Enzymic resolution of an α-methylene-γ-lactonic ester

Article abstract of DOI:10.1016/S0957-4166(00)00309-8

The possibility of using enzymes for the kinetic resolution of an α-methylene-γ-lactone was explored. The probe molecule, ethyl 2-methyl-4-methylene-tetrahydro-5-oxo-2-furancarboxylate was successfully resolved with Porcine pancreatic lipase and Candida rugosa lipase, in the presence of a cosolvent, leading to both enantiomers of the corresponding acid with high enantiomeric excesses. Copyright (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0957-4166(00)00309-8

TETRAHEDRON:
ASYMMETRY
Pergamon
Tetrahedron: Asymmetry 11 (2000) 3263–3267
Enzymic resolution of an a-methylene-g-lactonic ester
Giuliana Pitacco, Andrea Sessanta o Santi and Ennio Valentin*
Dipartimento di Scienze Chimiche, Universita` di Trieste, via Licio Giorgieri 1, I-34127 Trieste, Italy
Received 12 July 2000; accepted 8 August 2000
Abstract
The possibility of using enzymes for the kinetic resolution of an a-methylene-g-lactone was explored.
The probe molecule, ethyl 2-methyl-4-methylene-tetrahydro-5-oxo-2-furancarboxylate was successfully
resolved with Porcine pancreatic lipase and Candida rugosa lipase, in the presence of a cosolvent, leading
to both enantiomers of the corresponding acid with high enantiomeric excesses. © 2000 Elsevier Science
Ltd. All rights reserved.
The biological activity of natural products containing the a-methylene-g-lactone ring as
immunomodulators, antibiotics, antitumorals, antifungals, plant growth inhibitors and agents
for vasorelaxing activity is related to the presence of the enone system, whose b-carbon atom is
prone to undergo attack by a biological nucleophile belonging to an interacting enzyme.1–9
Many syntheses of these types of compounds have been described.10 However, the use of
enzymes for their kinetic resolution is, to our knowledge, so far unexplored. Therefore it seemed
interesting to verify the possibility of operating the resolution of chiral racemic g-lactones
containing the conjugated a-methylene function as well as an alkoxycarbonyl group as the site
of reaction by hydrolytic enzymes. The substrate chosen for initial studies was the ethyl ester of
2-methyl-4-methylene-tetrahydro-5-oxo-2-furancarboxylic acid 3,11 (Scheme 1) in which the
methylene group is sterically unaffected by the other substituents. The substrate was easily
prepared by a Reformatsky-type reaction between 2-(bromomethyl)acrylic acid 1 and pyruvic
acid ethyl ester, promoted by indium.12–15 The initially formed hydroxy hemiester ( )-2 was
isolated and cyclised into the desired butanolide ( )-3 under acidic conditions.
* Corresponding author. Tel: +39-040-6763917; fax: +39-040-6763903; e-mail: valentin@dsch.univ.trieste.it
0957-4166/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved.
PII: S0957-4166(00)00309-8

