An efficient route to either enantiomer of orthogonally protected trans-3-Aminopyrrolidine-4-carboxylic acid

Full text of DOI:10.1021/jo001534l

J . Org. Chem. 2001, 66, 3597-3599
3597
we report an improved synthesis of APC that is amenable
to large-scale preparation.
An Efficien t Rou te to Eith er En a n tiom er of
Or th ogon a lly P r otected
tr a n s-3-Am in op yr r olid in e-4-ca r boxylic Acid
Hee-Seung Lee, Paul R. LePlae, Emilie A. Porter, and
Samuel H. Gellman*
Department of Chemistry, University of Wisconsin,
Madison, Wisconsin 53706-1396
The new route is summarized in Scheme 1. Our
original route,2b like the new one, started from the known
â-ketoester 1.6 In the original route, reduction followed
by elimination produced the R,â-unsaturated ester, and
Michael addition of enantiomerically pure R-methylben-
zylamine then yielded a mixture of the four diastereo-
meric â-aminoesters. The desired isomer was isolated in
13% yield after tedious column chromatography.2b This
original route was serviceable for initial evaluation of
APC as a â-peptide building block, but this route was
not efficient enough for thorough exploration of the
structures and properties of APC-containing â-peptides.
The new route (Scheme 1) has been streamlined by
reducing the number of chemical operations and by
eliminating the need for chromatographic separations.
Development of this route was inspired by an asymmetric
synthesis of cis-2-aminocyclohexanecarboxylic acid re-
ported by Xu et al.7 â-Ketoester 1 is allowed to react with
(R)-R-methylbenzylamine in the presence of acetic acid,
and the resulting enamine is reduced in situ with NaBH3-
CN.8 This reduction produces a mixture of four diaster-
eomeric â-aminoesters in which 3 is the major product,
and we have found a two-step crystallization protocol that
allows isolation of hydrochloride salt 2 in diastereomeri-
cally pure form. The crude â-aminoester mixture is
dissolved in ethyl acetate and converted to a mixture of
hydrochloride salts by treatment with 4 N HCl in
dioxane. A single trans isomer crystallizes in relatively
pure form after this treatment (g98% de), although there
is contamination from the other trans isomer. Recrys-
tallization from acetonitrile yields a very pure form of
2 (g99% de) in 38% overall yield from 1. When (R)-R-
methylbenzylamine is used, the purified â-aminoester
hydrochloride is spectroscopically identical to material
previously identified by crystal structure determination
as the diastereomer shown in Scheme 1. Thus, use of (R)-
R-methylbenzylamine leads ultimately to a protected
form of (3S,4R)-trans-3-aminopyrrolidine-4-carboxylic acid.9
The new route is completed by alkaline ester hydroly-
sis, hydrogenolytic removal of the R-methylbenzyl group,
and Fmoc protection of the resulting amino group. These
three steps can be performed in rapid succession, and the
gellman@chem.wisc.edu
Received October 30, 2000
Oligo-â-amino acids (“â-peptides”) and other oligomers
with discrete and predictable folding propensities (“fol-
damers”) are subjects of increasing attention.1 Recent
work has established that short â-peptides (esix resi-
dues) containing conformationally restrained residues
display well-defined conformations in aqueous solution.2
In addition, â-peptides have been shown to display
interesting biological activities.3 Exploration of the struc-
tural and functional properties of â-peptides requires the
availability of a wide range of enantiomerically pure
â-amino acids. Despite the extensive effort that has been
devoted to â-amino acid synthesis,4 however, many
substitution patterns, particularly those that provide
conformational constraint, are not readily accessible.
We have shown that homooligomers of enantiomeri-
cally pure trans-2-aminocyclopentanecarboxylic acid
(ACPC) form a helix defined by 12-membered ring
hydrogen bonds between backbone amide groups (“12-
helix”).5 This finding prompted us to seek ACPC ana-
logues that bear an additional point of functionalization
on the five-membered ring. trans-3-Aminopyrrolidine-4-
carboxylic acid (APC) fulfills this need. We have recently
shown that short â-peptides constructed from APC and
ACPC adopt the 12-helical conformation in aqueous
solution2b and that a 17-residue â-peptide composed of
these two residues displays antimicrobial activity.3c Here
(1) (a) Seebach, D.; Matthews, J . L. J . Chem. Soc., Chem. Commun.
