First chemoenzymatic synthesis of (+)-2-carboxypyrrolidine-3-acetic acid, the nucleus of kainoid amino acids

Article abstract of DOI:10.1002/chir.21032

The distinctive nucleus of kainoid amino acids, (2S,3R)-(+)-2- carboxypyrrolidine- 3-acetic acid 6, was synthesized by a chemoenzymatic process, exploiting the diastereomeric cis/ trans methyl pyroglutamate derivatives 10a-c/11a-c as key intermediates. These mixtures, when subjected to a kinetic resolution mediated by α-chymotrypsin, reacted diastereo-, regio-, and enantioselectively to give the trans derivatives (+)-10a-c possessing the correct (2S,3R) configuration. Subsequently, the desired product (2S,3R)-(+)-6 could be obtained after well-established transformations.

Full text of DOI:10.1002/chir.21032

CHIRALITY 24:112–118 (2012)
First Chemoenzymatic Synthesis of (1)-2-Carboxypyrrolidine-3-Acetic
Acid, the Nucleus of Kainoid Amino Acids
1
1
1
2
FULVIA FELLUGA,1 CRISTINA FORZATO, PATRIZIA NITTI, GIULIANA PITACCO, FRANCO GHELFI, AND ENNIO VALENTIN1
*
1Department of Chemical and Pharmaceutical Sciences, University of Trieste, Trieste, Italy
2Department of Chemistry, University of Modena and Reggio Emilia, Modena, Italy
ABSTRACT
The distinctive nucleus of kainoid amino acids, (2S,3R)-(1)-2-carboxypyrrolidine-
3-acetic acid 6, was synthesized by a chemoenzymatic process, exploiting the diastereomeric cis/
trans methyl pyroglutamate derivatives 10a–c/11a–c as key intermediates. These mixtures,
when subjected to a kinetic resolution mediated by a-chymotrypsin, reacted diastereo-, regio-,
and enantioselectively to give the trans derivatives (1)-10a–c possessing the correct (2S,3R) con-
figuration. Subsequently, the desired product (2S,3R)-(1)-6 could be obtained after well-estab-
C
VV
lished transformations. Chirality 24:112–118, 2012.
2011 Wiley Periodicals, Inc.
KEY WORDS: kainoids; enzymatic hydrolysis; kinetic resolution; regioselectivity;
diastereoselectivity; enantiomeric excess
INTRODUCTION
although to our knowledge, this powerful synthetic approach
Kainoid amino acids1–4 are a large family of natural nonpro-
teinogenic pyrrolidine amino acids, whose parent member is
(2)-a-kainic acid 1, and which includes among the others the
C-4 epimer of 1 called (1)-a-allokainic acid 2, as well as vari-
ous derivatives related to acromelic acid 32,5 and (2)-domoic
acid 42,6 (Fig. 1). All of them show biological properties,
including insecticidal7 and antihelmintic8 activities, but most
significantly they are powerful neuroexcitatory agents.9–12
In particular, (2)-a-kainic acid 1 is a potent and specific
agonist at glutamate receptors and has been widely used as
a probe in the study of neurological diseases such as epi-
lepsy, Alzheimer’s disease, and Huntington’s chorea.13,14
These compounds are constrained analogs of L-glutamic
acids (L-Glu) 5, the major excitatory neurotransmitter in the
mammalian central nervous system. Structurally, they are
based on the nucleus of trans 2-carboxypyrrolidine-3-acetic
acid (CPAA) 6, all bearing an unsaturated substituent at C-4.
The three contiguous stereocenters are arranged in a typical
(C-2, C-3)-trans and (C-3, C-4)-cis relative configuration, with
the only exception being 2. A number of structure–activity
relationship studies have revealed that this relative geome-
try, together with the nature of the substituent at C-4, which
generally contains p-unsaturation, is crucial for the binding
at the kainic acid recognition site,15–18 and therefore for their
potent biological activity. In fact, the neuroexcitatory activity
of allokainic acid 2, in which the (C-3, C-4) relative configura-
tion is trans, is much lower than that of a-kainic acid 1, and
the molecule is inactive as antihelmintic.
has not been used in the asymmetric synthesis of CPAA 6.
In this study, we present the first chemoenzymatic synthe-
sis of 6 in its natural configuration. The synthetic strategy
that we devised is outlined in Scheme 1. The key intermedi-
ate is the optically active pyroglutamic derivative (2S,3R)-10
(R 5 Et, n-Bu, Bn), possessing the desired heterocyclic
framework and the correct stereochemistry.
Compounds 10 are accessible by conjugate addition of
methyl nitroacetate 8 to the appropriate glutaconic diester 7
and are prone to enzymatic kinetic resolution. In fact, as
reported in the literature,52,53 the (–)-enantiomers of ethyl
pyroglutamate and some 4-substituted derivatives of ethyl
pyroglutamate were recognized and hydrolyzed by a-chymo-
trypsin (a-CT).
MATERIALS AND METHODS
General
IR spectra were recorded on a Jasco FTIR 200 spectrometer. 1H NMR
spectra were run on a JEOL EX-400 (400 MHz) or on a Varian Innova 500
(500 MHz) using deuteriochloroform as the solvent, unless otherwise
stated. Chemical shifts are expressed in parts per million (d). The resonan-
ces were referenced to the solvent peak (CDCl3 at d 7.26, CD3OD at d
3.34, and D2O at d 4.87). Coupling constants are given in Hz. 13C NMR
spectra were recorded at 125.68 MHz on a Varian Innova 500 or at 67.95
on a JEOL 270 spectrometer. Signal attribution of compounds 10a–c and
11a–c was made by gradient correlation spectroscopy (COSY) and hetero-
nuclear single-quantum correlation (HSQC), and the relative configuration
of the same compounds was determined by nuclear overhauser effect spec-
troscopy (NOESY). Optical rotations were determined on a Perkin Elmer
Model 241 polarimeter, at 258C. Electrospray ionization mass spectrometry
(ESI-MS) were run on a Bruker Esquire 4000 instrument. MS were per-
formed on an ion trap Finnigan GCQ (70 eV) spectrometer. High resolu-
tion mass spectrometry (HRMS) were run on a Finngan MAT95XP spec-
trometer. Enzymatic hydrolyses were performed using a pH-stat Controller
The distinctive nucleus of kainoids, (2S,3R)-(1)-CPAA19
6
lacks the unsaturated substituent at C-4. Although 6 was
shown20 to not retain the kainate-type selectivity, it was rec-
ognized as an agonist of the N-methyl-D-aspartate (NMDA)
receptor.19
Whereas several strategies have been proposed for the
asymmetric total syntheses of kainoid amino acids2,21–36 and
other unnatural kainoids,37–45 only a few asymmetric synthe-
ses of optically active 6 with complete control of the stereo-
chemistry have been proposed.45–50
Contract grant sponsors: Ministero dell’Universita` e della Ricerca; University
of Trieste; Contract grant number: PRIN 2007.
*Correspondence to: Fulvia Felluga, Department of Chemical and Pharma-
ceutical Sciences, University of Trieste, via Giorgieri 1, Trieste 34127, Italy.
