Enzymatic Resolution of 5-Phenylselanyltetrahydro-2-furanone. Enantioselective Preparation of (R) and (S)-γ-Valerolactone

Article abstract of DOI:10.1081/SCC-120028354

Lipase-catalyzed lactonization of (2) provides both (R) and (S) enantiomers of 5-phenylselenyltetrahydro-2-furanone (1) in good enantiomeric excess. The kinetic resolution was examined using PPL (Porcine pancreatic lipase), PSL (Amano PS, Pseudomonas sp. lipase), MML (Mucor miehei lipase), CRL (Candida rugosa lipase), CAL-B (Candida Antarctica lipase, type B) and Novozym 435 (immobilized C. antarctica lipase type B) in different solvents. A tributyltin hydride reduction of enantiomerically enriched 1 gave both (R) and (S) enantiomers of S-4-pentanolide (γ-valerolactone).

Full text of DOI:10.1081/SCC-120028354

SYNTHETIC COMMUNICATIONSw
Vol. 34, No. 5, pp. 817–828, 2004
Enzymatic Resolution of
5-Phenylselanyltetrahydro-2-furanone.
Enantioselective Preparation of (R)
and (S)-g-Valerolactone
Giuliano C. Clososki,1 Carlos E. Costa,1 Lauri J. Missio,2
Quezia B. Cass,2 and Joa˜o V. Comasseto1,
*
1
´
Instituto de Quımica, Universidade de Sa˜o Paulo, Sa˜o Paulo, SP, Brazil
2
´
Departamento de Quımica, Universidade Federal de Sa˜o Carlos,
Sa˜o Carlos, SP, Brazil
ABSTRACT
Lipase-catalyzed lactonization of (2) provides both (R) and (S) enantio-
mers of 5-phenylselenyltetrahydro-2-furanone (1) in good enantiomeric
excess. The kinetic resolution was examined using PPL (Porcine
pancreatic lipase), PSL (Amano PS, Pseudomonas sp. lipase), MML
(Mucor miehei lipase), CRL (Candida rugosa lipase), CAL-B (Candida
Antarctica lipase, type B) and Novozym 435 (immobilized C. antarctica
lipase type B) in different solvents. A tributyltin hydride reduction of
enantiomerically enriched 1 gave both (R) and (S) enantiomers of S-4-
pentanolide (g-valerolactone).
*
´
Correspondence: Joa˜o V. Comasseto, Instituto de Quımica, Universidade de Sa˜o
Paulo, C.P. 26077, 05599-070-Sa˜o Paulo, SP, Brazil; E-mail: jvcomass@iq.usp.br.
817
DOI: 10.1081/SCC-120028354
0039-7911 (Print); 1532-2432 (Online)
www.dekker.com
Copyright # 2004 by Marcel Dekker, Inc.

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Clososki et al.
Key Words: g-Valerolactone; 5-Phenylselenyltetrahydro-2-furanone;
Porcine pancreatic lipase.
INTRODUCTION
The production of chiral building blocks, which could be used in the
synthesis of a wide range of biologically active substances, is in the forefront
of the synthetic organic chemistry.[1] The biocatalysis has been one of the
methods of choice to meet this goal.[2] Among the chiral fragments often
present in natural products of practical importance is the g-butyrolactone
backbone. This unity is present in many substances such as aromas,[3] insect
pheromones[4] and plants growth regulators.[5] An easy way to access the g-
butyrolactone unity is the selenolactonization route,[6] which was recently
made chiral by using chiral selenium electrophiles.[7]
In view of the already well established carbon–carbon bond formation
methodology involving free radical intermediates derived from selenolac-
tones,[8] the availability of chiral selenolactones enhances the synthetic
potential of this class of selenium compounds. A drawback of the published
methodologies to produce chiral selenolactones is that the expensive chiral
selenogroups would be lost during the free radical generation step. A possible
solution to this problem would be the enzymatic kinetic resolution of a
phenylselenolactone (Sch. 1). To our knowledge only a brief report on the
application of biocatalysis to prepare enantiopure selenium compounds was
published to date.[9]
With the aim of combining the synthetic potential of the biocatalysis with
that of the organoselenium chemistry, in this paper we present our preliminary
results on the use of enzymes to produce enantiomerically enriched (R) and
(S)-phenylselenolactones (1) starting from racemic phenylselenoesteres (2)
(Sch. 2). A tributyltin hydride reduction of enantiomerically enriched (R) and
(S)-1 gave both enantiomers of g-valerolactone (6), which has been widely
used as intermediate in the natural products synthesis[10] and in biosynthetic
studies.[10]
Scheme 1.

