K. Tetrahedron Lett. 1979, 20, 2003. (b) Cardellina, J. H., II.;
Moore, R. E. Tetrahedron Lett. 1979, 20, 2010.
Figure 1. Natural compounds containing N-acylated, O-alkylated
pyrrolin-2-ones.
10.1021/ol060500i CCC: $33.50
© 2006 American Chemical Society
Published on Web 04/22/2006

and the 3-methylated form of pyPhe-OMe is present in
palau’imide.11
Scheme 1. Synthesis of 4-Ethoxy-3-pyrrolin-2-ones
In our group, we are interested in the possibility of using
the pyrrolidine-2,4-diones as amino acid analogues. Because
pyrrolidine-2,4-diones are connected to the rest of the
molecule via an imide bond, we have focused on the creation
of this bond.
N-Acylation of sterically hindered chiral cyclic amides has
previously been described; especially the oxazolidinones have
attracted much interest due to their use as chiral auxiliaries.12
Similar pyrrolidinones have been N-acylated using an n-BuLi
protocol with electrophiles such as acid chlorides13 and Pfp-
esters.14
The corresponding N-acylation of 4-alkylated pyrroline-
2-ones has been reported previously in the syntheses of
althiomycin,15 pukeleimide A,16 dysidin,17 and mirabamide
E.6 However, only one group has reported an acylation in
which it was assumed that the stereochemistry at C-5 had
remained unchanged.6
In the O-alkylation, it was found that the use of lithium
bases (LiHMDS and n-BuLi) led to prolonged reaction times
due to a precipitation of the anion, which could not be
reversed by addition of 15-crown-5. However, among the
potassium bases tested, KHMDS afforded yields in the range
of 69-87%,20 with no racemization of the final product. This
was superior to both KOtBu in terms of yield and K2CO3 in
terms of enantiopurity.
For the acylation step, we had to be aware of two important
issues: (1) the poor nucleophilicity of the pyrrolinone anion
which has been described in an earlier report15a and (2) the
retention of enantiopurity in the products.
Because the anions of compounds 3 can obtain the
aromatic 4-alkoxy-1H-pyrrol-2-olate structure by intermo-
lecular proton transfer, we initially focused on avoiding
racemization in the deprotonation step.
Preliminary findings based on 1H NMR detection showed,
surprisingly, that no incorporation of deuterium was observ-
able even after quenching the anion of 3d with CF3COOD
at 20 °C. However, when the product after a water quench
were analyzed by HPLC, full racemization had actually taken
place at 20 °C (Table 1). Lowering the temperature to -30
°C led to a rise in the enantiomeric excess to 91%, and by
lowering the temperature further to below -45 °C, no
epimerization was observed.
The pyrrolidine-2,4-diones were synthesized efficiently via
a modified literature procedure18 (Scheme 1), in which a Boc-
protected amino acid was activated with EDC, condensed
with Meldrum’s acid, and finally cyclized to give the Boc-
protected pyrrolidine-2,4-diones (1). The following N-
deprotection was accomplished with TFA treatment giving
the parent pyrrolidine-2,4-diones (2). This sequence can be
run in 1 day, furnishing up to 30 g of the products.
In nature, the pyrrolinones appear in their O-methylated
form. However, preliminary experiments showed that the
methyl group was more difficult to remove than the ethyl
group,19 which was therefore used in the further work.
Conversion of the pyrrolidine-2,4-diones (2) into their
O-ethylated derivatives (3) was accomplished by deproto-
nation with KHMDS followed by alkylation with ethyl
tosylate in the presence of 18-crown-6.
(6) Paik, S.; Carmeli, S.; Cullingham, J.; Moore, R. E.; Patterson, G. M.
L.; Tius, M. A. J. Am. Chem. Soc. 1994, 116, 8116.
(7) Unson, M. D.; Rose, C. B.; Faulkner, D. J.; Brinen, L. S.; Steiner, J.
R.; Clardy, J. J. Org. Chem. 1993, 58, 6336.
(8) Carmeli, S.; Moore, R. E.; Patterson, G. M. L. Tetrahedron 1991,
47, 2087.
(9) Milligan, K. E.; Ma´rquez, B.; Williamson, R. T.; Davies-Coleman,
M.; Gerwick, W. H. J. Nat. Prod. 2000, 63, 965.
(10) Akaji, K.; Hayashi, Y.; Kiso, Y.; Kuriyama, N. J. Org. Chem. 1999,
64, 405.
(11) Luesch, H.; Yoshida, W. Y.; Moore, R. E.; Paul, V. J. Tetrahedron
2002, 58, 7959.
