A short synthesis of bicyclic dipeptides corresponding to Xxx-L-Pro Xxx-D-Pro having constrained trans-proline amides.

Article abstract of DOI:10.1021/ol9902069

[formula: see text] A short synthesis that generates two isomeric bicyclic dipeptides having constrained, trans-proline amide bonds has been developed. One of these bicyclic dipeptides corresponds to an Xxx-L-Pro dipeptide (4), while the other isomer corr

Full text of DOI:10.1021/ol9902069

ORGANIC
LETTERS
1999
Vol. 1, No. 8
1225-1228
A Short Synthesis of Bicyclic Dipeptides
Corresponding to Xxx-L-Pro Xxx-D-Pro
Having Constrained trans-Proline
Amides
Timothy P. Curran,* Lisa A. Marcaurelle, and Kathleen M. O’Sullivan
Department of Chemistry, College of the Holy Cross,
Worcester, Massachusetts 01610-2395
tcurran@holycross.edu
Received August 2, 1999
ABSTRACT
A short synthesis that generates two isomeric bicyclic dipeptides having constrained, trans-proline amide bonds has been developed. One of
these bicyclic dipeptides corresponds to an Xxx-L-Pro dipeptide (4), while the other isomer corresponds to an Xxx-D-Pro dipeptide (5). The two
1
isomers are readily distinguished by their H NMR spectra.
Owing to the stability obtained from π-bonding between the
N, C, and O atoms, amides only assume two conformations
that differ in orientation by 180°. The limited conformational
freedom of amides is an important part of protein structure.
Under normal circumstances most amide bonds in proteins
adopt only one of the two possible orientations, the trans-
amide conformation. The one exception to this are proline
amide bonds, which are known to assume either the cis- (1)
or trans-amide (2) conformation.1 Since proline amides
hormones.5 Of potential utility in studying biochemical events
involving proline would be peptides that constrain proline
to either the cis- or trans-amide conformation. In previous
work we have detailed a short synthesis that generates
constrained cis-proline dipeptides.6 Here, a short synthesis
that generates the corresponding trans-proline dipeptides is
presented.
(2) (a) Brandts, J. F.; Halvorson, H. R.; Brennan, M. Biochemistry 1975,
14, 953. For recent references, see: (b) Borden, K. L.; Richards, F. M.
Biochemistry 1990, 29, 3071. (c) Texter, F. L.; Spencer, D. B.; Rosenstein,
R.; Matthews, C. R. Biochemistry 1992, 31, 5687. (d) Shalongo, W.;
Jagannadham, M. V.; Heid, P.; Stellwegen, E. Biochemistry 1992, 31, 11390.
(e) Lang, K.; Schmid, F. X. J. Mol. Biol. 1990, 212, 185. (f) Kiefhaber, T.;
Kohler, H. H.; Schmid, F. X. J. Mol. Biol. 1992, 224, 217. (g) Kiefhaber,
T.; Kohler, H. H.; Schmid, F. X. J. Mol. Biol. 1992, 224, 231.
(3) (a) Brandl, C. J.; Deber, C. M. Proc. Natl. Acad. Sci. U.S.A. 1986,
83, 917. (b) Williams, K. A.; Deber, C. M. Biochemistry 1991, 30, 8919.
(c) Vogel, H.; Nilsson, L.; Rigler, R.; Meder, S.; Boheim, G.; Beck, W.;
Kurth, H. H.; Jung, G. Eur. J. Biochem. 1993, 212, 305. (d) Wess, J.;
Nanavati, S.; Vogel, Z.; Maggio, R. EMBO J. 1993, 12, 331. (e) Suchyna,
T. M.; Xu, L. X.; Gao, F.; Fourtner, C. R.; Nicholson, B. J. Nature 1993,
365, 847.
possess this conformational flexibility, it has been speculated
that cis-trans proline isomerization plays many important
biochemical roles, including controlling the rate of protein
folding,2 triggering receptor-mediated transmembrane signal-
ing,3 providing a recognition element in peptide antigens,4
and regulating both the activation and breakdown of peptide
(4) Richards, N. G. J.; Hinds, M. G.; Brennand, D. M.; Glennie, M. J.;
Welsh, J. M.; Robinson, J. A. Biochem. Pharm. 1990,40, 119.
(5) (a) Yaron, A.; Naider, F. Crit. ReV. Biochem. Mol. Biol. 1993, 28,
31. (b) London, R. E.; Stewart, J. M.; Cann, J. R. Biochem. Pharm. 1990,
40, 41. (b) Yaron, A. Biopolymers 1987, 26, S215.
(1) Grathwohl, C.; Wuthrich, K. Biopolymers 1981, 20, 2623.
(6) Curran, T. P.; McEnaney, P. M. Tetrahedron Lett. 1995, 36, 191.
10.1021/ol9902069 CCC: $18.00 © 1999 American Chemical Society
Published on Web 09/15/1999