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Enzymatic Resolution of 5-Phenylselanyltetrahydro-2-furanone
819
Scheme 2.
RESULTS AND DISCUSSION
In the search for methodologies to prepare the phenylselenoesteres (2),
propyl-4-hydroxy-5-phenylselanylpentanoate (2a) was synthesized by two
different routes as shown in Sch. 2.
By route a compound 2a was obtained in 33% overall yield from
4-pentenoic acid (3) by reacting epoxide 5 with the phenylselenolate anion at
2408C for 2 min. At room temperature and at reaction times superior to 3 min,
the main product formed was the selenolactone 1. Even at low temperature the
lactone 1 was formed, being separated from 2 by column chromatography on
silica gel impregnated with triethylamine. By route b compound 3 was initially
transformed into racemic 5-phenylselenyltetrahydro-2-furanone 1 through a
selenolactonization with phenylselanil bromide. Hydrolysis of 1 with KOH
followed by an esterefication with propyl bromide afforded 2a in 56% overall
yield from 3. Since route b showed to be a superior method to prepare 2, it was
chosen to synthesize compounds 2b and 2c. Care must be taken to manipulate
2, since heating and trace of acids can lactonize it.
With racemic 2 in hand we performed a systematic evaluation of solvent
and lipases to promote the kinetic resolution (Sch. 3).
In a typical experiment, a solution containing 0.2 g of the substrate and
0.2 g of the enzyme in organic solvent (10 mL) were stirred at 308C. The
progress of the reaction was monitored by HPLC and it was stopped when the
conversion into lactone 1 reached ca. 50%. Then, the lipase was removed by

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Clososki et al.
Scheme 3.
centrifugation and the resulting solution concentrated. The organic residues
were subjected to silica gel chromatography to obtain 1 and unreacted
compound 2. The enantiomeric excesses (e.e.) of 1 were determined by HPLC
equipped with an amilose tris(3,5-dimethylphenylcarbamate) chiral phase
column, prepared as reported by Cass and Batigalhia.[12] The E (enantiomeric
ratio) values were calculated from the e.e. of products and the conversion
values (c) according to Sih, Sharples and Fajan’s equation (E ¼ ln
[1 2 c(1 þ e.e.p)/ln [1 2 c(1 2 e.e.p)]).[13] The absolute stereochemistry of
the (R) and (S) phenylselenolactones could be attributed by comparing the
[a]D values of the products obtained by us with those of the literature.[14] The
results obtained in the enzymatic lactonizations are summarized in Table 1.
Initially, In order to determine the best enzyme to perform the kinetic
lactonization of the racemic phenylselenoesteres 2, reaction of compound 2a
was examined using PPL (Sigma’s porcine pancreatic lipase), PSL (Amano’s
PS–Pseudomonas sp. lipase), MML (Novozyme’s Mucor miehei lipase), CRL
(Sigma’s Candida rugosa lipase), CAL-B (Roche’s C. antarctica lipase, type
B) and Novozym 435w (Novozyme’s immobilized Candida Antarctica lipase
type B) in diethyl ether. It was observed that the most efficient enzyme to
perform this lactonization was Novozym 435w which after 3 hours led to 97%
conversion, but with no enantiomeric excess (Table 1, Entry 6). The best
enzyme in terms of enantioselectivity was found to be PPL, which transformed
2a into (R)-1 in 74% e.e. (Table 1, Entry 3).
In an attempt to improve the e.e. of the enzymatic reactions, compound 2a
was allowed to react with PPL in hexane, cyclohexane, toluene, t-butyl methyl
ether, but these reactions presented inferior results. As observed for substrate
2a, the E values obtained in the enzymatic lactonization of 2b and 2c showed
that the PPL/diethyl ether is the most efficient combination to perform this
transformation. In addition, the similar E values calculated for 2a–c showed
that the enantioselectivity of these enzymatic lactonizations was not
dependent on the size of the substituents attached to the ester group.
Starting from unreacted (S)-2a, an acid-catalyzed lactonization using
p-toluenesulphonic acid (PTSA) was firstly attempted to synthesize (S)-1.