(12) (a) Evans, D. A.; Bartroli, J.; Shih, T. L. J. Am. Chem. Soc. 1981,
103, 2127. (b) Ager, D. J.; Prakash, I.; Schaad, D. R. Aldrichimica Acta
1997, 30, 3. and references therein.
(13) Davies, S. G.; Dixon, D. J.; Doisneau, G. J.-M.; Prodger, J. C.;
Sanganee, H. J. Tetrahedron: Asymmetry 2002, 13, 647.
(14) Andrus, M. B.; Li, W.; Keyes, R. F. J. Org. Chem. 1997, 62, 5542.
(15) (a) Inami, K.; Shiba, T. Bull. Chem. Soc. Jpn. 1985, 58, 352. (b)
Toogood, P. L.; Hollenbeck, J. J.; Lam, H. M.; Li, L. Bioorg. Med. Chem.
Lett. 1996, 6, 1543. (c) Zarantonello, P.; Leslie, C. P.; Ferritto, R.;
Kazmierski, W. M. Bioorg. Med. Chem. Lett. 2002, 12, 561.
(16) James, G. D.; Mills, S. D.; Pattenden, G. J. Chem. Soc., Perkin
Trans. 1 1993, 2581.
(17) Ko¨hler, H.; Gerlach, H. HelV. Chim. Acta 1984, 67, 1783.
(18) (a) Jouin, P.; Castro, B.; Nisato, D. J. Chem. Soc., Perkin Trans. 1
1987, 1177. (b) Ma, D.; Ma, J.; Ding, W.; Dai, L. Tetrahedron: Asymmetry
1996, 7, 2365. (c) Courcambeck, J.; Bihel, F.; de Michelis, C.; Que´le´ver,
G.; Kraus, J. L. J. Chem. Soc., Perkin Trans. 1 2001, 12, 1421.
(19) Preliminary removal conditions were 10% HBr in MeCN.
For the acylations, Fmoc-protected amino acids were
preferred over Boc-protected acids because the O-ethyl group
was to be removed under acidic conditions,19 and it was
therefore considered advantageous with a base labile N-
protecting group.
Initial acylation experiments were conducted at -78 °C.
These experiments quickly showed that, in contrast to what
we observed in the alkylation reactions, lithium bases
(LiHMDS, n-BuLi) were now superior to the potassium
(20) To a solution of tetramic acid (1 g, 1.0 equiv) in THF (30 mL) at
0 °C was added dropwise KHMDS (0.50 M in toluene, 1.05 equiv), and
the suspension was stirred for 10 min at 0 °C. To this was added EtOTs
(1.1 equiv) and 18-crown-6 (1.1 equiv), and the mixture was slowly heated
to room temperature. Upon completion of the reaction (TLC, EtOAc), the
mixture was evaporated with 50 mL of silica, and flash chromatography
(EtOAc) provided the pure product.
2104
Org. Lett., Vol. 8, No. 10, 2006

philicity and possibility of racemization inherent in the anion
structure by maintaining the reaction temperature around -50
°C.
Table 1. Racemization Investigations
Applying these optimized conditions to a range of
substrates (Table 3) furnished the desired dipeptide analogues
Table 3. Acylation of 4-Alkoxy-3-pyrrolin-2-ones
entry
temperature (°C)
ee (%)a
1
2
3
4
20
0
-30
-45
0
79
91
>99
compounda
isolated yield (%)b
a Determined by HPLC; Chiralcel AD column, gradient 7 f 5% iPrOH
in hexane, RT(S)-H-pyPhe-OEt ) 7.6 min, RT(R)-H-pyPhe-OEt ) 10.7 min.
Fmoc-Phe-pyGly-OMe (4a)
Fmoc-Val-pyAla-OEt (4b)
Fmoc-Leu-pyAla-OEt (4c)
Fmoc-Phe-pyAla-OEt (4d)
Fmoc-Ala-pyVal-OEt (4e)
Fmoc-Leu-pyVal-OEt (4f)
Fmoc-Phe-pyVal-OEt (4g)
Fmoc-Ala-pyLeu-OEt (4h)
Fmoc-Val-pyLeu-OEt (4i)
Fmoc-Phe-pyLeu-OEt (4j)
Fmoc-Ala-pyPhe-OEt (4k)
Fmoc-Val-pyPhe-OEt (4l)
Fmoc-Leu-pyPhe-OEt (4m)
Boc-Phe-pyPhe-OEt (4n)
91
92
88
85
89
78
78
84
81
80
84
88
85
92
(KOtBu, KHMDS) and magnesium (i-PrMgCl) bases. This
difference may be due to a diminished reactivity of the
nucleophile when a larger counterion is present. For the final
optimizations, we used the commercially available H-pyGly-
OMe (3e) to minimize steric hindrance.
