A convenient chemoenzymatic synthesis of (R)-(-) and (S)-(+)-homo-β- proline

Article abstract of DOI:10.1016/j.tetasy.2004.08.036

Both enantiomers of the heterocyclic GABA analogue homo-β-proline (3-pyrrolidineacetic acid) were synthesized by a chemoenzymatic method involving the use of two enantiocomplementary enzymes in the disymmetric hydrolyses of 3-nitromethylglutaric acid diethyl ester.

Full text of DOI:10.1016/j.tetasy.2004.08.036

Tetrahedron:
Asymmetry
Tetrahedron: Asymmetry 15 (2004) 3323–3327
A convenient chemoenzymatic synthesis of (R)-(ꢀ) and
(S)-(+)-homo-b-proline
Fulvia Felluga,* Valentina Gombac, Giuliana Pitacco and Ennio Valentin
`
Dipartimento di Scienze Chimiche, Universita di Trieste, via Licio Giorgieri 1, I-34127Trieste, Italy
Received 24 August 2004; accepted 31 August 2004
Abstract—Both enantiomers of the heterocyclic GABA analogue homo-b-proline (3-pyrrolidineacetic acid) were synthesized by a
chemoenzymatic method involving the use of two enantiocomplementary enzymes in the disymmetric hydrolyses of 3-nitromethyl-
glutaric acid diethyl ester.
ꢀ 2004 Elsevier Ltd. All rights reserved.
1. Introduction
A few asymmetric syntheses of (R)- and (S)-homo-b-
proline have already been described, based either on
the use of (S)-(ꢀ)-1-phenylethylamine as a chiral auxil-
iary,4,5 or on the use of aspartic acid as the chiral start-
ing material,6 through procedures involving several
steps. Herein we report the chemoenzymatic synthesis
of both enantiomers of homo-b-proline in high enantio-
meric excesses.
GABA (c-aminobutyric acid)1 is a major inhibitory neu-
rotransmitter in the mammalian central nervous system
(CNS), as approximately 40% of synapses in the CNS
are GABAÕergic. Several neurological and psychiatric
disorders, such as anxiety, ParkinsonÕs disease, epilepsy
and some forms of schizofrenia, are correlated with a
disfunctioning of the GABA system.1c
Many unnatural c-amino acids have been designed for
their promising therapeutic use in the treatment of these
diseases, acting either as specific agonists at postsynaptic
GABAA receptors2 or as inhibitors of the GABA-up-
take mechanism.3
2. Results and discussion
The synthetic approach to the target compounds started
from diethyl 3-(nitromethyl)pentanedioate 2, which was
the product of a Michael addition of nitromethane to
diethyl glutaconate 1, catalyzed by 1,1,3,3-tetra-
methylguanidine (Scheme 1).7
(R) and (S)-Homo-b-proline [(R)- and (S)-3-pyrrolidine-
acetic acid] are conformationally restrained c-amino
acids, which bind to the GABA transport system in
the brain. They are known to act as potent inhibitors
of the neuronal and glial uptake mechanism of
GABA.3,4 They bind the GABA receptor sites2 with
an opposite stereochemistry, as the (R)-enantiomer has
a major affinity for the GABAA receptor, while the
(S)-enantiomer is more potent as an inhibitor of the
GABAB receptor. For this reason, the obtainment of
both antipodes of this cyclic GABA analogue is an inter-
esting synthetic target.
The racemic c-lactam 4, precursor of ( )-homo-b-pro-
line, was obtained in good yield from the intermediate
2 by a reduction/cyclization sequence carried out over
Raney nickel, through the intermediacy of the amino
diester 3, not isolated.8
The prochiral nitrodiester 2 is an ideal substrate for
enzymatic transformations. Desymmetrization of pro-
chiral compounds is known to constitute an efficient
route to single enantiomers in good yields (>50%) and,
therefore, this method is well recognized in biocatalysis
as a valuable tool for inducing asymmetry.9 In particu-
lar, many 3-substituted glutaric acid diesters have been
transformed by hydrolytic enzymes to the corresponding
*
Corresponding author. Tel.: +39 040 5583924; fax: +39 040
5583903; e-mail: felluga@dsch.univ.trieste.it
0957-4166/$ - see front matter ꢀ 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetasy.2004.08.036

3324
F. Felluga et al. / Tetrahedron: Asymmetry 15 (2004) 3323–3327
CO2Et
CO2Et
CO2Et
CO2Et
i
ii
CO2Et
NO2
CO2Et
NH2
[( )-3]
O
CO2Et
N
H
84%
95%
2
( )-4
1
Scheme 1. Reagents and conditions: (i): CH3NO2 (5equiv), 1,1,3,3-tetramethylguanidine (0.2equiv), 12h, rt; (ii) H2, RaNi, 1atm, 3h, rt.
optically active half esters with enantiotopic
specificity.10
obtained by coupling their carboxylic functions with
(S)-(ꢀ)-1-phenylethylamine, followed by integration of
the corresponding nitromethylenic proton signals, which
resonated at 4.25, 4.35 (each dd) and 4.17, 4.37 (each dd)
ppm, respectively.