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Enzymatic Resolution of 5-Phenylselanyltetrahydro-2-furanone
821

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Clososki et al.
Scheme 4.
However, under these conditions only racemic 1 was obtained, probably
formed through the mechanistic pathway shown in Sch. 4.
In view of the high reactivity of Novozym 435w (Table 1, Entry 6) we
used this lipase to perform the lactonization of (S)-2a. Under these conditions
S-phenylselanyltetrahydro-2-furanone 1 was obtained with 64% enantiomeric
excess (Sch. 5).
Both racemic and chiral 1 were then submitted to tributyltin hydride
reduction leading to g-valerolactone 6 (Sch. 6). According to the literature[8]
this reaction can be performed without free-radical initiator. However, it was
observed that the use of catalytic amount of the 1,10-azo-bis-(ciclohexane-
carbonitrile) reduced the reaction time and afforded 6 in a better yield.
It is well-known that there are occasional difficulties in removing tin
species from the products of stannane mediated radical reactions, particularly
when excess of the hydride is used to improve the rate of reduction.[15] Both
distillation and chromatographic separations, performed over silica-gel using
different solvent systems, were not efficient to purify g-valerolactone 6. The
best work-up procedure was found by dissolving the dry residue of the
reduction reaction in acetonitrile and then washing several times with dry
hexane to remove organotins, as previously reported by Hamon and
Richards.[16] The organice extract was then concentrated and compound 6
Scheme 5.

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Enzymatic Resolution of 5-Phenylselanyltetrahydro-2-furanone
823
Scheme 6.
Scheme 7.
purified by column chromatography. The g-valerolactone 6 was separated into
its enantiomers by means of gas chromatography using a b-cyclodextrin
column.
Since the selenium removal should not affect the stereogenic center, the
absolute stereochemistry of the g-valerolactones could be confirmed by
comparing the [a]D values of the products obtained by us with those of the
literature. In particular, the [a]D of compound (S)-6 obtained in 74% e.e. was
226.7, close to 229.6, the reported value for the enantiomerically pure (2)-
(S)-g-valerolactone.[17]
In conclusion, the first kinetic resolution of a phenylselenolactone pro-
moted by enzymes was performed, opening the perspective for a more
economical route to these interesting building blocks which, as shown for (R)
and (S)-g-valerolactone, could be used in the enantioselective synthesis of
biologically active g-butyrolactones, by means of well known selenium
methodologies,[8,18] in particular the free radical mediated chain elongation
(Sch. 7).[8a]
EXPERIMENTAL
General
The NMR spectra were recorded on Bruker DRX-500 and Varian FT-300
spectrometers using TMS as internal reference (1H NMR) and the central peak
of the CDCl3 signal (13C NMR). IR spectra were obtained with a Perkin-Elmer
1600 grating infrared spectrophotometer. The GC analyses were performed on
a Hewlett-Packard 5890(II) and on a Shimadzu GC-17 (chiral analysis). The