E-mail: ffelluga@units.it
Received for publication 4 July 2011; Accepted 8 September 2011
DOI: 10.1002/chir.21032
Published online 19 December 2011 in Wiley Online Library
(wileyonlinelibrary.com).
Biotransformations are a powerful tool to control the con-
figuration of chiral centers in stereoselective synthesis,51
C
VV
2011 Wiley Periodicals, Inc.

FIRST CHEMOENZYMATIC SYNTHESIS OF CPAA
113
NMR (67.94 MHz, CDCl3) d, ppm 37.3 (t), 66.3 (t), 66.9 (t), 124.6 (d),
128.2 (d), 128.3 (d), 128.4 (d), 128.5 (d), 128.6 (d), 128.7 (d), 135.4 (s),
135.9 (s), 140.0 (d), 165.5 (s), 169.6 (s); HRMS (EI): Calcd for C19H18O4:
310.1205; Found: 310.1200; ESI-MS: 311.3 [M1H]1.
General procedure for the synthesis of compounds 9a–c. To a
solution of the appropriate diester 7a–c (20 mmol) and methyl nitroace-
tate 8 (2.86 g, 24 mmol) in CH3CN (10 ml), DBU (3.65 g, 24 mmol) was
added dropwise at 08C. The mixture was left on stirring at RT until disap-
pearance of the staring material (TLC, petroleum ether/ethyl acetate
4:1). The solvent was removed in vacuo and replaced with ether (10 ml),
and the solution was washed with aq. 1 N HCl, saturated NaHCO3, and
brine. The yellow oily residue so obtained was used in the next step
without any purification, 100% yield.
Ethyl methyl 2-nitro-3-(ethoxycarbonyl)methylpentanedioate [9a]. IR
(neat), 1737, 1734, 1564, 1377 cm21; 1H NMR (400 MHz, CDCl3) d, ppm:
1.30, 1.29 (2 t, J 5 7.0 Hz, 3 H each, 2 CH3CH2O), 2.61 (m, 4 H, 2
CH2COOEt), 3.30 (m, 1 H, H-3), 3.80 (s, 3 H, OCH3), 4.27 (2q, J 5 7.0
Hz, 2 H each, 2 OCH2), 5.66 (d, J 5 5.9 Hz, 1 H, CHNO2); 13C NMR
(67.94 MHz, CDCl3) d, ppm 14.0 (2q), 32.6 (d), 34.1 (t), 34.2 (t), 53.5
(q), 61.0 (2t), 88.2 (d), 164.0 (s), 170.9 (2s); MS, m/z: 306 (M11, 29%),
260 (M2NO2, 11), 230 (21), 216 (15), 201 (10), 181 (24), 171 (27), 156
(100), 128 (22), 113 (23); ESI-MS: 306 [M1H]1, 328 [M1Na]1; HRMS
(EI): Calcd for: C12H19NO8: 305.1111; Found: 305.1126.
Fig. 1. Selected examples of kainoid amino acids 1–4; L-glutamic acid 5;
(1)-CPAA 6.
PHM290 Radiometer, Copenhagen. Chiral high resolution gas liquid chro-
matography (GLC) analyses were run on a Shimadzu GC-14B instrument,
the capillary columns being ChiraldexTM type G-TA, g-cyclodextrin (40 m
3 0.25 mm; carrier gas Helium, 180 kPa, split 1:100) at 1508C isoth.; thin
layer chromatography (TLC)’s were performed on Polygram1 Sil G/UV254
silica gel precoated plastic sheets (eluent: light petroleum–ethyl acetate).
Flash chromatography was run on silica gel, 230–400 mesh ASTM (Kie-
selgel 60, Merck), using light petroleum 40–708C/ethyl acetate mixtures
as the eluent.
a-CT (53.1 U/mg) was purchased from Fluka. Ethyl nitroacetate,
diethyl pent-2-enedioate (diethyl glutaconate), and pent-2-dienoic acid
(glutaconic acid) were purchased from Sigma–Aldrich. Methyl nitroace-
tate 8 was prepared by reacting ethyl nitroacetate in methanol with tri-
methylchlorosilane following a literature procedure.54 Tetrahydrofuran
(THF) was distilled from Na-benzophenone.
n-Butyl
methyl
2-nitro-3-(n-Butoxycarbonyl)methylpentanedioate
1H NMR (400 MHz,
[9b]. Oil, IR (neat) 1738, 1734, 1564, 1367 cm21
;
CDCl3) d, ppm: 0.96 (t, 6H, 2 CH3CH2), 1.38 (sext, 4H, 2 CH2), 1.61
(quint, 4H, 2 CH2), 2.61 (m, 4H, 2 CH2CO2Bun), 3.30 (m, 1H, H-3), 3.78
(s, 3H, OCH3), 4.10 (2t, 4H, CH2O), 5.66 (d, J 5 5.6 Hz, 1H, CHNO2);
13C NMR (67.94 MHz, CDCl3) d, ppm: 13.5 (2q), 19.0 (2t), 30.4 (2t), 32.6
(d), 34.1 (2t), 53.5 (q), 65.0 (2t), 88.2 (d), 163.9 (2s), 171.0 (s); MS, m/z:
288 (10%), 232 (12), 202 (11), 185 (24), 169 (30), 156 (23), 142 (25), 128
(19), 113 (61), 105 (29), 86 (39), 59 (100), 41 (44), 30 (100). HRMS (EI):
Calcd for: C16H27NO8: 361.1737; Found: 361.1745.
Benzyl
methyl
2-nitro-3-(benzyloxycarbonyl)methylpentanedioate
1H NMR (400 MHz,
[9c]. Oil, IR (neat), 1738, 1734,1564, 1367 cm21
;
CDCl3) d, ppm: 2.68 (2m, 4 H, 2 CH2COOBn), 3.36 (m, 1H, H-3), 3.80 (s,
3H, CH3O), 5.13 (s, 4H, 2 CH2Ph), 5.66 (d, J 5 5.8 Hz, 1H, CHNO2),
7.36 (m, 10H, 2 Ph); 13C NMR (67.94 MHz, CDCl3) d, ppm: 32.5 (d),
33.0 (2t), 53.4 (q), 66.6 (2t), 88.05 (d), 128.0 (4d), 128.2 (6d), 135.2 (2s),
163.75 (2s), 170.6 (s); MS, m/z: 430 (M11, 1%), 383 (M2NO2, 1), 293
(8), 202 (10), 181 (12), 156 (82), 128 (66), 107 (90), 91 (100), 79 (28);
HRMS (EI): Calcd for: C22H23NO8: 429.1424; Found: 429.1413.
Syntheses
General procedure for synthesis of 7b,c. To a solution of (E)-
glutaconic acid (2.0 g, 15.4 mmol) in n-butanol (5 ml) and benzyl alcohol
(5 ml), trimethylchlorosilane (7.8 ml, 61.6 mmol) was added,54 and the
mixture was left on standing at room temperature (RT) overnight. The
alcohol was distilled off, and the residue was used without purification.