During the screening of the possible enzymes able to dis-
criminate between the enantiotopic ester groups of com-
pound 2, we have found that crude pig liver esterase
(PLAP, pig liver acetone powder) hydrolyzed the pro-
(S) ester group of 2 (Scheme 2), to give monoester (S)-
(ꢀ)-5 with 99% ee and 82% yield in only 2h. When pure
pig liver esterase (PLE) was used, the same (S)-enantio-
mer was isolated, although with a slightly lower enantio-
meric excess (92%). These results are in accordance with
the already observed pro-(S) enantiopreference of pig
liver esterase in the hydrolysis of a series of 3-monosub-
stituted glutaric acid diesters.10h On the other hand,
when Porcine pancreatic lipase (PPL) was used in the
desymmetrization of diester 2, monoester (R)-(+)-5 with
98% ee was isolated, after 72h, in 79% yield. In neither
case, contrary to reported examples,10d was the 3-substi-
tuted pentanedioic acid detected as the product of
hydrolysis of both ester groups in 2.
H
N
H
N
S
R
S
S
Me
Me
CO2Et
NO2
CO2Et
NO2
Ph
O
Ph
O
(—)-10
(—)-11
Other enzymes were also checked, such as a-chymotryp-
sin, which hydrolyzed the substrate with no enantio-
selectivity, or lipases from Candida rugosa, Aspergillus
niger and Pseudomonas species, which proved com-
pletely inactive.
The conversion of the half esters (R)-(+)-5 and (S)-(ꢀ)-5
into the corresponding enantiomerically pure (S)-(ꢀ)-
and (R)-(+)-homo-b-proline, respectively, 9 was per-
formed within a few steps, as described in Scheme 2.
Reduction of the nitro group under an atmospheric
pressure of hydrogen, in the presence of Raney nickel
as catalyst, at room temperature for 3h, afforded the
corresponding amino substituted diesters, which were
not isolated. Heating the crude reaction mixture in an
ethanol/toluene solution (1:1) for 2h ensured the com-
plete lactamization of the reduction intermediates into
the desired lactams (R)-(+)-6 and (S)-(ꢀ)-6. Interest-
ingly, the cyclization reaction occurred exclusively at
The enantiomeric excesses of (R)-(+)-5 and (S)-(ꢀ)-5
were determined by 1H NMR (C6D6, 400MHz) analysis
of their diastereoisomeric amides (ꢀ)-10 and (ꢀ)-11,10f
CO2H
CO2H
ii
i
CO2Et
CO2Et
2
82%
79%
NO2
(R)-(+)-5
NO2
(S)-(—)-5
1
iii
91%
iii 91%
the ester function, as verified by the analysis of the H
NMR spectra of the crude reaction mixtures.
RO2C
RO2C
O
O
Removal of the lactamic carbonyl function was per-
formed on the N-Boc protected c-lactams (R)-(+)- and
(S)-(ꢀ)-7, obtained from the corresponding acidic lac-
tams (R)-(+)-6 and (S)-(ꢀ)-6 by esterification11 of the
carboxylic function and treatment with di-tert-butyl
dicarbonate.12 Thus, treatment of the N-protected c-lac-
tams (R)-(+)-7 and (S)-(ꢀ)-7 with BH3–DMS13 in THF
afforded the corresponding ethyl (R)-(ꢀ)- and (S)-(+)-3-
pyrrolidineacetate 8, after removal of the tert-butoxy-
carbonyl group. These latter compounds were eventu-
ally hydrolyzed to the target molecules (R)-(ꢀ)- and
(S)-(+)-homo-b-proline 9, obtained in approximately
50% overall yield starting from 2.