1997, 2015. (b) Gellman, S. H. Acc. Chem. Res. 1998, 31, 173. (c)
DeGrado, W. F.; Schneider, J . P.; Hamuro, Y. J . Peptide Res. 1999,
54, 206. (d) Gademann, K.; Hintermann, T.; Schreiber, J . V. Curr. Med.
Chem. 1999, 6, 905. (e) Kirshenbaum, K.; Zuckermann, R. N.; Dill, K.
A. Curr. Opin. Struct. Biol. 1999, 9, 530. (f) Stigers, K. D.; Soth, M. J .;
Nowick, J . S. Curr. Opin. Chem. Biol. 1999, 3, 714. (g) Barron, A. E.;
Zuckermann, R. N. Curr. Opin. Chem. Biol. 1999, 3, 681.
(2) (a) Appella, D. H.; Barchi, J . J .; Durell, S.; Gellman, S. H. J .
Am. Chem. Soc. 1999, 121, 2309. (b) Wang, X.; Espinosa, J . F.; Gellman,
S. H. J . Am. Chem. Soc. 2000, 122, 4821.
(3) (a) Werder, M.; Hausre, H.; Abele, S.; Seebach, D. Helv. Chim.
Acta 1999, 82, 1774. (b) Hamuro, Y.; Schneider, J . P.; DeGrado, W. F.
J . Am. Chem. Soc. 1999, 121, 12200. (c) Porter, E. A.; Wang, X.; Lee,
H.-S.; Weisblum, B.; Gellman, S. H. Nature 2000, 404, 565.
(4) For recent literature on â-amino acid synthesis, see: J uaristi,
E.; Lo´pez-Ruiz, H. Curr. Med. Chem.1999, 6, 983. J uaristi, E. Enan-
tioselective synthesis of â-amino acids; Wiley-VCH: New York, 1997.
Davis, F. A.; Reddy, G. V.; Liang, C.-H. Tetrahedron Lett. 1997, 38,
5139. Ishitani, H.; Ueno, M.; Kobayashi, S. J . Am. Chem. Soc. 1997,
119, 7153. Sibi, M. P.; Shay, J . J .; Liu, M.; J asperse, C. P. J . Am. Chem.
Soc. 1998, 120, 6615. Tang, T. P.; Ellman, J . A. J . Org. Chem. 1999,
64, 12. Zhu, G.; Chen, Z.; Zhang, X. J . Org. Chem. 1999, 64, 6907.
Dexter, C. S.; J ackson, R. F. W. J . Org. Chem. 1999, 64, 7579.
(5) (a) Appella, D. H.; Christianson, L. A.; Klein, D. A.; Huang, X.;
Barchi, J . J .; Powell, D. R.; Gellman, S. H. Nature 1997, 387, 381. (b)
Appella, D. H.; Christianson, L. A.; Klein, D. A.; Richards, M. R.;
Powell, D. R.; Gellman, S. H. J . Am. Chem. Soc. 1999, 121, 7574. (c)
Barchi, J . J .; Huang, X.; Appella, D. H.; Christianson, L. A.; Durell, S.
R.; Gellman, S. H. J . Am. Chem. Soc. 2000, 122, 2711.
(6) Blake, J .; Willson, C. D.; Rapoport, H. J . Am. Chem. Soc. 1964,
86, 5293.
(7) Xu, D.; Prasad, K.; Repic, O.; Blacklock, T. J . Tetrahedron:
Asymmetry 1997, 8, 1445. For thorough reviews on the use of
R-methylbenzylamine in the preparation of enantiomerically pure
molecules, see: J uaristi, E.; Escalante, J .; Leo´n-Romo, J . L.; Reyes,
A. Tetrahedron: Asymmetry 1998, 9, 715. J uaristi, E.; Leo´n-Romo, J .
L.; Reyes, A.; Escalante, J . Tetrahedron: Asymmetry 1999, 10, 2441.
(8) Attempts to reduce the enamine with NaBH4 were unsuccessful.
Reduction of a related enamine with NaB(OAc)3H provides predomi-
nantly the cis â-amino ester: Cimarelli, C.; Palmieri, G. J . Org. Chem.
1996, 61, 5557.