(E)-Di-n-butyl-pent-2-enedioate [7b]. Pale yellow oil, IR (neat) 1739,
1724, 1661 cm21
;
1H NMR (500 MHz, CDCl3) d, ppm 0.84 (t, 6 H, 2
General procedure for the synthesis of c-lactams 10a/11a–10c/
11c. To a solution of 9a–c (2 mmol) in ethyl acetate (15 ml), Raney1
2800 Nickel was added, and the mixture was hydrogenated at ambient
pressure until disappearance of the starting material (TLC, eluent ethyl
acetate). The catalyst was filtered off on a pad of celite, and the solvent
was evaporated and replaced with toluene. The mixture was refluxed to
assure a complete cyclization. After removal of the solvent in vacuo, the
residue was purified on column (eluent ethyl acetate/light petroleum,
gradient from 20 to 100%).
CH3), 1.30 (m, 4 H, 2 CH2), 1.52 (m, 4 H, 2 CH2), 3.13 (dd, J 5 6.9, 1.5
Hz, 2 H, 2 H-4), 4.04, 4.02 (2t, J 5 6.6 Hz, 2 H each, 2 CH2O), 5.84 (dt, J
5 15.6, 1.5 Hz, 1 H, H-2), 6.94, 6.88 (dt, J 5 15.6, 7.3 Hz, 1 H, H-3); 13C
NMR (67.94 MHz, CDCl3) d, ppm 13.5 (2q), 19.0 (t), 19.1 (t), 30.5 (t),
30.6 (t), 37.35 (t), 64.3 (t), 65.0 (t), 124.5 (d), 139.6 (d), 165.9 (s), 169.9
(s); ESI: 243.1 [M1H]1, 265 [M1Na]1; HRMS (electron impact ioniza-
tion [EI]): Calcd. for C13H22O4: 242.1518; Found: 242.1533.
(E)-Dibenzyl-pent-2-enedioate [7c]. Pale yellow oil, IR (neat) 3088,
3065, 3033, 1733, 1724, 1661, 1607, 1506, 1498, 1455, 742, 698 cm21
.
1H
Trans/cis-3-ethoxycarbonylmethyl-2-methoxycarbonyl-5-oxopyrrolidine
[10a/11a]. 9:1 ratio, 65% yield after chromatographic purification,
NMR (500 MHz, CDCl3) d, ppm: 3.29 (dd, J 5 7.4, 1.5 Hz, 2 H, H-4),
5.16, 5.20 (2s, 2 H each, 2 OCH2Ph), 6.00 (d, J 5 15.7 Hz, 1 H, H-2),
7.06, 7.10 (dd, J 5 15.7, 7.7 Hz, 1 H, H-3), 7.37 (m, 10 H, 2 Ph); 13C
pale yellow oil; IR (neat) 3350, 3244, 1736,1702 cm21 1H NMR, (500
;
MHz, CDCl3) resonances are given separately for the two isomers: 10a,
Scheme 1. An RSA showing the general strategy for the synthesis of (1)-CPAA 6. [Color figure can be viewed in the online issue, which is available at
wileyonlinelibrary.com.]
Chirality DOI 10.1002/chir

114
FELLUGA ET AL.
d, ppm: 1.29 (t, 3 H, CH3CH2O), 2.12 (dd, J 5 6.6, 17.2 Hz, 1 H, H-4),
2.54 (dd, J 5 8.3, 16.1 Hz, 1 H, CHHCO2Et), 2.67 (dd, J 5 8.8, 17.2 Hz, 1
H, H-4), 2.78 (dd, J 5 5.6, 16.1 Hz, 1 H, CHHCO2Et), 2.93 (m, 1 H, H-3),
3.70 (s, 3H, OCH3), 3.99 (d, J 5 5.5 Hz, 1 H, H-2), 4.25 (q, 2 H,
CH3CH2O), 6.21 (bs, 1H, NH); 11a (signals not hidden under those of
10a) d, ppm: 2.23 (dd, J 5 7.0, 17.0 Hz, 1 H, H-4), 2.44 (ddd, J 5 5.4,
8.2, 17.0, 2 H, CH2COOEt), 2.47 (dd, J 5 8.2, 17.0 Hz, 1 H, H-4), 3.19 (m,
1 H, H-3), 3.68 (s, 3 H, CH3O), 4.33 (d, J 5 7.1, 1 H, H-2); 13C NMR
(125.68 MHz, CDCl3) 10a, d, ppm: 13.8 (q), 34.8 (d), 35.6 (t), 37.9 (t),
51.5 (q), 60.0 (d), 61.3 (t), 171.0 (s), 171.3 (s), 176.6 (s); 11a (signals
not hidden under those of 10a) d, ppm: 34.4 (d), 34.7 (t), 58.3 (d), 170.6
(s), 171.2 (s), 177.5 (s); MS, m/z: 229 (M1., 5%), 197 (14), 183 (11), 170
(100), 156 (31), 155 (10), 142 (43), 141 (14), 96 (88), 41 (10). HRMS
(EI): Calcd for: C10H15NO5: 229.0950; Found: 229.0955.
ee), together with 5% of (6)-14. IR (neat) cm21: 3600–3200 (very
broad, multiple bands), 1725, 1652; 1H NMR (500 MHz, D2O)
resonances are given separately for the three compounds: 12a, d,
ppm: 1.21 (t, J 5 7.1 Hz, 3 H, CH3CH2O), 2.21 (dd, J 5 6.2, 17.0
Hz, 1 H, H-4), 2.52 (m, 1 H, CHHCO2Et), 2.70 (m, 1 H, H-4),
2.79 (dd, J 5 6.0, 16.0 Hz, 1 H, CHHCO2Et), 2.87 (m, 1 H, H-3),
4.11 (d, J 5 6.1, 1 H, H-2), 4.25 (q, J 5 7.1 Hz, 2 H, CH3CH2O);
(2)-13, d, ppm: 2.27 (dd, J 5 6.2, 17.5 Hz, 1 H, H-4), 2.72 (dd, J
5 8.0, 16.6 Hz, 1H, CHHCO2H), 2.76 (dd, J 5 9.0, 17.5 Hz, 1 H,
H-4), 2.85 (dd, J 5 6.2, 16.6 Hz, 1H, CHHCO2H), 2.94 (m, 1H, H-
3), 4.16 (d, J 5 5.5 Hz, 1 H, H-2); 14 (signals not hidden under
those of 12a and 13): d, ppm: 2.26 (dd, J 5 7.4, 17.0 Hz, 1 H,
H-4), 2.60 (m, 3 H, H-4 and CH2COOH), 3.26 (m, H-3), 4.44 (d, J
5 8.4 Hz, H-2); 13C NMR (67.94 MHz, D2O) 12a, d, ppm: 14.5
(q), 36.8 (d), 37.1 (t), 39.1 (t), 60.8 (d), 62.0 (t), 173.2 (s), 175.0
(s), 179.7 (s); (2)-13, d, ppm: 35.6 (d), 36.2 (t), 38.6 (t), 61.5 (d),
176.4 (s), 176.5 (s), 180.9 (s); (6)-14, d, ppm: 34.0 (d), 35.4 (t),
35.5 (t), 59.8 (d), 175.5 (s), 176.45 (s), 181.5 (s).