N
N
R1
R1
(S)-(—)-6: R = R1 = H
(S)-(—)-4: R = Et, R1 = H
(S)-(—)-7: R = Et, R1 = Boc
(R)-(+)-6: R = R1 = H
(R)-(+)-4: R = Et, R1 = H
(R)-(+)-7: R = Et, R1 = Boc
iv
iv
100% 100%
v
v
90%
90%
vi
70%
vi
70%
RO2C
RO2C
N
H
N
H
(R)-(—)-8: R = Et
(R)-(—)-9: R = H
(S)-(+)-8: R = Et
(S)-(+)-9: R = H
vii
vii
100%
100%
Scheme 2. Reagents and conditions: (i) PPL (1:1 w/w), pH7.4, rt, 3d;
(ii) PLAP (1:1 w/w), pH7, rt, 2h; (iii) Raney nickel, EtOH, 1atm, rt,
3h, then refluxing in 1:1 ethanol/toluene, 2h; (iv) EtOH, TMSCl, rt,
12h; (v) (BOC)2O (2equiv), DMAP, Et3N (1equiv), CH2Cl2 1h; (vi)
BH3–DMS (2equiv), dry THF, 0ꢁC; (vii) Amberlite 400 (OHꢀ), then
AcOH.
The change in the configurational descriptors in going from the linear
compounds (R)-(+)- and (S)-(ꢀ)-5 to the cyclic ones (S)-(ꢀ)- and (R)-
(+)-6 is due to a change in the order of priority of the groups
attached to the stereocentre.

F. Felluga et al. / Tetrahedron: Asymmetry 15 (2004) 3323–3327
3325
3. Conclusion
added to the reaction mixture and the solution heated
to a complete cyclization. After filtering on a pad of Cel-
ite, the solvents were evaporated in vacuo and the resi-
due chromatographed on a column (eluent: light
petroleum–ethyl acetate, from 4:1 to 1:1) to give c-lac-
tam 4 (0.65g, 95% yield). IR, cmꢀ1 (neat): 3055 (NH),
In conclusion, the synthetic pathway proposed offers an
easy and efficient route to both enantiomers of homo-b-
proline with high enantiomeric excess and satisfactory
yields.
1
1735 (CO2Et), 1693 (NHCO); H NMR, d, ppm: 6.14
(br s, 1H, NH), 4.13 (q, 2H, CH2O), 3.59 (dd, J = 8.0
and 9.7Hz 1H, H-5), 3.08 (dd, J = 6.2 and 9.7Hz, 1H,
H-5), 2.87 (sept, 1H, H-4), 2.51 (dd, J = 8.8 and 16.8
Hz, 1H, H-3), 2.47 (dd, J = 1.5 and 8.8Hz, 2H,
CH2CO2Et), 2.04 (dd, J = 7.3 and 16.8Hz, H-3), 1.24
(t, 3H, CH3); 13C NMR, d, ppm: 177.4 (s), 171.6 (s),
60.7 (t), 47.5 (t), 38.7 (t), 36.2 (t), 31.1 (d), 14.2 (q);
ESI-MS (m/z): 172.0 [M+H]+, 194.1 [M+Na]+, 209.9
[M+K]+. Anal. Calcd for C8H13NO3: C, 56.13; H,
7.65; N, 8.18. Found: C, 56.1; H, 7.6; N, 8.2.
4. Experimental
4.1. General
IR spectra were recorded on a Jasco FT-IR 200 spec-
1
trometer. H NMR and 13C NMR spectra were run on
a Jeol EX-400 (400MHz for proton, 100.1MHz for car-
bon), using deuterochloroform as a solvent and tetra-
methylsilane as the internal standard, unless otherwise
stated. Optical rotations were determined on a Perkin–
Elmer Model 241 polarimeter, at 25ꢁC. ESI-MS spectra
were obtained on a PE-API spectrometer at 5600V by
infusion of methanolic solutions. Enzymatic hydrolyses
were performed using a pH-stat Controller PHM290
Radiometer (Copenhagen). TLCÕs were performed on
Polygramꢂ Sil G/UV254 silica gel pre-coated plastic
sheets (eluent: light petroleum–ethyl acetate). Flash chro-
matography was run on silica gel, 230–400 mesh ASTM
(Kieselgel 60, Merck), using mixtures of light petroleum
40–70ꢁC and ethyl acetate as the eluent. CHN analyses
were run on a 1106 Carlo Erba Elemental Analyzer.
4.2. Enzymatic hydrolyses of compound 2
4.2.1. (R)-(+)- and (S)-(ꢀ)-3-(Nitromethyl)pentanedioic
acid monoethyl ester 5. A suspension of the diester 2
(1.0g, 4.0mmol) in phosphate buffer (pH7) (20mL)
was hydrolyzed with PPL (0.5g) at room temperature
under vigorous stirring. The pH was kept at its initial
value by automatic continuous addition of 1M NaOH.