(9) The stereochemical nomenclature was ambiguous in our previous
description of APC synthesis (ref 2b), but the structures drawn in that
paper are stereochemically accurate.
10.1021/jo001534l CCC: $20.00 © 2001 American Chemical Society
Published on Web 04/18/2001

3598 J . Org. Chem., Vol. 66, No. 10, 2001
Notes
Sch em e 1a
a
Reagents and conditions: (a) (R)-(+)-R-methylbenzylamine, AcOH, EtOH, rt; (b) NaBH3CN, 75 °C and then 4 N HCl in dioxane,
EtOAc, 0 °C; (c) saturated Na2CO3, EtOAc; (d) LiOH‚H2O, THF/CH3OH/H2O (6/3/1), 0 °C and then acidic workup; (e) H2 (45 psi), 10%
Pd-C, 95% EtOH; (f) Fmoc-OSu, NaHCO3, acetone/H2O (2/1), 0 °C to rt.
times with diethyl ether. The combined organic extracts were
washed with brine, dried over MgSO4, and concentrated to give
a colorless oil. The oil was applied to a plug of silica gel and
washed with 2:1 hexane/ethyl acetate. The filtrate was concen-
trated to obtain a colorless oil. The oil was dissolved in ethyl
acetate (250 mL), and 4 N HCl in dioxane (15.6 mL) was added
dropwise at room temperature. The resulting solution was cooled
to 0 °C and allowed to stand for 3 h at 0 °C. A precipitate formed
during this time. The solid was filtered and washed two times
with 100 mL portions of ethyl acetate to provide the desired
material in 42% crude yield (g98% de) from 1. The diastereo-
meric excess was determined by GC-MS (0.25 mm × 12.0 m
Supelco SPB-1 fused silica capillary column; temperature gradi-
ent ) 150 °C, 2 min; 150-220 °C, 10 °C/min; 220-250 °C, 0.9
°C/min; 250-280 °C, 10 °C/min; 280 °C, 1 min; injector temper-
ature ) 280 °C; detector temperature ) 300 °C; flow rate ) 1.2
mL/min); the (3S,4R)-trans isomer had a retention time of 13.3
min, and the (3R,4S)-trans isomer had a retention time of 13.0
min). This crude product could be purified by recrystallization
from acetonitrile. The solid was suspended in acetonitrile (200
mL) and heated to reflux for 1 h. The mixture was then cooled
to 0 °C for 3 h. The resulting precipitate was isolated by filtration
and washed two times with 30 mL portions of acetonitrile. The
solid was further dried under vacuum to give 9.4 g of 2 as a
white crystalline solid (g99.0% de, 38% yield from 1): mp 190-
final product can be purified by crystallization from
n-heptane/ethyl acetate. The protection pattern of di-
amino acid derivative 4 is suitable for Fmoc-based
synthesis of the â-peptide backbone on a solid support,
with deprotection of the pyrrolidine ring nitrogens upon
acidolytic cleavage from the resin. The synthetic route
outlined in Scheme 1 is amenable to multigram synthesis
of â-peptide building block 4 from 1 in 1 week. We are
currently exploring the attachment of functionally diverse
side chains to the pyrrolidine nitrogen atom.
Exp er im en ta l Section
Gen er a l P r oced u r es. Melting points were determined on a
capillary melting point apparatus and were uncorrected. Optical
rotations were measured using sodium light (D line, 589.3 nm).
THF was distilled from sodium benzophenone ketyl under N2.
Unless otherwise noted, all other commercially available re-
agents and solvents were purchased from Aldrich and used
without further purification, except for 4 N HCl in dioxane,
which was purchased from Pierce, and Fmoc-Osu, which was
purchased from Advanced ChemTech. Analytical thin-layer
chromatography (TLC) was carried out on Whatman TLC plates
precoated with silica gel 60 (250 µm layer thickness). Visualiza-
tion was accomplished using either a UV lamp, potassium
permanganate stain (2 g of KMnO4, 13.3 g of K2CO3, 3.3 mL of
5% (w/w) NaOH, 200 mL of H2O), or phosphomolybdic acid
(PMA) stain (10% phosphomolybdic acid in ethanol). Column
chromatography was performed on EM Science silica gel 60
(230-400 mesh). Solvent mixtures used for TLC and column
chromatography are reported in v/v ratios.