Trans/cis-3-n-butyloxycarbonylmethyl-2-methoxycarbonyl-5-oxopyrrolidine
[10b/11b]. 9:1 ratio, 60% yield, after chromatographic purification, pale
yellow oil, IR (neat) 3348, 3242, 1732, 1707 cm21 1H NMR (500 MHz,
;
CDCl3) resonances are given separately for the two isomers: 10b, d, ppm:
0.89 (t, 3 H, CH3CH2), 1.33 (m, 2 H, CH3CH2CH2), 1.57 (m, 2 H,
CH3CH2CH2), 2.07 (dd, J 5 6.6, 17.2 Hz, 1 H, H-4), 2.49 (dd, J 5 8.3, 16.1
Hz, 1 H, CHHCO2Bun), 2.62 (dd, J 5 8.8, 17.2 Hz, 1 H, H-4), 2.70 (dd, J 5
5.6, 16.1 Hz, 1 H, CHHCO2Bun), 2.86 (m, 1 H, H-3), 3.73 (s, 3 H, OCH3),
3.97 (d, J 5 5.5 Hz, 1 H, H-2), 4.05 (t, 2 H, OCH2), 7.15 (1H, bs, 1 H, NH);
11b (signals not hidden under those of 10b) d, ppm: 2.17 (dd, J 5 10.0,
17.2 Hz, 1 H, H-4), 2.41 (ddd, J 5 5.5, 8.1, 17.0 Hz, 2 H, CH2COOEt), 2.47
(dd, J 5 8.4, 17.2 Hz, 1 H, H-4), 3.12 (m, 1 H, H-3), 3.70 (s, 3 H, CH3O),
4.31 (d, J 5 7.1 Hz, 1H, H-2); 13C NMR (125.67 MHz, CDCl3) 10b, d,
ppm: 13.5 (q), 19.0 (t), 30.4 (t), 35.0 (d), 35.8 (t), 38.4 (t), 52.6 (q), 60.1
(d), 64.7 (t), 171.1 (s), 171.6 (s), 176.7 (s); 11b, d, ppm: d 14.0 (q), 22.5
(t), 29.5 (t), 34.7 (d), 52.2 (q), 58.5 (d), 177.65 (s); MS, m/z: 225 (5%), 198
(29), 183 (11), 156 (13), 142 (100), 141 (15), 96 (21), 57 (10), 41 (14).
HRMS (EI): Calcd for: C12H19NO5: 257.1263; Found: 257.1278.
Treatment of this sample with 2 N HCl for 24 h gave, after evapora-
tion, a crude from which (2)-13 crystallized (MeOH) as a hygroscopic
semisolid material. [a]2D3 217.6 (c 1.7, H2O), 65% ee.
(1)-(2S,3R)-3-(Ethoxycarbonyl)methyl-2-methoxycarbonyl-5-oxopyrroli-
dine [(1)-10a]. Compound (1)-10a with >99% ee was obtained by
stopping the enzymatic hydrolysis at ꢀ80% conversion, [a]2D3 132.2 (c
0.5, CHCl3).
(2)-(2R,3S)-3-(n-Butoxycarbonyl)methyl-5-oxopyrrolidine-2-carboxylic
acid [(2)-12b]. Compound 12b was obtained with 72% ee and 18%
yield by stopping the enzymatic hydrolysis at ꢀ20% conversion; [a]D23
26.5 (c 0.5, CH3OH); white solid, m.p. 130–28C; IR (nujol): 3230, 3100–
2600 (very broad), 1731, 1651 cm21 1H NMR (500 MHz, CD3OD) d,
;
ppm: 0.98 (t, 3 H, CH3CH2), 1.42 (sext, 2 H, CH3CH2CH2), 1.65 (quint, 2
H, CH3CH2CH2), 2.17 (dd, J 5 6.5, 17.2, 1 H, H-4), 2.60 (dd, J 5 8.1, 16.0
Hz, 1 H, CHHCO2Bun), 2.65 (dd, J 5 9.1, 17.2, 1 H, H-4), 2.76 (dd, J 5
5.8, 16.0 Hz, 1 H, CHHCO2Bun), 2.88 (m, 1 H, H-3), 4.03 (d, J 5 5.6 Hz,
1 H, H-2), 4.13 (t, J 5 6.7 Hz, 2 H, OCH2); 13C NMR (67.94 MHz,
CD3OD) d, ppm: 14.9 (q), 21.1 (t), 32.7 (t), 37.7 (d), 37.9 (t), 40.2 (t),
62.7 (d), 66.6 (t), 174.0 (s), 175.6 (s), 180.3 (s); MS, m/z: 198 (11%), 169
(31), 142 (100), 127 (31), 114 (17), 96 (41), 68 (10), 67 (18), 55 (11), 41
(36). HRMS (EI): Calcd for: C11H17NO5: 243.1107; found: 243.1110.
(1)-(2S,3R)-3-(n-Butoxycarbonyl)methyl-2-methoxycarbonyl-5-oxopyrroli-
dine [(1)-10b]. Compound (1)-10b was obtained by stopping the en-
zymatic hydrolysis at 80% conversion, 20% yield, > 99% ee, [a]2D3 125.0
(c 0.5, CHCl3).
Trans/cis-3-benzyloxycarbonylmethyl-2-methoxycarbonyl-5-oxopyrrolidine
[10c/11c]. 4:1 ratio, 60% yield, after chromatographic purification, semi-
solid material; IR (Nujol), 3219, 1748, 1711, 1669 cm21 1H NMR (500
;
MHz, CDCl3) resonances are given separately for the two isomers: 10c, d,
ppm: 2.14 (dd, J 5 6.4, 17.2 Hz, 1 H, H-4), 2.59 (dd, J 5 8.8, 16.1 Hz, 1 H,
CHHCO2Bn), 2.66 (dd, J 5 8.8, 17.2 Hz, 1 H, H-4), 2.81 (dd, J 5 5.9, 16.1
Hz, 1 H, CHHCO2Bn), 2.93 (m, 1 H, H-3), 3.75 (s, 3 H, CH3O), 4.00 (d, J 5
5.5 Hz, 1 H, H-2), 5.11 (apparent s, 2 H, CH2Ph), 6.47 (bd, 1 H, NH), 7.35
(m, 5 H, Ph); 11c (signals not hidden under those of 10c) d, ppm: 2.19
(dd, J 5 10.3, 16.8 Hz, 1 H, H-4), 2.47 (apparent d, J 5 7.5 Hz, 2 H,
CH2COOEt), 2.49 (dd, J 5 8.6, 16.8, Hz, 1 H, H-4), 3.20 (m, 1 H, H-3), 3.67
(s, 3H, CH3O), 4.35 (d, J 5 7.9 Hz, 1H, H-2); 13C NMR (125.67 MHz,
CDCl3) 10c, d, ppm: 34.9 (d, C-4), 35.6 (t), 38.3 (t), 52.7 (q, OCH3), 59.9 (d,
C-5), 66.7 (t, OCH2Ph), 128.4 (d) 128.6 (d), 128.7 (d), 135.5 (s), 171.0 (s),
171.6 (s), 176.6 (s); 11c, d, ppm: 34.1 (d), 35.0 (t), 38.3 (t), 52.3 (q), 58.4
(d), 66.7 (t), 128.4 (d), 128.6 (d), 128.7 (d), 135.5 (s), 170.9 (s), 171.3 (s),
176.7 (s); MS, m/z: 200 (13%), 182 (8), 154 (16), 91 (100), 69 (13), 57 (12),
41 (10). HRMS (EI): Calcd for: C15H17NO5: 291.1107; Found: 291.1115.