The obvious approach for constraining an Xxx-Pro dipep-
tide to the trans-amide conformation is to tether the δ-carbon
of the pyrrolidine ring of proline to the R-carbon on the
preceding amino acid using a linker Y, as shown in general
structure 3. A number of species that correspond to general
The preparation of 4 and 5 is outlined in Scheme 1.
Coupling of cis-2,5-dicarbethoxypyrrolidine (18, obtained in
two steps from diethyl meso-1,4-dibromoadipate8) with N-R-
Cbz-N-â-Boc-L-aminoalanine (19, obtained either by addition
structure 3 have been prepared and described in recent years,
and they are shown in Figure 1.7 The target molecules in
the present work are 4 and 5, which employ a lactam
methylene (CO-NH-CH2) linker. Two attractive features of
this linker are that the lactam is easy to assemble and that it
limits the conformational mobility of the bicyclic ring system.
The two isomers 4 and 5 significantly differ in their
configuration at the carbon bearing the carboxylic acid. In 4
the carboxylic acid has the configuration identical to that of
L-proline, while in 5 the carboxylic acid has the configuration
identical to that of D-proline. Thus, 4 is a constrained, trans-
Xxx-L-Pro dipeptide, while 5 is a constrained, trans-Xxx-
D-Pro dipeptide. Another attractive feature of the synthesis
detailed below is that it generates an easily separable mixture
of derivatives of 4 and 5.
Scheme 1a
a (a) EDC, CH2Cl2 (95%); (b) LiOH, H2O, EtOH (78%); (c)
4-nitrophenol, DCC, CH2Cl2 (54%); (d) CF3CO2H, CH2Cl2; (e)
pyridine (d and e: 44% 25, 8% 26).
of the Boc group to commercially available N-R-Cbz-L-
aminoalanine9 or by a facile two-step synthesis starting from
Cbz-Asn-OH10) yields dipeptide 20. Treatment of 20 with 1
equiv of LiOH leads to hydrolysis of one of the two esters
to generate a mixture of half-esters 21 and 22. Esterification
of this mixture with DCC and 4-nitrophenol afforded an
inseparable mixture of diesters 23 and 24. Last, treatment
of the mixture of 23 and 24 with CF3CO2H in CH2Cl2,
followed by addition of the resulting amine salts to anhydrous
pyridine at 23 °C, led to the formation of a 5.5:1 mixture of
the isomeric, bicyclic dipeptides 25 and 26. The abundance
of 25 relative to 26 must arise during the treatment of 20
with LiOH. Either one of the esters reacts more quickly than
the other because of assistance from the neighboring ami-
noalanine residue or reaction at one of the esters is hindered
Figure 1. Bicyclic dipeptides conforming to general structure 3.
1226
Org. Lett., Vol. 1, No. 8, 1999