After consumption of 1equiv of NaOH, the mixture
was centrifuged and extracted with diethyl ether
(2 · 10mL). The aqueous layer was acidified to pH2
with 2M HCl and extracted with chloroform
(5 · 10mL). The combined organic layers were dried
Raneyꢂ 2800 nickel, slurry in water, active catalyst, was
purchased from Aldrich. Porcine pancreatic lipase type
II (PPL), and Porcine liver acetone powder (PLAP) were
supplied by Sigma, while a-chymotrypsin (a-CT; 53.1U/
mg) and esterase from pig liver (PLE, 220U/mg) were
purchased from Fluka.
over Na2SO4 and evaporated to give (R)-(+)-5, oil,
25
D
0.70g, 79% yield, 98% ee; ½aŠ ¼ þ2:2 (c 1, MeOH);
IR cmꢀ1(neat): 3600–2800 (br, OH), 1785, 1731
1
(CO2H, CO2Et), 1553, 1378 (NO2); H NMR, d, ppm:
9.42 (br s, 1H, OH), 4.86 (apparent d, J = 5.9Hz, 2H,
CH2NO2), 4.13 (q, 4H, CH2O), 3.04 (quint, 1H, H-3),
2.61 (apparent d, J = 6.6Hz, 2H, 2 H-2 or 2 H-4), 2.54
(apparent d, J = 6.6Hz, 2H, 2 H-2 or 2 H-4), 1.25 (t,
3H, CH3); 13C NMR (100.4MHz): 176.6 (s), 170.9 (s),
77.3 (t), 61.1 (t), 35.2 (t), 35.0 (t), 30.3 (d), 14.0 (q);
ESI-MS (m/z): 218.0 [MꢀH].+ Anal. Calcd for
C8H13NO6: C, 43.84; H, 5.98; N, 6.39. Found: C, 43.9;
H, 6.0; N, 6.35.
4.1.1. Synthesis of diethyl 3-(nitromethyl)pentanedioate
2. A mixture of nitromethane (8.2g, 135mmol),
diethyl 2-pentenedioate 1 (5.0g, 27mmol) and 1,1,3,3-
tetramethylguanidine7 (0.625g, 5.3mmol) was stirred
at room temperature for 24h. Excess nitromethane
was removed in vacuo, the residue dissolved in diethyl
ether and washed with 5% HCl and the organic phase
dried over Na2SO4 and chromatographed on column
(eluent: light petroleum–ethyl acetate, 9:1) to give pure
2 as a pale yellow oil, 5.5g, 84% yield. IR, cmꢀ1 (neat):
1728 (CO2Et), 1545 and 1382 (NO2); 1H NMR, d, ppm:
4.62 (d, J = 6.3Hz, 2H, CH2NO2), 4.14 (q, 4H, 2
CH2O), 3.05 (quint, 1H, H-3), 2.51 (d, 4H, J = 6.8Hz,
2 H-2 and 2 H-4), 1.25 (t, 6H, 2 CH3); 13C NMR,
d, ppm: 170.4 (s), 77.2 (t), 60.4 (t), 34.8 (t), 30.1 (d),
13.6 (q); ESI-MS (m/z): 248.1 [M+H]+, 270.0
[M+Na]+, 286.0 [M+K]+. Anal. Calcd for C10H17NO6:
C, 48.58; H, 6.93; N, 5.67. Found: C, 48.7; H, 6.9; N,
5.6.
Under the same conditions as above, diester 2 (1.0g,
4.0mmol) was hydrolyzed with PLAP (0.5g) in phos-
phate buffer (pH7) (20mL) to give (S)-(ꢀ)-5, oil,
25
0.72g, 82% yield, 99% ee; ½aŠ ¼ ꢀ2:2 (c 1, MeOH).
D
The same compound (S)-(ꢀ)-5 with a 92% ee was ob-
tained by running the hydrolysis of the diester 2 (1.0g,
4.0mmol) with PLE (10mg, 400U/mmol) in 20mL
phosphate buffer (pH7), under the conditions described
25
D
above; ½aŠ ¼ ꢀ2:0 (c 1, MeOH).