(2)-(2R,3S)-3-(Benzyloxycarbonyl)methyl-5-oxopyrrolidine-2-carboxylic
acid [(2)-12c]. Compound (2)-12c was obtained with 61% ee and
18% yield by stopping the enzymatic hydrolysis at ꢀ20% conversion,
white solid, m.p. 99–1008C; [a]2D3 27.3 (c 0.5, CH3OH) IR (nujol) 3300–
2600 (multiple broad bands), 1734, 1650 cm21 1H NMR (500 MHz,
;
CD3OD), d, ppm: 2.16 (dd, J 5 6.2, 17.0 Hz, 1 H, H-4), 2.60 (dd, J 5 8.7,
17.2 Hz, 1 H, CHHCO2Bn), 2.66 (dd, J 5 7.4, 17.0 Hz, 1 H, H-4), 2.86 (m,
2 H, CHHCO2Bn and H-3), 4.02 (d, J 5 5.1 Hz, 1 H, H-2), 5.17 (s, 2H,
CH2Ph), 7.38 (m, 5H, Ph); 13C NMR (67.94 MHz, CD3OD), d, ppm: 36.9
(d). 37.0 (t), 39.2 (t), 61.8 (d), 67.6 (t), 129.3 (d), 129.4 (d), 129.6 (d),
137.4 (s), 172.9 (s), 174.8 (s), 179.4 (s); MS, m/z: 277 (M1Á, 3%), 200
(13), 182 (8), 154 (16), 91 (100). HRMS (EI): Calcd for: C14H15NO5:
277.0950; found 277.0939.
(1)-(2S,3R)-3-(benzyloxycarbonyl)methyl-2-methoxycarbonyl-5-oxopyrroli-
dine [(1)-10c]. Compound 10c was obtained by stopping the enzymatic
hydrolysis at 80% conversion, 20% yield, 95% ee, [a]2D3 11.7 (c 1.5, CHCl3).
(1)-(2S,3R)-3-(methoxycarbonyl)methyl-2-methoxycarbonyl-5-oxopyrroli-
dine-2-carboxylate [(1)-15]. To compare the sign of optical rotations
with literature data, compounds (2)-12a–c were esterified at the car-
boxy group at C-2 and transesterified at the alkoxycarbonyl group at C-3
with methanol in the presence of TMSCl54 to give (2S,3R)-(1)-15; [a]2D3
132.0 (c 0.5, CHCl3); lit.47: [a]D23 132.9 (c 0.34, CHCl3). Other spectro-
scopic and analytical data were in accordance with the literature.47
General procedure for enzymatic hydrolyses. A suspension of
the appropriate diester (1.0 g) in phosphate buffer (pH 7.4; 20 ml) was
treated with a-CT (0.2 g) at RT under vigorous stirring. The pH was kept
at its initial value by automatic continuous addition of 1 M NaOH. At the
desired conversion value, the unreacted diester was extracted from the
suspension with ethyl acetate (10 ml, three times) using a centrifuge for
the separation of the layers. For the isolation of the carboxylic acids, the
aqueous layer was acidified to pH 2 with 2 N HCl, evaporated to dryness,
and the solid residue was triturated with MeOH.
The hydrolysis of 10c/11c (1 g, 3.5 mmol) with a-CT (200 mg) was
first run for 30 min. Extraction with ethyl acetate resulted in the recov-
ery of a 1:20 mixture of 10c/11c (780 mg), which was resubjected to
hydrolysis with 160 mg of a-CT under the conditions described.
(2)-(2R,3S)-3-(Ethoxycarbonyl)methyl-5-oxopyrrolidine-2-carboxylic acid
[(2)-12a], (2)-(2R,3S)-3-carboxymethyl-5-oxopyrrolidine-2-carboxylic acid
[(2)-13], and (6)-cis-3-carboxymethyl-5-oxopyrrolidine-2-carboxylic acid
[14]. Enzymatic hydrolysis of 10a/11a, stopped at 20% conver-
sion, gave a 1:2 mixture of (2)-12a (70% ee) and (2)-13 (63%
Transformation of 10a into (1)-(2S,3R)-CPAA 6. To a solution
of (2S,3R)-(1)-10a (0.46 g, 2.0 mmol), in 5 ml of dichloromethane, di-
Chirality DOI 10.1002/chir

FIRST CHEMOENZYMATIC SYNTHESIS OF CPAA
115
tert-butyldicarbonate (0.88 g, 4.0 mmol) DMAP (0.24 g, 2.0 mmol) and
triethylamine (0.28 ml, 2.0 mmol) were added, and the resulting solution
stirred at RT until disappearance of the substrate (TLC, ethyl acetate).
The reaction mixture was washed with 5% citric acid, 5% NaHCO3, and
brine, and the solvent was removed in vacuo. The oily residue was col-
umn chromatographed (light petroleum/ethyl acetate from 100 to 10%,
slow gradient) from which pure (2S,3R)-(1)-16a was isolated, 0.520 g,
80% yield.
(1)-(2S,3R)-1-tert-butoxycarbonyl-3-(ethoxycarbonyl)methyl-2-methoxyca-
rbonyl-5-oxopyrrolidine [(1)-16a]. [a]2D3 12.6 (c 0.6, CHCl3); IR (neat)
1
1795, 1750, 1730 cm21; H NMR (400 MHz, CDCl3) d, ppm: 1.23 (t, 3 H,
CH3CH2), 1.48 (s, 9 H, Boc), 2.29 (dd, J 5 3.8, 17.8 Hz, 1 H, H-4), 2.48
(dd, J 5 8.0, 17.3 Hz, 1 H, CHHCO2Et), 2.64 (dd, J 5 6.3, 17.3 Hz, 1 H,
CHHCO2Et), 2.67 (m, 1 H, H-3), 2.87 (dd, 1 H, J 5 8.9, 17.8 Hz, 1 H, H-
4), 3.79 (s, 3 H, CH3O), 4.11 (t, 2 H, CH2O), 4.31 (d, J 5 3.5 Hz, 1 H, H-
2). 13C NMR (67.94 MHz, CDCl3) d, ppm: 14.1 (q), 27.8 (3q), 31.1 (d),
37.4 (t), 38.6 (t), 52.8 (q), 61.8 (t), 64.0 (d), 84.0 (s), 149.2 (s), 170.4 (s),
170.5 (s), 172.0 (s); ESI-MS: 330.1 [M1H1].