(3S,4R)-1-(N-ter t-Bu toxyca r bon yl)-3-[(1′R)-p h en yleth y-
la m in o]-4-eth oxyca r bon ylp yr r olid in e Hyd r och lor id e (2).
To a stirred solution of â-ketoester 16 (16.0 g, 62.3 mmol) in
absolute ethanol (250 mL) under N2 were added (R)-(+)-R-
methylbenzylamine (16.0 mL, 124.5 mmol) and glacial acetic acid
(7.1 mL, 124.5 mmol) to obtain a cloudy solution. The reaction
mixture was stirred at room temperature until the formation of
the enamine was complete (3 h, monitored by TLC, product Rf
0.55, 7:3 hexane/ethyl acetate). Sodium cyanoborohydride (16.5
g, 249.2 mmol) was then added to the reaction mixture at room
temperature, and the resulting solution was heated to 75 °C and
stirred for 14 h under N2. (This reaction must be carefully
monitored by TLC; disappearance of the enamine indicates
completion of reaction. Higher temperature and/or longer reac-
tion time leads to formation of an R,â-unsaturated ester side
product.) The ethanol was removed via rotary evaporation in a
well-ventilated hood (CAUTION-possible HCN evolution!).
Water (250 mL) was added. The mixture was extracted three
191 °C; [R]23 ) 4.8 (c 1.05, MeOH); 1H NMR (DMSO-d6, 300
D
MHz, 24 °C) δ 10.12-9.75 (br, 2H), 7.70-7.28 (m, 5H), 4.54 (m,
1H), 4.05 (q, J HH ) 7.2 Hz, 2H), 3.80-3.59 (m, 3H), 3.54-3.22
(m, 3H), 1.62 (d, J HH ) 6.3 Hz, 3H), 1.37 (s, 9H), 1.13 (t, J HH
)
7.2 Hz, 3H); 1H NMR (DMSO-d6, 500 MHz, 60 °C) δ 10.22 (br s,
2H), 7.66-7.65 (m, 2H), 7.42-7.83 (m, 3H), 4.48 (m, 1H), 4.05
(q, J HH ) 7.0 Hz, 2H), 3.78-3.68 (m, 3H), 3.54-3.35 (m, 3H),
1.67 (d, J HH ) 7.0 Hz, 3H), 1.36 (s, 9H), 1.13 (t, J HH ) 7.0 Hz,
3H); 13C NMR (DMSO-d6, 125.7 MHz, 60 °C) δ 170.20, 152.70,
136.54, 128.68, 128,58, 127.90, 78.95, 60.92, 56.61, 56.00, 47.86,
46.69, 44.70, 27.82, 19.76, 13.50.
(3S,4R)-1-(N-ter t-Bu t oxyca r b on yl)-3-[(1′R)-p h en ylet h -
yla m in o]-4-eth oxyca r bon ylp yr r olid in e (3). A sample of
compound 2 was mixed with excess saturated Na2CO3 solution,
extracted into ethyl acetate, dried over MgSO4, and concentrated
in vacuo: 1H NMR (CDCl3, 300 MHz) δ 7.34-7.16 (m, 5H), 4.22-
4.07 (m, 2H), 3.80 (q, J HH ) 6.0 Hz, 1H), 3.71-3.53 (m, 1H),
3.53-3.22 (m, 3H), 2.94-2.78 (m, 2H), 1.39 (d, rotamer, 9H),
1.32 (d, J HH ) 6.3 Hz, 3H), 1.29-1.18 (m, 3H);1H NMR (DMSO-
d6, 500 MHz, 60 °C) δ 7.35-7.25 (m, 3H), 7.22-7.18 (m, 2H),
4.12-4.05 (m, 2H), 3.78 (q, J HH ) 7.0 Hz, 1H), 3.54 (m, 1H),
3.36 (m, 1H), 3.24 (m, 2H), 3.00-2.90 (m, 2H), 1.36 (s, 9H), 1.24
(d, J HH ) 6.5 Hz, 3H), 1.54 (t, J HH ) 7.0 Hz, 3H); 13C NMR
(DMSO-d6, 125.7 MHz, 24 °C) δ 172.57, 172.43, 153.25, 153.14,
145.90, 128.11, 126.60, 126.51, 78.31, 60.28, 58.84, 58.02, 55.61,

Notes
J . Org. Chem., Vol. 66, No. 10, 2001 3599
51.39, 51.17, 48.53, 47.79, 46.70, 39.33, 24.56, 13.86; 13C NMR
(DMSO-d6, 125.7 MHz, 60 °C) δ 172.09, 153.02, 145.72, 127.79,
126.27, 126.21, 78.06, 59.94, 58.43, 55.50, 51.19, 48.28, 46.52,
27.83, 24.12, 13.57; MS-MALDI m/z 363.3 (M + H).