4.1.2. Ethyl ( )-2-oxo-4-pyrrolidineacetate 4. To a
solution of compound 2 (1.0g, 4.0mmol) in ethanol
(15mL), Raney nickel was added and the mixture
hydrogenated at atmospheric pressure until disappear-
ance of the starting material (TLC, eluent: light petro-
leum–ethyl acetate, 4:1). Toluene (10mL) was then
4.3. Determination of the enantiomeric excess of
(R)-(+)-5 and (S)-(ꢀ)-5
4.3.1. Ethyl (10S,3R)-(ꢀ)- and (10S,3S)-(ꢀ)-3-(nitro-
methyl)-4-(N-1-phenylethylcarbamoyl)butanoate 10 and
11. To a solution of the half ester (R)-(+)-5 (0.05g,

3326
F. Felluga et al. / Tetrahedron: Asymmetry 15 (2004) 3323–3327
25
D
0.2mmol) in 5mL of CH2Cl2 with 1 drop of DMF
added, oxalyl chloride (20lL, 0.2mmol) was added.
The solution was stirred at room temperature for 2h
and then the solvents removed in vacuo, after which
(S)-(+)-1-phenylethylamine (0.05g, 0.4mmol) was
added at once. After 10min of stirring, ether was added
and the solution washed with 1M HCl. After drying the
organic phase, the solvent was evaporated to give the
(+)-6, a semisolid material, 0.60g, 91% yield. ½aŠ
¼
þ2:9 (c 0.5, MeOH); IR, cmꢀ1 (Nujol): 3600–2600
(broad band, OH), 1726 (CO2H), 1674 (NHCO); 1H
NMR (CD3OD), d, ppm: 3.59 (dd, J = 8.0 and 9.7Hz,
1H, H-5), 3.08 (dd, J = 6.2 and 9.7Hz, 1H, H-5), 2.87
(sept, 1H, H-4), 2.51 (dd, J = 8.8 and 16.8Hz, 1H, H-
3), 2.47 (dd, J = 1.5 and 8.8Hz, 2H, CH2CO2H), 2.04
(dd, J = 7.3 and 16.8Hz, H-3); 13C NMR (CD3OD), d,
ppm: 175.9 (s), 173.5 (s), 43.5 (t), 36.5 (t), 36.4 (t), 31.0
(d); ESI-MS (m/z): 144.0 [M+H]+, 166.1 [M+Na].+
Anal. Calcd for C6H9NO3: C, 50.35; H, 6.34; N, 9.79.
Found: C, 50.35; H, 6.4; N, 9.8.
corresponding amide (ꢀ)-10 in quantitative yield. Mp
25
D
112–114ꢁC. ½aŠ ¼ ꢀ24:9 (c 0.22, CHCl3); IR, cmꢀ1
(Nujol): 3310 (NH), 3066 (Ph), 1720 (CO2Et), 1640
(CONH), 1546 (NO2), 1496 (Ph), 1378 (NO2); 1H
NMR (C6D6), d, ppm: 7.24 (m, 5H, ArH), 5.27 (quint,
1H, CHPh), 5.13 (br s, 1H, NH), 4.35 dd, J = 5.9 and
12.4Hz, 1H, CHNO2), 4.25 dd, J = 6.2 and 12.4Hz,
1H, CHNO2), 4.00 (q, 2H, CH2O), 3.06 (quint, 1H, H-
3), 2.38 (part ABof an ABX system, J = 6.2, 7.0 and
16.8Hz, 2H, 2 H-2), 1.94 (part ABof an ABX system,
J = 6.6, 7.0 and 15.4Hz, 2H, 2 H-4), 1.30 (d, 3H,
J = 7.0Hz, CH3CHPh), 1.05 (t, 3H, CH3); 13C NMR
(C6D6), d, ppm: 171.3 (s), 168.9 (s), 144.1 (s), 128.3
(2d), 128.0 (d), 127.6 (2d), 77.9 (t), 60.6 (t), 48.9 (d),
36.8 (t), 35.1 (t), 31.4 (d), 21.8 (q), 14.0 (q); ESI-MS
(m/z): 339.1 [M+H]+, 362.2 [M+Na]+, 378.6 [M+K]+.
Anal. Calcd for C16H22N2O6: C, 56.80; H, 6.55; N,
8.28. Found: C, 56.9; H, 6.45; N, 8.3.
The opposite enantiomer (S)-(ꢀ)-6 was obtained under the
25
same conditions from (R)-(+)-5. ½aŠ ¼ ꢀ2:8 (c 1, MeOH).