Following the experimental procedure described,41,45 (1)-16a (0.520
g, 1.6mmol) was transformed into (1)-(2S,3R)-6 as its hydrochloride.
Optical rotation was measured at pH 3 for the comparison with lit.45
[a]2D3 118.0 (c 0.1, D2O); lit.45: [a]D23 119.0 (c 1, H2O).
Scheme 2. Synthesis of the racemic lactams as diastereomeric mixtures
10a–c/11a–c.
Scheme 3. Enzymatic hydrolysis of the diastereoisomeric lactams 10a–c/11a–c.
Chirality DOI 10.1002/chir

116
FELLUGA ET AL.
TABLE 1. Main results of the hydrolysis reactionsa of mixtures of 10a-c and 11a-c with a-CT
ca. 20% Conversionb
Acid [ee %]c,d (yield, %)e
ca. 80% Conversionb
Substrate (relative ratio)
E
Time
Time
Unreacted Ester [ee %]c (yield, %)e
f
10a 1 11a (9:1)
10b 1 11b (9:1)
10ch
–
10 min
2 h
8 h
(2)-13 [65] (16)g
(2)-12b [72] (15)
(2)-12c [61] (21)
30 min
8 h
36 h
(1)-10a [>99] (16)
(1)-10b [>99] (16)
(1)-10c [95] (22)
10
5
aReaction conditions: 1.0 g substrate, 0.5 g enzyme, 0.1 phosphate buffer, pH 7.4 (5 ml/mmol), RT.
bConversions calculated with respect to the trans diastereomers 10a–c.
cDetermined by chiral HRGC.
dAfter esterification of the carboxylic group.
eYields of isolated products.
fNot determined (see text).
gAfter 2 N HCl treatment of the crude extracts (see text).
hAfter preliminary hydrolysis with a-CT (see text).
hydrolysis products (2)-12a (70% ee) and (2)-13 (61% ee) in
a 1:2 ratio This prevented the calculation of the E (Enantio-
meric Ratio)55 value. Treatment of the mixture with 2 N HCl
converted (2)-12a into (2)-13, allowing the only trans diacid
(2)-13 to be precipitated as a pure isomer with 65% ee (16%
yield). The products (2)-12a and (2)-13 were accompanied
RESULTS AND DISCUSSION
Synthesis of the Racemic Substrates and Enzymatic
Resolution
The trans pyroglutamic acid derivatives 10a–c were pre-
pared in their racemic form by DBU mediated addition of
methyl nitroacetate 8 to the appropriate glutaconic diesters
7a–c. The reaction furnished the corresponding nitrotriest-
ers 9a–c. The nitro group of 9a–c was subsequently reduced
with hydrogen over Raney-Ni and spontaneously cyclized to
form the lactams. Unfortunately, this last step was not com-
pletely diastereoselective and led to trans/cis mixtures of
10a–c and 11a–c in the 9:1, 9:1, and 4:1 ratio, respectively
(Scheme 2).
As the trans and cis diastereomers of the pyrrolidinone die-
sters 10a–c and 11a–c were inseparable by flash chromatog-
raphy, each mixture was submitted to enzymatic resolution.
As a consequence, the enzyme of choice should have been
not only enantioselective but also diastereo- and regioselec-
tive, having to discriminate between trans and cis diastereom-
ers and between two different ester groups. After screening a
series of hydrolytic enzymes, a-CT proved the most selective,
leading to the products indicated in Scheme 3.
1
by a small amount (5%, by H NMR) of the cis diacid (6)-14
which was analyzed as its dimethyl diester (6)-15.
The reactions performed with the mixtures 10b/11b and
10c/11c were much cleaner, in as much as only the me-
thoxycarbonyl group at C-2 was hydrolyzed in both cases,
furnishing the corresponding trans monocarboxylic acids
(2)-12b with 72% ee and (2)-12c with 61% ee, respectively.
The reaction rates of the respective trans isomers resulted to
be significantly slower than with 10a, probably as a conse-
quence of the increasing steric encumbrance of the ester
function at C-3. Instead, hydrolysis of the cis isomers was
just as fast. This allowed an easy recovery of the products. In
the case of the mixture 10b/11b, the reaction was stopped
at 20% conversion, and therefore, the acidic fraction extracted
contained the trans hemiester (2)-12b and the cis diacid
(6)-14. On standing (2)-12b precipitated as crystalline
pure compound.
With the benzyl derivative mixture 10c/11c, the hydroly-
sis of the cis isomer 11c to the racemic dicarboxylic acid 14
was also extremely fast when compared to that of 10c.
Therefore, to get rid of the cis isomer, the aqueous hydroly-
sis mixture was extracted at pH 7.4 with ethyl acetate after
20 min. In that manner, most of 11c was removed, and the
remaining mixture 10c/11c (20:1 ratio and 78% yield) was
resubjected to hydrolysis with a-CT. At 20% conversion, the
recovered hemiester (2)-12c was crystallized and isolated
as a pure diastereomer with 61% ee.
As usual in kinetic enzymatic resolutions, the hydrolysis
reactions were performed at two different conversion values,
namely, about 20%, to separate the resulting acid and about
80%, to get the unreacted ester, both with the highest possi-
ble ee’s. The results are reported in Table 1.
At 20% conversion (Scheme 3 and Table 1), the trans iso-
mers 10a–c, which prevailed in their respective mixtures,
were transformed enantioselectively by the enzyme, but at a
lower rate than the cis isomers 11a–c, which were hydrolyzed
with no regio- and stereoselectivity. The only exception was
the case of the mixture 10a/11a (R 5 Et) where both dia-
stereoisomers reacted very rapidly, reaching 20% conversion
in 10 min. As depicted in Scheme 3, the hydrolysis of 10a
was not regioselective, with concomitant formation of two
When the hydrolysis reactions were allowed to proceed to
80% conversion, the unreacted diesters (1)-10a–c were iso-
lated with high enantiomeric excesses, >99% in the case of
(1)-10a,b and 95% for (1)-10c. As no traces of cis isomers
Scheme 4. Transformation of (1)-10a into (1)-6.
Chirality DOI 10.1002/chir

FIRST CHEMOENZYMATIC SYNTHESIS OF CPAA
117
3. Hollmann M, Heinemann S. Cloned glutamate receptors. Annu Rev Neu-
rosci 1994;17:31–108.
have been found, we think that they were completely hydro-
lyzed, and the products were removed under the basic
workup.
4. McGeer EG, Olney JW, McGeer PL. Kainic acid as a tool in neurobiol-
ogy. New York: Raven Press; 1983.
From all these results, it can be deduced that a-CT was
diastereoselective, transforming the cis and trans isomers at
different rates. As to the regioselectivity, a different behavior
of the enzyme toward the trans and cis diastereomers was
observed. In fact, the trans isomers always reacted with the
methoxycarbonyl group at C-2, while the cis isomers afforded
the diacid as racemates, as both ester groups were
hydrolyzed.
5. Taguchi T, Tomotoshi K, Mizumura K. Excitatory actions of mushroom
poison (acromelic acid) on unmyelinated muscular afferents in the rat.
Neurosci Lett 2009;456:69–73.