by the neighboring aminoalanine. In either case, the chiral
aminoalanine must exert an influence on the course of the
reaction. The two bicyclic dipeptides 25 and 26 were easily
separated and purified by flash chromatography.11
appear as a doublet of doublets at 3.3 ppm and a multiplet
at 3.65 ppm, with the hydrogen appearing as the doublet of
doublets not showing any coupling to the lactam NH.
Similarly 28, which has the same bicyclic ring system as
26, was prepared from 19 and L-proline methyl ester. Its 1H
NMR and COSY spectra also bear many similarities to its
structural relative 26. Like 26, the two hydrogens of the
lactam methylene CH2 appear as multiplets centered at 3.6
and 3.9 ppm, and both resonances show coupling to the
neighboring lactam methylene NH. The near identical
appearance of the lactam methylene resonances in 25 and
27, and in 26 and 28, support the assignments of 25 and 26.
They also indicate that the bicyclic ring systems here have
little conformational mobility and assume one defined
conformation in solution. Finally, the spectroscopic data for
25-28 show that the appearance of the lactam methylene
in the 1H NMR spectrum can be used to assign the
stereochemistry in these bicyclic dipeptides.
The two isomers 25 and 26 were readily distinguished by
the appearance of the lactam methylene CH2 group in their
respective 1H NMR spectra. For the first isomer to elute off
the chromatography column, the 1H NMR and COSY spectra
showed the resonances of this CH2 appearing as a doublet
of doublets at 3.44 ppm and a multiplet at 3.63 ppm. The
hydrogen appearing as the doublet of doublets was only
coupled to its geminal partner and the neighboring methine;
it was not coupled to the lactam NH. The absence of coupling
here indicates that the dihedral angle between the lactam NH
and one of the hydrogens on the CH2 is approximately 90°.
Conversely, for the second isomer to elute off the chroma-
1
tography column, the H NMR and COSY spectra showed
the resonances of the CH2 appearing as two multiplets
centered at 3.57 and 3.95 ppm. Both multiplets in the second
isomer were coupled to the lactam NH. Inspection of
Dreiding models of both 25 and 26 showed that only 25
could assume a conformation in which one of the hydrogens
on the lactam CH2 had a nearly 90° dihedral angle with the
neighboring lactam NH. Thus, it was concluded that 25 was
the first isomer to elute off the column, while 26 was the
second isomer.
Also supporting the assignments of 25 and 26 were
molecular modeling experiments in which low-energy con-
formations of 25-28 were obtained using an MM2 energy
minimization. In the low-energy conformations of 25 and
27, the calculated dihedral angles for the lactam methylene
CH2 were -81° and 33°, and -85° and 30°, respectively.
For 26 and 28, the calculated dihedral angles for the lactam
methylene CH2 were -132° and -16°, and -127° and -11°,
respectively. These calculated dihedral angles are in agree-
To confirm this conclusion, model compounds 27 and 28,
which could be prepared with established configurations at
all chiral centers, were examined. 27, which has the same
bicyclic ring system as 25, was prepared from 19 and
D-proline methyl ester using the route outlined in Scheme
1. The 1H NMR and COSY spectra of 27 bear many
1
similarities to the H NMR and COSY spectra of 25. Like
25, the two hydrogens of the lactam methylene CH2 in 27
(7) (a) 6: Lombart, H.-G.; Lubell, W. D. J. Org. Chem. 1996, 61, 9437.
(b) 6: Mueller, R.; Revesz, L. Tetrahedron Lett. 1994, 35, 4091. (c) 7:
Nagai, U.; Sato, K.; Nakamura, R.; Kato, R. Tetrahedron 1993, 49, 3577.
(d) 8: Colombo, L.; Di Giacomo, M.; Papeo, G.; Carugo, O.; Scolastico,
C.; Manzoni, L. Tetrahedron Lett. 1994, 35, 4031. (e) 9: De Lombaert, S.;
Blanchard, L.; Stamford, L. B.; Sperbeck, D. M.; Grim, M. D.; Jenson, T.
M.; Rodriguez, H. R. Tetrahedron Lett. 1994, 35, 7513. (f) 10-11: Claridge,
T. D. W.; Hulme, C.; Kelly, R. J.; Lee, V.; Nash, I. A.; Schofield, C. J.
Bioorg. Med. Chem. Lett. 1996, 6, 485. (g) 10-11: Slomczynska, U.;
Chalmers, D. K.; Cornille, F.; Smythe, M. L.; Beusen, D. D.; Moeller, K.
D.; Marshall, G. R. J. Org. Chem. 1996, 61, 1198. (h) 12-13: Cornille,
F.; Slomczynska, U.; Smythe, M. L.; Beusen, D. D.; Moeller, K. D.;
Marshall, G. R. J. Am. Chem. Soc. 1995, 117, 909. (i) 14-15: Li, W.;
Hanau, C. E.; d’Avignon, A.; Moeller, K. D. J. Org. Chem. 1995, 60, 8155.
(j) 16: Hanessian, S.; Ronan, B.; Laoui, A. Bioorg. Med. Chem. Lett. 1994,
4, 1397. (k) 17: Flynn, G. A.; Giroux, E. L.; Dage, R. C. J. Am. Chem.
Soc. 1987, 109, 7914.
Figure 2. Low-energy conformations of 27 (A) and 28 (B). For
clarity, the Cbz group has been truncated to an acetamide. The
carbons are darkly shaded, the nitrogens are lightly spotted, the
oxygens are heavily spotted, and the hydrogens are white.
(8) (a) Cignarella, G.; Nathansohn, G. J. Org. Chem. 1961, 26, 1500.
(b) Lowe, G.; Ridley, D. D. J. Chem. Soc., Perkin Trans. 1 1973, 2024. (c)
Kemp, D. S.; Curran, T. P. J. Org. Chem. 1988, 53, 5729.
(9) Available from Bachem Bioscience, Inc., Catalog no. C-3315.
(10) Waki, M.; Kitajima, Y.; Izumiya, N. Synthesis 1981, 266.
Org. Lett., Vol. 1, No. 8, 1999
1227