acetate to afford 1.13 g (72%) of 4 as a white solid: mp 113-
115 °C; [R]23 ) -18.3 (c 1.2, MeOH); 1H NMR (CD3OD, 300
D
MHz) δ 7.77 (d, J HH ) 7.2 Hz, 2H), 7.62 (d, J HH ) 7.5 Hz, 2H),
7.37 (t, J HH ) 7.2 Hz, 2H), 7.28 (t, J HH ) 7.2 Hz, 2H), 4.48-4.27
(m, 3H), 4.18 (t, J HH ) 7.2 Hz, 1H), 3.72-3.46 (m, 3H), 3.14 (m,
1H), 2.88 (m, 1H), 1.44 (s, 9H); 13C NMR (CD3OD, 75.4 MHz, 24
°C) δ 174.63, 158.13, 156.02, 145.24, 145.16, 142.56, 128.74,
128.11, 126.17, 126.10, 120.90, 81.29, 67.77, 54.73, 54.06, 51.73,
51.20, 28.70; 13C NMR (CD3OD, 125.7 MHz, 50 °C) δ 158.20,
156.11, 145.28, 145.22, 142.56, 128.71, 128.09, 126.06, 120.87,
81.12, 67.81, 55.10, 51.61, 28.77; MS-MALDI m/z 475.3 (M +
Na), 491.2 (M + K).
(3S,4R)-1-(N-ter t-Bu t oxyca r b on yl)-3-(9H -flu or en -9-yl-
m eth oxycar bon ylam in o)-4-h ydr oxycar bon ylpyr r olidin e (4).
Compound 2 (1.39 g, 3.49 mmol) was dissolved in THF/MeOH/
H2O (6/3/1, v/v/v, 40 mL), and the solution was cooled to 0 °C.
LiOH‚H2O (732 mg, 17.4 mmol) was added. The mixture was
stirred at 0 °C for 3 h. Aqueous HCl (1 N, 18 mL) was added at
0 °C. The solvent was then removed on a vacuum rotary
evaporator to give a white solid (Rf 0.32, 1:9 MeOH/CH2Cl2). The
white solid was dissolved in 150 mL of 95% ethanol in a
hydrogenation flask. Pd-C (10%, 1.1 g) was added. The resulting
mixture was shaken under H2 (45 psi) for 24 h. After the reaction
was complete (disappearance of starting material, as monitored
by TLC), the mixture was filtered through Celite, and the filtrate
was concentrated to obtain a white solid. This solid was dissolved
in acetone/H2O (2/1, v/v, 150 mL) and cooled to 0 °C, and Fmoc-
Osu (1.53 g, 4.54 mmol) and NaHCO3 (2.93 g, 34.9 mmol) were
added. The reaction mixture was stirred at 0 °C for 1 h and then
allowed to stir at room temperature overnight. Water (50 mL)
was added. The acetone was removed under reduced pressure.
The aqueous layer was stirred for 1 h with diethyl ether (50 mL),
the layers were separated, and the aqueous layer was acidified
with 1 N aqueous HCl, extracted with ethyl acetate, dried over
MgSO4, and concentrated to give a foamy solid. The crude
product was purified by crystallization from n-heptane/ethyl
The enantiomer of 4 was prepared by starting with (S)-(-)-
R-methylbenzylamine: [R]23 ) +18.3 (c 1.2, MeOH).
D
Ack n ow led gm en t. This research was supported by
the National Institutes of Health (GM56414) and the
Leukemia Society of America. P.R.L. was supported in
part by a fellowship from Kodak, and E.A.P. was
supported in part by a Biotechnology Training Grant
from NIGMS.
Su p p or tin g In for m a tion Ava ila ble: 1H NMR, 13C NMR,
and mass spectrometry data for 2-4. This material is available
free of charge via the Internet at http://pubs.acs.org.
J O001534L