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Clososki et al.
mass spectra were performed on Shimadzu GC-MS QP5050. Optical rotations
were measured on a Jasco DIP 370 digital polarimeter. The enzymatic
reactions were monitored in a Shimadzu LC10AD HPLC.
Propyl-4-hydroxy-5-phenylselanylpentanoate (2a)
To a solution of diphenyl diselenide (1.72 g, 5.5 mmol) in ethanol (30 mL)
was added solid sodium borohydride in small portions until the characteristic
yellow color of the diphenyl diselenide faded. The resulting sodium phenyl-
selenolate solution was cooled to 2408C and epoxide 5 (1.58 g, 10 mmol) was
added. After stirring for 2 min, ethyl acetate (150 mL) was added and the
mixture was washed with 10% NaHCO3 aqueous solution (3 ꢀ 20 mL). The
residue was purified by flash silica gel column chromatography eluting with
1
hexane/ethyl acetate (7 : 3) to give 2a (1.78 g, 57%). H NMR (500 MHz,
CDCl3) d: 0.92 (t, J ¼ 7.44 Hz; 3H), 1.64 (qui, J ¼ 7.44 Hz; 2H), 1.73–1.97
(m, 2H), 2.4 (t, J ¼ 6.85 Hz; 2H), 2.76 (d, J ¼ 3.94; 2H), 2.88–2.95 (m, 2H),
3.59–3.76 (m, 1H), 4.01 (t, J ¼ 7.44 Hz; 1H), 7.20–7.23 (m, 2H); 7.51–7.57
(m, 3H). 13C NMR (125 MHz, CDCl3) d: 10.4, 21.9, 30.7, 31.5, 36.1, 66.0,
69.4, 125.9, 129.1, 129.8, 132.6, 173.8. IR (KBr) (cm21): 3431, 2967, 1739.
Anal. Calcd for C14H20O3Se: C, 53.34; H, 6.39. Found: C, 53.35; H, 6.22.
5-Phenylselanyltetrahydro-2-furanone (1)
A solution of phenylselanil bromide (5.37 g, 22,7 mmol) in dry THF
(70 mL) was added dropwise to a solution of 4-pentenoic acid 3 (2.0 g,
20 mmol) in dry THF (150 ml) at 2788C under nitrogen. Triethylamine
(20 ml) was added and the mixture was stirred for 17 h at room temperature.
Then, the solvent was concentrated under reduced pressure and NH4Cl
saturated solution (10 mL) was added. The mixture was extracted with AcOEt
(3 ꢀ 50 mL), dried (MgSO4), filtered and evaporated under reduced pressure.
The residue was chromatographed on silica gel (hexane/AcOEt ¼ 9 : 1) to
give 1 (3.9 g, 75%). 1H NMR (500 MHz, CDCl3) d: 1.89–1.98 (m, 1H), 2.35–
2.43 (m, 1H), 2.50–2.57 (m, 2H), 2.98–3.04 (m, 1H), 3,24–3.29 (m, 1H),
4.64 (dtd, J ¼ 7.36 Hz, J ¼ 6.99 Hz, J ¼ 4.78 Hz, 1H), 7.26–7.30 (m, 2H),
7.51–7.55 (m, 2H). 13C NMR (125 MHz, CDCl3) d: 27.4, 28.6, 31.7, 79.2,
127.5, 128.7, 133.0, 135.4 and 176.4. IR (film) (cm21): 2933, 1773, 730. MS
(m/z) (% rel.): 43 (100%), 55 (51%), 77 (49%), 85 (97%), 157 (13%), 256
(11%). Anal. Calcd for C11H12O2Se: C, 51.78; H, 4.74. Found: C, 51.67; H,
4.94. The analytical separation of the enantiomers of racemic and chiral
5-phenylselanyltetrahydro-2-furanone (1) were achieved in chiral HPLC;