6. Clayden J, Read B, Hebditch KR. Chemistry of domoic acid, isodomoic
acids, and their analogs. Tetrahedron 2005;61:5713–5724.
7. Maeda M, Kodama T, Tanaka T, Yoshizumi H, Takemoto T, Nomoto K,
Fujita T. Structures of isodomoic acids A, B and C, novel insecticidal
amino acids from the red alga chondria armata. Chem Pharm Bull
1986;34:4892–4895.
The absolute configuration of the diesters (1)-10a–c was
assessed as (2S,3R) by transforming them into the corre-
sponding dimethyl diester (1)-15 (Scheme 3), whose con-
figuration and optical rotation value are known from the lit-
erature.47 Finally, the diester (2S,3R)-(1)-10a, having >
99% ee, was transformed into the target product (2S,3R)-
(1)-6 (Scheme 4). Protection of the amidic nitrogen with a
Boc group56 gave compound (1)-16a, which after selective
exhaustive lactam carbonyl reduction41,45 afforded the
desired product, having a positive optical rotation, in ac-
cordance with the data reported45 for the natural NMDA
agonist.
8. Watase H, Tomiie Y, Nitta I. Structure of kainic acid and its isomer, allo-
kainic acid I. Nature (London) 1958;181:761–762.
9. Meldrum B, Garthwaite J. Excitatory amino acid neurotoxicity and neuro-
degenerative disease. Trends Pharm Sci 1990;11:11379–11387.
10. Johnson RL, Koerner JF. Excitatory amino acid neurotransmission. J
Med Chem 1988;31:2057–2066.
11. Konno K, Hashimoto K, Ohfune Y, Shirahama H, Matsumoto T. Acro-
melic acids A and B. Potent neuroexcitatory amino acids isolated from
Clitocybe acromelalga. J Am Chem Soc 1988;110:4807–4815.
12. Konno K, Shirahama H, Matsumoto T. Isolation and structure of aromelic
acid A and B. New kainoids of Clitocybe acromelalga. Tetrahedron Lett
1983;24:939–942.
13. Wang Q, Yu S, Simonyi A, Sun GY, Sun AY. Kainic acid-mediated
excitotoxicity as a model for neurodegeneration. Mol Neurobiol 2005;31:3–16.
As to the other enzymes investigated on the same mix-
tures, Porcine pancreas lipase was also active, showing the
same stereopreference as a-CT, but with lower enantiose-
lectivity. Pig liver esterase hydrolyzed the trans isomers
10a–c with no enantioselectivity, while the cis isomers
11a–c were hydrolyzed with enantioselectivity but no
regioselectivity, giving, at about 20% conversion value, (2)-
14 with 61% ee. At 80% conversion value, the unreacted die-
sters 11a–c were isolated in > 99% ee. However, owing to
the small amount of the cis isomers in the parent mixtures,
this result does not seem very useful from the synthetic
point of view, and the product was only detected by HRGC
analysis. Further different enzymes (Lipases from Candida
antarctica A, C. antarctica B (Novozyme 435), Amano
Lipase PS from Burkholderia cepacia, Amano Lipase P12
from Aspergillus niger, Amano Acylase from Aspergillus mel-
leus, and Esterase from horse liver) were screened unsuc-
cessfully and given their low stereoselectivity or inactivity,
and they were disregarded.
14. Sperk G. Kainic acid seizures in the rat. Prog Neurobiol 1994;42:1–32.
15. Hashimoto K, Ohfune Y, Shirahama H. Synthesis of conformationally re-
stricted analogs of kainic acid. Is the conformation of the C4-substituent
of kainoid important to its neuroexcitatory activity? Tetrahedron Lett
1995;36:6235–6238.
16. Hansen JJ, Krogsgaard-Larsen P. Structural, conformational, and stereo-
chemical requirements of central excitatory amino acid receptors. Med
Res Rev 1990;10:55–94
17. Bunch L, Krogsgaard-Larsen P. Subtype selective kainic acid receptor
agonists: discovery and approaches to rational design. Med Res Rev
2009;29:3–28.
18. Sonnenberg JD, Koch HP, Willis CL, Bradbury F, Dauenhauer D,
Bridges RT, Chamberlin AR. The role of the C-4 side chain of kainate
and dihydrokainate in EAA receptor and transporter selectivity. Bioorg
Med Chem Lett 1996;6:1607–1612.
19. Tsai C, Schnider JA, Lehmann J. Trans-2-carboxy-3-pyrrolidineacetic acid
(CPAA),
a novel agonist at NMDA-type receptors. Neurosci Lett
1988;92:298–302.
20. Johnston GAR, Curtis DR, Davies J, McCulloch RM. Spinal interneurone
excitation by conformationally restricted analogues of L-glutamic acid. Na-
ture 1974;248:804–805.
21. Wood ME, Fryer AM. Synthesis of kainoids and kainoid analogs. Adv
Nitrogen Heterocycl 1998;3:159–218.
CONCLUSIONS
In conclusion, a-CT proved to be the most efficient
enzyme in the hydrolysis of the diastereomeric mixtures of
10a–c and 11a–c, furnishing the desired precursors of the
pyrrolidine amino acid (1)-CPAA 6. Different levels of selec-
tivity were exhibited when using this enzyme. It was regiose-
lective and enantioselective only toward the trans isomers
which reacted slower than the cis isomers, thus allowing sep-
aration of the cis product on workup. Although the enantio-
meric ratio was low for the three hydrolysis reactions,
the slowly reacting diesters were accessible in good enantio-
meric excesses at high conversion values and with the
correct stereochemistry required for the final target
compound, which was subsequently obtained by standard
transformations.
22. Kitamoto K, Sampei M, Nakayama Y, Sato T, Chida N. Novel sequential
sigmatropic rearrangements of allylic diols: application to the total syn-
thesis of (2)-kainic acid. Org Lett 2010;12:5756–5759.
23. Farwick A, Helmchen G. Enantioselective total synthesis of (2)-a-kainic
acid. Org Lett 2010;12:1108–1111.
24. Sakaguchi H, Tokuyama H, Fukuyama T. Total synthesis of (2)-kainic
acid via intramolecular Michael addition: a second-generation route. Org
Lett 2008;10:1711–1714.
25. Tomooka K, Akiyama T, Man P, Suzuki M. Asymmetric synthesis of (2)-
and (1)-kainic acid using a planar chiral amide as a chiral building block.
Tetrahedron Lett 2008;49:6327–6329.
26. Sakaguchi H, Tokuyama H, Fukuyama T. Stereocontrolled total synthesis
of (2)-kainic acid. Org Lett 2007;9:1635–1638.
27. Thuong MBT, Sottocornola S, Prestat G, Broggini G, Madec D, Poli G.
New access to kainic acid via intramolecular palladium-catalyzed allylic
alkylation. Synlett 2007;1521–1524.
28. Jung YC, Yoon CH, Turos E, Yoo KS, Jung KW. Total syntheses of (2)-
a-kainic acid and (1)-a-allokainic acid via stereoselective CÀÀH insertion
and efficient 3,4-stereocontrol. J Org Chem 2007;72:10114–10122.