D
4.4.2. Ethyl (R)-(+)-2-oxo-4-pyrrolidineacetate 4. The
acidic lactam (R)-(+)-6 (0.61g, 4.1mmol) was dissolved
in EtOH, and trimethylchlorosilane11 (1.0g, 8.2mmol)
was added. The mixture was left at room temperature
overnight, after which evaporation of the solvent to dry-
ness gave ethyl (R)-(+)-2-oxo-4-pyrrolidineacetate 4 in
25
D
a quantitative yield, ½aŠ ¼ þ2:8 (c 1, MeOH).
The opposite enantiomer (S)-(ꢀ)-4 was obtained from
25
(S)-(ꢀ)-6, ½aŠ ¼ ꢀ2:8 (c 1, MeOH).
D
The same reaction was carried out on the opposite enan-
tiomer (S)-(ꢀ)-5 to give the corresponding amide (ꢀ)-11.
4.4.3. Ethyl (R)-(+)-2-oxo-1-tert-butoxycarbonyl-4-pyrr-
olidineacetate 7. To a solution of (R)-(+)-5 (0.400g,
2.3mmol), having 99% ee, in 5mL of dichloromethane,
di-tert-butyldicarbonate (0.64g, 4.8mmol), DMAP
(0.16g, 2.4mmol) and triethylamine (0.20mL, 2.4mmol)
were added12 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 removed in vacuo
to give 0.58g (90% yield) of pure ethyl (+)-2-oxo-1-
25
D
Mp 103–105ꢁC. ½aŠ ¼ ꢀ54 (c 1, CHCl3); IR, cmꢀ1
(Nujol): 3308 (NH), 3066 (Ph), 1725 (CO2Et), 1639
(CONH), 1546 (NO2), 1498 (Ph), 1378 (NO2); 1H
NMR (C6D6), d, ppm: 7.24 (m, 5H, ArH), 5.43 (br s,
1H, NH), 5.25 (quint, 1H, CHPh), 4.37 (dd, J = 5.8
and 12.4Hz, 1H, CHNO2), 4.17 (dd, J = 5.5 and
12.4Hz, 1H, CHNO2), 3.98 (q, 2H, CH2O), 3.05 (quint,
J = 6.6Hz, 1H, H-3), 2.39 (apparent d, J = 6.6Hz, 2H,
CH2CO2Et), 1.98 (2 pseudoq, part ABof an ABX sys-
tem, JAB = 15.4Hz, 2H, CH2CONH), 1.27 (d, 3H,
J = 7.0Hz, CH3CHPh), 1.03 (t, 3H, CH3); 13C NMR
(C6D6), d, ppm: 171.3 (s), 168.9 (s), 144.1 (s), 128.3
(2d), 128.0 (d), 127.6 (2d), 77.9 (t), 60.6 (t), 48.9 (d),
36.8 (t), 35.1 (t), 31.4 (d), 21.8 (q), 14.0 (q). ESI-MS
(m/z): 339.1 [M+H]+, 362.0 [M+Na]+, 378.5 [M+K]+.
Anal. Calcd for C16H22N2O6: C, 56.80; H, 6.55; N,
8.28. Found: C, 56.8; H, 6.4; N, 8.3.
tert-butoxycarbonyl-4-pyrrolidineacetate
½aŠ ¼ þ2:7 (c 1.1, MeOH); IR (film): 1788, 1742
(R)-(+)-7.
25
D
(NCO2But, CO2Et), 1716 (NCO); 1H NMR, d, ppm:
4.06 (q, 2H, CH2O), 3.88 (dd, J = 7.9 and 10.9Hz, 1H,
H-5), 3.32 (dd, J = 6.6 and 10.9Hz, 1H, H-5), 2.64 (m,
1H), 2.58 (m, 1H), 2.39 (d, J = 6.6Hz, CH2CO2Et),
2.23 (m, 1H), 1.17 (t, 3H, CH3); 13C NMR, d, ppm:
172.6 (s), 170.9 (s), 149.6 (s), 82.6 (s), 60.5 (t), 51.1 (t),
38.6 (t), 37.8 (t), 27.7 (3q), 26.9 (d), 13.9 (q); ESI-MS
(m/z): 272.1 [M+H]+, 294.0 [M+Na]+, 310.0 [M+K]+.
Anal. Calcd for C13H21NO5: C, 57.55; H, 7.80; N,
5.16. Found: C, 57.5; H, 7.8; N, 5.2.
4.4. Transformation of (S)-(ꢀ)-5 into (R)-(ꢀ)-3-pyrrol-
idineacetic acid 9
The opposite enantiomer (S)-(ꢀ)-7 was obtained from
25
(S)-(ꢀ)-5, ½aŠ ¼ ꢀ2:6 (c 0.8, MeOH).