In these experiments, we found that the electrophile
(Fmoc-AA-OPfp) had to be added slowly (over 15 min) to
avoid degradation and that using an excess of either
electrophile or nucleophile did not lead to an increase in yield
(Table 2).
a Fmoc-AA-OPfp and Boc-Phe-ONp were used as electrophiles. b Only
1
one diastereomer was observed by H NMR (de > 95%).
Table 2. Optimizing the N-Acylation Reaction
in very good isolated yields and with complete retention of
enantiopurity.
In addition to the Fmoc-protected amino acids, Boc-Phe-
ONp was employed in the reaction giving 4n in 92% yield
and again with full retention of enantiopurity.
It can be seen that steric hindrance may have some
influence on this reaction, as the highest yields are obtained
for smaller reactants such as pyGly, Ala, pyAla, Val, and
pyVal. pyPhe is ranked almost as good as pyAla, reflecting
that the benzylic side chain may be able to bend away from
the reaction center.
To show that these novel dipeptide analogues can be
employed in further synthesis, the Fmoc protecting group
in compound 4d was removed by DBU treatment affording
5d in excellent yield (Scheme 2).
entry equiv electrophile temperature (°C) isolated yield (%)
1
2
3
4
0.5
1.1
2.0
1.1
-78
-78
-78
-55 f -45
72
76
77
91
Satisfyingly, it was observed that raising the temperature
from -78 °C to -50 °C led to the increase in nucleophilic
reactivity which was necessary for obtaining a synthetically
useful reaction.21 Thus, it is possible to balance the nucleo-
Scheme 2. N-Deprotection
(21) n-BuLi (1.58 M in hexanes, 1.0 equiv) was added dropwise to a
solution of H-pyAA-OEt (3) (50 mg, 1.0 equiv) in THF (3 mL) at -55 °C,
and stirring was continued for 10 min. To this yellow solution was added
Fmoc-AA-OPfp (1.1 equiv) dissolved in THF (2 mL) by a syringe pump
over 15 min. The mixture was allowed to stir for an additional 5 min. Over
this 30 min period, the temperature had been allowed to reach -45 °C.
The mixture was quenched with AcOH (0.1 mL) and evaporated with 5
mL of silica. Column chromatography (30f50% EtOAc in heptane)
afforded the pure compounds (4) as white solids.
Org. Lett., Vol. 8, No. 10, 2006
2105

Figure 3. Torsional angles used in the computational study.
In conclusion, we have developed a facile synthesis of
dipeptide analogues containing pyrrolin-2-ones. These trans-
formations have been shown to afford a robust and general
method, where the products could be achieved in high yield
and excellent enantiopurity.
Figure 2. All low-energy conformations for 5j. (Hydrogens are
omitted for clarity.)
As expected, the computational investigations of these
analogues have suggested that they should adopt a linear,
extended conformation with some conformational constraints
in the pyrrolinone ring.
We are currently investigating the physical and biological
properties of these compounds, and results will be reported
in due time.
To investigate the structure of the desired dipeptide
analogues, conformational searches on the N-deprotected
compounds (5b-m) were carried out.22
In all cases, the conformational search gives rise to the
expected almost planar pyrrolinone ring (Figures 2 and 3)
with values for æ and ψ varying between -161.8 to -173.2
and 171.0 to 177.3, respectively.
Acknowledgment. We gratefully thank The Lundbeck
Foundation, Novo Nordisk Corporate Research Affairs, The
Danish Research Council for Technology and Production
Sciences, The Torkil Holm Foundation, The Danish Natural
Science Research Council, The Augustinus Foundation, and
The Ib Henriksen Foundation for the financial support which
made this work possible.
In the low energy conformations of H-Phe-pyAA-OEt
(5d,g,j), an electrostatic interaction could be observed
between the π-systems on the lactam carbonyl on the
pyrrolinone ring system and the phenyl from the benzyl
group, respectively (Figure 2).
In all low energy conformations of compounds 5a-m, the
backbone exists in the extended form, and the different
conformations represent rotation of the side chains R1, R2,
and the OEt group.
Supporting Information Available: Experimental pro-
cedures, full spectroscopic data for all new compounds, in
addition to detailed procedures and results from the molecular
modeling study. This material is available free of charge via
the Internet at http://pubs.acs.org.
(22) See Supporting Information for details and results of the molecular
modeling study.
OL060500I
2106
Org. Lett., Vol. 8, No. 10, 2006