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Enzymatic Resolution of 5-Phenylselanyltetrahydro-2-furanone
825
Stationary phase: amilose tris(3,5-dimethylphenylcarbamate) coated onto
˚
APS-Hypersil (5 mm particle size and 120 A pore size, 20% w/w). Mobile
phase: [Hexane : Ethanol (9 : 1)]. Flow rate: 0.5 mL/min.
Propyl-4-hydroxy-5-phenylselanylpentanoate (2a)
To the selenolactone 1 (1.01 g, 4 mmol) at r.t. was added an aqueous
solution of KOH (12 mL, 0.28 g, 4.2 mmol) and the mixture was refluxed for
4 h. The water was removed by azeotropic distillation (three times) with DMF
(30 mL). To the dried solid were added dry DMF (16 mL) and n-propyl
bromide (0.72 mL, 8 mmol), and the mixture was stirred for 24 h at room
temperature. Diethyl ether (50 mL) was added and the organic layer was
washed with water (5 ꢀ 25 mL), dried (MgSO4), filtered and evaporated under
reduced pressure. The residue was purified by flash silica gel column
chromatography eluting with hexane/ethyl acetate (7 : 3) to give 2a (0.94 g,
75%). The spectral data are agreement with those reported for compound 2a
prepared as described above.
Methyl-4-hydroxy-5-phenylselanylpentanoate (2b)
Following the same procedure described for the preparation of 2a,
compound 2b was obtained in 76% yield using MeI as the alkylating agent.
NMR 1H (500 MHz, CDCl3): d (ppm) 1.5 (b, 1H); 1.9–2.0 (m, 1H); 2.37–2.45
(m, 1H); 2.47–2.62 (m, 2H); 3.05 (dd, J ¼ 13 Hz, J ¼ 8 Hz; 1H); 3.2 (dd,
J ¼ 13 Hz, J ¼ 4.5 Hz; 1H), 3.5 (s, 3H); 4.62–4.68 (m, 1H); 7.26–7.30
(m, 3H); 7.54–7.56 (m, 2H). NMR 13C (125 MHz, CDCl3): d (ppm) 26.7,
28.8, 31.9, 69.3, 79.4, 127.7, 129.4, 133.3, 174.2. I.R. (film) (cm21): 3467,
2948, 1730, 1580, 1437, 1171, 1062, 744. Anal. Calcd for C12H16O3Se: C,
50.18; H, 5.61. Found: C, 50.29; H, 5.47.
Benzyl-4-hydroxy-5-phenylselanylpentanoate (2c)
Following the same procedure described for the preparation of 2a,
compound 2c was obtained in 74% yield using benzyl bromide as the
alkylating agent. NMR 1H (500 MHz, CDCl3): d (ppm) 1.75 (br, 1H); 1.9–2.0
(m, 1H); 2.36–2.45 (m, 1H); 2.47–2.62 (m, 2H); 3.0 (dd, J ¼ 13 Hz,
J ¼ 8 Hz; 1H); 3.29 (dd, J ¼ 13 Hz, J ¼ 5 Hz; 1H), 4.62–4.68 (m, 1H); 4.69
(s, 2H); 7.24–7.38 (m, 8H); 7.52–7.58 (m, 2H). NMR 13C (125 MHz, CDCl3):
d (ppm) 26.6, 28.7, 31.9, 69.2, 79.4, 127, 127.6, 127.6, 128.7, 128.7, 129.3,