4.4.1. (R)-(+)-2-Oxo-4-pyrrolidineacetic acid 6. To a
solution of (S)-(ꢀ)-5 (1.0g, 4.6mmol) in ethanol
(15mL), Raney nickel was added and the mixture
hydrogenated at atmospheric pressure until disappear-
ance of the starting material (TLC, eluent: ethyl ace-
tate). The mixture was then added with toluene
(10mL) and heated to obtain a complete cyclization.
After removal of the solvents in vacuo, the residue was
dissolved in 5% NaHCO3 and extracted with diethyl
ether (3·). The aqueous layer was acidified to pH3,
evaporated to dryness to give a residue which, on tritu-
ration with methanol, afforded the acidic lactam (R)-
D
4.4.4. Ethyl (R)-(ꢀ)-3-pyrrolidineacetate 8. To a solu-
tion of (R)-(+)-7 (0.4g, 1.5mmol), having 99% ee, in
dry THF (20mL), 1.5mL of a 2M THF solution of
BH3–DMS (3mmol)13 were slowly added at ꢀ10ꢁC,
while stirring under Ar. After the addition was com-
plete, the temperature was left to rise to room tempera-
ture and the mixture stirred until disappearance of the
starting material (TLC, eluent: light petroleum/ethyl
acetate, 1:1). Ethanol was then added (10mL) with
a few drops of 1M HCl after which the solution was

F. Felluga et al. / Tetrahedron: Asymmetry 15 (2004) 3323–3327
3327
Pharmacological Aspects; Krogsgaard-Larsen, P., Scheel-
Krueger, J., Kofod, H., Eds.; Munksgaard: Copenhagen,
1979.
refluxed for 1h. The solvents were removed in vacuo, the
residue dissolved in 5% NaHCO3 and the solution
extracted with ethyl acetate to give (R)-(ꢀ)-8 (0.16g,
25
D
70% yield); ½aŠ ¼ ꢀ14 (c 0.25, MeOH). 1H NMR
2. (a) Krogsgaard-Larsen, P.; Frolund, B.; Ebert, B.
The GABA Receptors, 2nd ed.; Humana: Totowa, New
Jersey, 1997, pp 37–81; (b) Bowery, N. G.; Hudson, A. L.;
Price, G. W. Neuroscience 1987, 20, 365–383; (c) The
GABA Receptors; Enna, S. J., Ed.; Humana: Clifton, NJ,
1983; (d) Hill, D. R.; Bowery, N. G. Nature 1981, 290,
149–152.
3. (a) Krogsgaard-Larsen, P.; Hjeds, H.; Falch, E.; Jorgen-
sen, F. S.; Nielsen, L. Adv. Drug Res. 1988, 17, 381–456;
(b) Krogsgaard-Larsen, P.; Falch, E.; Larsson, O. M.;
Schousboe, A. Epilepsy Res. 1987, 1, 77–93; (c) Krogs-
gaard-Larsen, P.; Falch, E.; Hjeds, H. Prog. Med. Chem.
1985, 22, 67–120.
(D2O), d, ppm: 4.23 (q, 2H, CH2O), 3.64 (dd, J = 7.7
and 11.7Hz, 1H, H-2), 3.48 (m, 1H, H-5), 3.32 (m,
1H, H-5), 3.03 (dd, J = 8.8 and 11.7Hz, 1H, H-2), 2.75
(m, 1H, H-3), 2.65 (part ABof an AXB system,
J = 6.6, 8.4 and 16.5Hz, 2H, CH2CO2Et), 2.30 (m, 1H,
H-4), 1.72 (m, 1H, H-4), 1.31 (t, 3H, CH3); 13C NMR
(D2O), d, ppm: 172.9 (s), 61.4 (t), 50.5 (t), 45.8 (t),
37.2 (t), 35.1 (d), 30.7 (t), 14.3 (q). ESI-MS (m/z):
157.9 [M]+, 159.1 [M+H]+. Anal. Calcd for
C8H15NO2: C, 61.12; H, 9.62; N, 8.91. Found: C, 61.1;
H, 9.5; N, 8.9.
4. Nielsen, L.; Brehm, L.; Krogsgaard-Larsen, P. J. Med.
Chem. 1990, 33, 71–77.
The opposite enantiomer (S)-(+)-8 was obtained from
5. (a) Galeazzi, R.; Geremia, S.; Mobbili, G.; Orena, M.