1
ment with the H NMR coupling constant data for 25-28.
oped. The two isomeric peptides are easily separated by
chromatography and are readily distinguished by H NMR.
1
Figure 2 shows the low-energy conformations for 27 and
28.
The spectroscopic data also indicate that these two bicyclic
structures adopt well-defined conformations. The use of these
constrained prolines as probes of the enzymatic activity of
the protease CD26 are currently being pursued.
In summary, a short synthesis that generates two isomeric,
constrained trans-proline amide dipeptides has been devel-
(11) 25 and 26 are separable by flash chromatography (EtOAc). 25: 1H
NMR (300 MHz, CDCl3) δ 7.35 (5H, s), 6.07 (1H, d, J ) 5.1 Hz), 6.05
(1H, d, J ) 5.9 Hz), 5.12 (2H, s), 4.94 (1H, m), 4.67 (2H, m), 4.18 (2H,
q, J ) 7.1 Hz), 3.63 (1H, m), 3.44 (1H, dd, J ) 11.6, 11.6 Hz), 2.65 (1H,
m), 2.30 (1H, m), 2.17 (1H, m), 2.06 (1H, m), 1.30 (3H, t, J ) 7.1 Hz).
TLC (EtOAc) Rf ) 0.47. Anal. Calcd for C19H23N3O6: C, 58.60; H, 5.95;
N, 10.79. Found: C, 58.26; H, 6.17; N, 10.62. 26: 1H NMR (300 MHz,
CDCl3) δ 7.35 (5H, s), 6.39 (1H, s), 5.82 (1H, d, J ) 4.3 Hz), 5.08 (2H,
dd, J ) 9.3, 12.1 Hz), 4.73 (2H, m), 4.15 (2H, m), 3.95 (2H, m), 3.57 (1H,
m), 2.67 (1H, m), 2.13 (3H, m), 1.24 (3H, t, J ) 7.1 Hz). TLC (EtOAc) Rf
) 0.24. Anal. Calcd for C19H23N3O6: C, 58.60; H, 5.95; N, 10.79. Found:
C, 58.48; H, 6.10; N, 10.59.
Acknowledgment. This work was supported by a Cottrell
College Science Award of Research Corporation (CC3786)
and by a grant from the National Institutes of Health (NIH-
AREA Grant AI41227-01). The NMR spectrometer used in
the experiments was purchased with a grant from the
National Science Foundation (NSF-ILI USE-8852774).
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