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Clososki et al.
133.2, 140.8, 176.6. I.R. (film) (cm21): 3461; 2925; 1944; 1724; 1580; 1154;
1079; 733. Anal. Calcd for C18H20O3Se: C, 59.51; H, 5.55. Found: C, 59.40;
H, 5.58.
Typical Procedure for the Lipase-Catalyzed Kinetic
Lactonization of 2
To a solution of substrate 2 (0.2 g) in the dry organic solvent (10 mL) was
added the lipase (0.2 g) and the mixture was stirred at 308C. The progress of
reaction was monitored by HPLC (Supelcosil LC-18 15 cm ꢀ 4.6 mm ꢀ 5 mm
column. Mobile phase: Acetonitrile : H2O 0.65 : 0.35. Flow rate: 1 mL/min)
and it was stopped when conversion into lactone (R)-1 reached ca. 50%. Then,
the lipases were removed by centrifugation and the resulting solution
concentrated under reduced pressure. The residue was purified by flash silica
gel column chromatography eluting with hexane/diethyl ether (1 : 1) as the
eluent. From substrate 2a and PPL in diethyl ether (0.06 g, 37%) of compound
(R)-1. [74% e.e., [a]2D5 ¼ 2 0.5, (c ¼ 2.56)] and (0.10 g, 52%) of compound
(S)-2a [64% e.e (estimated from the lactonized derivative), [a]2D5 ¼ þ16.14,
(c ¼ 3.11)] were obtained. The spectral data for compounds (R)-1 and (S)-2
are in agreement with those of racemic 1 and 2.
Typical Procedure for Lactonization of 2 with Novozym 435w
Compound (S)-2 (1 g) was dissolved in dry diethyl ether (50 mL) and
Novozym 435w (1 g) was added. The mixture was stirred for 6 h at 308C,
filtered and the solvent evaporated under reduced pressure. The residue was
purified by flash silica gel column chromatography eluting with hexane/
diethyl ether (1 : 1). Starting from (S)-2a, compound (S)-1 (0.32 g, 90%) was
obtained [64% e.e., [a]2D5 ¼ þ 0.3, (c ¼ 2.31)]. The spectral data are in
agreement with those of racemic 1.
(S)-4-Pentanolide (g-Valerolactone) (6)
In a round-bottomed flask, compound 1 (0.255 g, 1 mmol) was dissolved
in toluene (3 mL). A reflux condenser was coupled and nitrogen was swept
through the system for about 5 min. Then tributyltin hydride (0.54 mL, 0.58 g,
2 mmol) and catalytic amount of the 1,10-azobis-(ciclohexanecarbonitrile)
were added into the mixture and the flask was immersed into a preheated oil
bath (120–1308C). The mixture was refluxed for 2 h and then cooled.

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Enzymatic Resolution of 5-Phenylselanyltetrahydro-2-furanone
827
Acetonitrile (10 mL) was added and the resulting mixture was washed with
hexane (10 ꢀ 10 mL) and the solvent was evaporated. The residue was puri-
fied by flash silica gel column chromatography eluting with hexane/diethyl
1
ether (1 : 1) to give S-4-pentanolide (g-valerolactone) 6 (61 mg, 61%) H
NMR (300 MHz, CDCl3) d: 1.43 (d, J ¼ 6.3 Hz, 3H), 1.78–1.91 (m, 1H),
2.54–2.59 (m, 2H), 4.6–4.71 (m, 1H). 13C NMR (75 MHz, CdCl3) d: 21.0,
29.1, 29.8, 77.4, 177.6 IR (film) (cm21): 2998, 1777, 1432, 1130. MS (m/z)
(%rel.): 41 (47%), 43 (34%), 56 (100%), 85 (45%) and 100 (8%). The
analytical separation of the enantiomers of racemic and chiral g-valerolactone
was achieved in a gas chromatograph equipped with a CHIRASIL-DEX CD
(CROMPACKw) chiral phase capillary column. Chiral separations were
performed using the following gradient temperature program: 608C (3 min) to
1908C (10 min) at 58C/min. Carrier gas at 15 psi, the injector and detector
temperatures were maintained at 2308C and 2508C, respectively.
ACKNOWLEDGMENTS
The authors thank FAPESP and CNPq for support and Amano Pharma-
ceutical Co. and Novozymes Inc. for their generous gifts of lipases. Authors
also thank Prof. Liliana Marzorati for critical revision of this article.
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Received in the USA September 5, 2003

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