Tetrahedron: Asymmetry 1996, 7, 79–88; (b) Galeazzi, R.;
Mobbili, G.; Orena, M. Tetrahedron 1996, 52, 1069–1084;
(c) Cardillo, B.; Galeazzi, R.; Mobbili, G.; Orena, M.
Synlett 1995, 11, 1159–1160.
6. Thomas, C.; Orecher, F.; Gmeiner, P. Synthesis 1998,
1491–1496.
7. Pollini, G. P.; Barco, A.; De Giuli, G. Synthesis 1972, 9,
44–45.
25
D
(S)-(ꢀ)-5, ½aŠ ¼ þ13:2 (c 1, MeOH).
4.4.5. (R)-(ꢀ)-3-Pyrrolidineacetic acid 9. Ester (R)-(ꢀ)-
8 was treated as described in the literature,5a on Amber-
lite IRA 400 (OHꢀ form) and eluted with an equivalent
amount of aqueous acetic acid (6%), to quantitatively
25
D
give (R)-(ꢀ)-3-pyrrolidineacetic acid 9 ½aŠ ¼ ꢀ9:1 (c
2, H2O) [lit.4 ꢀ9.3 (c 1, H2O); lit.5a ꢀ9.1 (c 1, H2O),
8. Seebach, D.; Brenner, M. Helv. Chim. Acta 1999, 82,
2365–2379.
1
lit.6 ꢀ10.1 (c 1, H2O)]. H NMR (D2O), d, ppm: 3.62
(dd, J = 7.7 and 11.7Hz, 1H, H-2), 3.48 (m, 1H, H-5),
3.34 (m, 1H, H-5), 3.00 (dd, J = 8.8 and 11.7Hz, 1H,
H-2), 2.76 (m, 1H, H-3), 2.64 (m, 2H, CH2CO2Et),
2.30 (m, 1H, H-4), 1.74 (m, 1H, H-4). 13C NMR
(D2O), d, ppm: 178.6 (s), 50.5 (t), 46.0 (t), 38.9 (t),
35.1 (d), 30.4 (t). ESI-MS: 131.0 [M+], 144.1 [M+H+],
162.0 [M+K+].
9. (a) Davies, B. G.; Bojer, V. Nat. Prod. Rep. 2001, 18, 618–
640; (b) Faber, K. Biotransformations in Organic Chemis-
try, 4th ed.; Springer-Verlag: Berlin, 2000; (c) Schoffers,
E.; Golebiowski, A.; Johnson, C. R. Tetrahedron 1996, 52,
3769–3826; (d) Haraldsson, G. G. The Application of
Lipases in Organic Synthesis. In The Chemistry of Func-
tional Groups, Supplement B: The Chemistry of Acid
Derivatives; Patai, S., Ed.; John Wiley and Sons: Chich-
ester, 1992; Vol. 2.
10. (a) Moen, A. R.; Hoff, B. H.; Hansen, L. K.; Aanthonsen,
T.; Jacobsen, E. E. Tetrahedron: Asymmetry 2004, 15,
1551–1554; (b) Chenevert, R.; Tchedam, N.; Yannick, S.
R.; Goupil, D. Tetrahedron: Asymmetry 1998, 9, 4325–
4329; (c) Loubinoux, B.; Sinnes, J.-L.; OÕSullivan, A. C.;
Winkler, T. Tetrahedron 1995, 51, 3549–3558; (d) Huges,
D. L.; Bergan, J. J.; Amato, J. S.; Bhupathy, M.; Leazer, J.
L.; Mc Namara, J. M.; Sidler, D. R.; Reider, P. J.;
Grabowski, E. J. Org. Chem. 1990, 55, 6252–6259; (e)
Lam, L. K.-P.; Jones, B. Can. J. Chem. 1988, 66, 1422–
1424; (f) Santaniello, E.; Chiari, M.; Ferraboschi, P.;
Trave, S. J. Org. Chem. 1988, 53, 1567–1569; (g) Lam, L.
K.-P.; Hui, A. H. F.; Jones, B. J. Org. Chem. 1986, 51,
2047–2050; (h) Francis, C. J.; Jones, B. J. Chem. Soc.,
Chem. Commun. 1984, 9, 579–580.
Enantiomer (S)-(+)-3-pyrrolidineacetic acid
9 had
25
D
½aŠ ¼ þ9:3 (c 2, H2O) [lit.4 +9.6 (c 1, H2O), lit.5a
+9.2 (c 1, H2O); lit.6 +8.6 (c 1, H2O)].
Acknowledgements
Financial support from MIUR (Rome, PRIN 2001–
2003) is gratefully acknowledged.
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