3264
G. Pitacco et al. / Tetrahedron: Asymmetry 11 (2000) 3263–3267
Scheme 1.
The enzymic hydrolyses of ( )-3 were carried out in aqueous solution and in a 1:9 mixture of
acetone:water,16,17 both at pH 7.4, using a series of commercially available hydrolases. The most
relevant results are summarized in Table 1.
Addition of a small amount (10%) of acetone resulted in a remarkable increase of the E18,19
value and hence in a higher enantioselectivity. In fact, using Porcine pancreatic lipase, the
corresponding lactonic acid (+)-4 was isolated with 89% e.e., whereas the use of Candida rugosa
lipase furnished the laevorotatory enantiomer (−)-4 with 82% e.e. A second enzymic resolution
carried out on these mixtures improved the e.e. of (+)-4† to 97% and that of (−)-4‡ to 99% e.e.
The e.e’s of the acids were determined by chiral HRGC of their respective ethyl esters (+)-3 and
(−)-3, obtained by esterification of the acids with EDC and HOBT. a-Chymotrypsin also proved
efficient in hydrolysing the ester group, furnishing the acid (+)-4 with 77% e.e., while Aspergillus
niger lipase, Pseudomonas sp. lipase and pig liver esterase were not enantioselective and
Pseudomonas fluorescens lipase furnished (+)-4 with only 29% e.e.
The absolute configuration of (+)-4 was determined to be S by means of CD spectroscopy.
The ethyl ester (+)-3 was reduced with sodium borohydride to a 1:1 mixture of the butanolides
(+)-7 and (+)-8 through the intermediacy of the corresponding lactonic esters (−)-5 and (+)-6,
respectively (Scheme 2). Separation of the hydroxy lactones (+)-7 and (+)-8 afforded both
diastereomers as pure compounds. A comparison was then made between the CD spectrum of
(+)-7§ and that of (3S,5S)-(+)-5-hydroxymethyl-3-methyl-4,5-dihydro-2(3H)-furanone 9.20 Both
compounds exhibited a positive Cotton effect for the n“p* transition [(+)-7, Dm217 +0.2; UV,
umax 210 nm, mmax 200, EtOH; (+)-9, Dm
+0.3, EtOH; UV, umax 206 nm, mmax 200, EtOH].
220
Although the g-carbon atom in (+)-7 bears a methyl group in place of a hydrogen atom, the
order of polarizability of the substituents is the same in both compounds, thus allowing a
comparison between the curves to be made.21,22 As a consequence, the absolute configuration of
(+)-7 is also (3S,5S) and that of (+)-4 is S.
† (+)-4: [h]D25=+11.8 (c 0.64, MeOH); CD: Dm202=−4.9, Dm226=+3.2 (MeOH); UV: umax 224 nm, mmax 2400 (MeOH);
1H NMR (400 MHz) l, ppm (CDCl3): 9.5 (1H, bs, OH), 6.22 (1H, dt, J 2.9, 2.4 Hz, ꢀCH), 5.66 (1H, dt, J 2.5 Hz,
ꢀCH), 3.27 (1H, dt, J 17.4, 2.4 Hz, H-3), 2.84 (1H, dt, J 17.4, 2.9 Hz, H-3), 1.65 (3H, s, Me); 13C NMR l, ppm
(CDCl3): 175.1 (s), 169.4 (s), 133.5 (s), 123.2 (t), 81.1 (s), 38.5 (t), 24.1 (q).
‡ (−)-4: ([h]2D5 −12.3 (c 0.66, MeOH), CD: Dm201=+6.6, Dm227 −3.9 (MeOH); UV: umax 224 nm, mmax 2400 (MeOH).
1
§ (+)-7: [h]2D5 +5.0 (c 0.90, EtOH); CD: Dm217 +0.2 (EtOH); UV: umax 210 nm, mmax 200 (EtOH); H NMR (400 MHz)
l, ppm (CDCl3): 3.69 (1H, d, J 12.2 Hz, CHOH), 3.43 (1H, d, J 12.2 Hz, CHOH), 2.83 (2H, m and bs, H-3, OH),
2.08 (2H, 2 pseudoquartets of an AB system, 2 H-4), 1.31 (s, 3H, Me), 1.25 (d, 3H, Me); 13C NMR l, ppm (CDCl3):
179.5 (s), 84.2 (s), 67.5 (t), 36.6 (t), 35.0 (d), 22.3 (q), 15.3 (q).

Table 1
Enzymic resolution of (9)-3a
H2O
Conv. (%) Ester (% e.e.)b Acid (% e.e.)c
H2O/acetone 9:1
/
E
React. time
(min)
E
React. time
(min)
Conv. (%) Ester (% e.e.)b Acid (% e.e.)c
Candida rugosa
lipase
Porcine
4
7
16
45
15
37
(+)-3 (10)
(−)-3 (38)
(−)-4 (58)
(+)-4 (66)
12
110
30
21
28
(+)-3 (22)
(−)-3 (35)
(−)-4 (82)
(+)-4 (89)
24
pancreatic
lipase
a-Chymotrypsin 2 24
11
(−)-3 (5)
(+)-4 (41)
9
24
19
(−)-3 (18)
(+)-4 (77)
(
a Reaction conditions: 100 mg of substrate, 100 mg of enzyme, 0.1 M phosphate buffer at pH 7.4 (13 mL), acetone (1.3 mL), rt. The experiments were
monitored with a PHM290 Meterlab™ pH-stat-controller.
b Determined by HRGC, capillary column ChiraldexTM type G-TA, trifluoroacetyl g-cyclodextrin.
c Determined by HRGC on the ethyl ester derivative.
326–267

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G. Pitacco et al. / Tetrahedron: Asymmetry 11 (2000) 3263–3267
Scheme 2.
The chemical yields of the enzymic kinetic resolutions were accurately evaluated. It is
important to emphasise that no significant loss of material was found. In fact, the enantiomer-
ically pure acid was isolated in yields varying from 93 to 97% and the unreacted ester was
recovered in yields ranging from 90 to 95% (both yields were calculated on the basis of the
expected value at each conversion value).
These results would suggest that no irreversible Michael-type reaction occurred between the
acceptor enone function of the substrate and some biological nucleophiles present in the
enzymatic systems used.
Acknowledgements
Financial support by the MURST, CNR (Rome) and the University of Trieste are gratefully
acknowledged.
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