LITERATURE CITED
1. Moloney MG. Excitatory amino acids. Nat Prod Rep 2002;19:597–616.
2. Parsons AF. Recent developments in kainoid amino acid chemistry.
Tetrahedron 1996;52:4149–4174.
29. Chalker JM, Yang A, Deng K, Cohen T. Two syntheses of (2)-kainic acid
via highly stereoselective zinc-ene cyclizations. Org Lett 2007;9:3825–3828.
Chirality DOI 10.1002/chir

118
FELLUGA ET AL.
30. Pandey SK, Orellana A, Greene AE, Poisson J-F. High-pressure Diels–
Alder approach to natural kainic acid. Org Lett 2006;8:5665–5668.
Synthesis of enantiopure a-allokainoids, b-kainoids, 2,3-methanoprolines
and other 3,4-disubstituted prolines. Tetrahedron: Asymmetry
1996;7:2613–2626.
31. Morita Y, Tokuyama H, Fukuyama T. Stereocontrolled total synthesis of
(2)-kainic acid. Regio- and stereoselective lithiation of pyrrolidine ring
with the (1)-sparteine surrogate. Org Lett 2005;7:4337–4340.
44. Conway GA, Park JS, Maggiora L, Mertes MP, Galton N, Michaelis EK.
Palladium(II)-catalyzed olefin-coupling reactions of kainic acid: effects of
substitution on the isopropenyl group on receptor binding. J Med Chem
1984;27:52–56.
32. Trost BM, Rudd MT. Ruthenium-catalyzed cycloisomerizations of diy-
nols. J Am Chem Soc 2005;127;4763–4776.
45. Oba M, Saegusa T, Nishiyama N, Nishiyama K. Synthesis of nonprotei-
nogenic amino acids using Michael addition to unsaturated orthopyroglu-
tamate derivative. Tetrahedron 2008;65:128–133.
33. Hodgson DM, Hachisu S, Andrews MD. Synthesis of a-kainic acid from
a 7-azabicyclo[2.2.1]heptadiene by tandem radical addition-homoallylic
radical rearrangement Org Lett 2005;7:815–817.
46. Karoyan P, Chassaing G. Short asymmetric synthesis of (2S,3S)- and
(2S,3R)-3-prolinoglutamic acids: 2-carboxy-3-pyrrolidineacetic acids
(CPAA). Tetrahedron Lett 2002;43:253–255.
34. Poisson JF, Orellana A, Greene AE. Stereocontrolled synthesis of (2)-
kainic acid from trans-4-hydroxy-L-proline. J Org Chem 2005;70:10860–
10863.
47. Bailey JH, Cherry DT, Dyer J, Moloney MG, Bamford MJ, Keeling S,
35. Martinez MM, Hoppe D. Novel approach to the (2)-sparteine-mediated
synthesis of kainoids: total synthesis of (2)-a-kainic acid by (2)-sparte-
ine-mediated deprotonation. Eur J Org Chem 2005;1427–1443.
Lamont RB. Pyrrolidinones derived from (S)-pyroglutamic acid. Part 1.
Conformationally constrained glutamate.
2000;2783–2792.
J Chem Soc Perkin 1
36. Anderson JC, O’Loughlin JMA, Tornos JA. Asymmetric aza-[2,3]-Wittig
sigmatropic rearrangements: chiral auxiliary control and formal asymmet-
ric synthesis of (2S,3R,4R)-4-hydroxy-3-methylproline and (2)-kainic
acid. Org Biomol Chem 2005;3:2741–2749.
48. Sato T, Matsubayashi K, Yamamoto K, Hishikawa H, Ishibashi K, Ikeda
M. Synthesis of a simple kainic acid analog by means of carbamoyl-
methyl radical cyclization. Heterocycles 1995;40:261–270.
49. Langlois N, Rojas A. Synthesis of (2S,4S)-2-carboxy-4-pyrrolidineacetic
acid, a conformationally constrained 2-amino adipic acid analog. Tetrahe-
dron Lett 1993;34:2477–2480.
37. Hodgson DM, Hachisu S, Andrews MD. Stereocontrolled syntheses of
kainoid amino acids from 7-azabicyclo[2.2.1]heptadienes using tandem
radical addition-homoallylic radical rearrangement.
2005;70:8866–8876.
J Org Chem
50. Langlois N, Andriamialisoa RZ. Diastereoselective synthesis of (2S,3R)-2-
carboxy-3-pyrrolidineacetic acid, a simple kainic acid analog. Tetrahedron
Lett 1991;32:3057–3058.
38. Scott ME, Lautens M. Total synthesis of (2)-(a)-kainic acid via a diaster-
eoselective methylenecyclopropane ring expansion. Org Lett
2005;7:3045–3047.
51. Faber K. Biotransformations in organic chemistry. Berlin: Springer; 2004.
39. Nakamura A, Lectard S, Hashizume D, Hamashima Y, Sodeoka Y. Dia-
stereo- and enantioselective conjugate addition of a-ketoesters to nitroal-
kenes catalyzed by a chiral Ni(OAc)2 complex under mild conditions.
J Am Chem Soc 2010;132:4036–4037.
52. Gillan T, Mor G, Pepper FW, Cohen SG. Preferred hydrolysis of methyl
D-pyroglutamates (5-carbomethoxy-2-pyrrolidones) by a-chymotrypsin.
Bioorg Chem 1977;6:329–340.
53. Bertoli A, Fanfoni L, Felluga F, Pitacco G, Valentin E. Chemoenzymatic
synthesis of optically active a-methylene-g-carboxy-g-lactams and g-lac-
tones. Tetrahedron: Asymmetry 2009;20:2305–2310.
40. Collado I, Ezquerra J, Mateo AI, Pedregal C, Rubio A. Stereocontrolled
synthesis of 5a- and 5b-substituted kainic acids.
J Org Chem
1999;64:4304–4314.
54. Brook MA, Chan TH. A simple procedure for the esterification of carbox-
ylic acids. Synthesis 1983:201–203.
41. Collado I, Ezquerra J, Mateo AI, Rubio A. Stereocontrolled synthesis of 4-
substituted (6)-kainic acids. J Org Chem 1998;63:1995–2001.
55. Chen CS, Fujimoto Y, Girdaukas G, Sih CJ. Quantitative analyses of bio-
42. Baldwin JE, Fryer AM. Prithchard GJ. Parallel synthesis of novel hetero-
chemical kinetic resolutions of enantiomers.
1982;104:7294–7299.
J Am Chem Soc
aromatic acromelic acid analogues from kainic acid.
2001;66:2588–2596.
J Org Chem
56. Felluga F, Gombac V, Pitacco G, Valentin E. A convenient chemoenzy-
matic synthesis of (R)-(2) and (S)-(1)-homo-b-proline. Tetrahedron:
Asymmetry 2004;15:3323–3327.
43. Ezquerra J, Escribano A, Rubio A, Remuin˜a´n MJ, Vaquero JJ. 4-Benzyl-
2,3-didehydroprolinate as a homochiral template for Michael additions.
Chirality DOI 10.1002/chir