Enantioselective bioreduction of β-keto sulfones with the fungus Curvularia lunata

Article abstract of DOI:10.1016/S0957-4166(01)00048-9

β-Keto sulfones bearing bulky groups are reduced with high enantioselectivities to the corresponding optically active β-hydroxy sulfones by the fungus Curvularia lunata CECT 2130; the cells can be re-used without loss of their catalytic activity.

Full text of DOI:10.1016/S0957-4166(01)00048-9

TETRAHEDRON:
ASYMMETRY
Pergamon
Tetrahedron: Asymmetry 12 (2001) 513–515
Enantioselective bioreduction of b-keto sulfones with the fungus
Curvularia lunata
Vicente Gotor, Francisca Rebolledo* and Ramo´n Liz
Departamento de Qu´ımica Orga´nica e Inorga´nica, Universidad de Oviedo, 33071 Oviedo, Spain
Received 17 January 2001; accepted 26 January 2001
Abstract—b-Keto sulfones bearing bulky groups are reduced with high enantioselectivities to the corresponding optically active
b-hydroxy sulfones by the fungus Curvularia lunata CECT 2130; the cells can be re-used without loss of their catalytic activity.
© 2001 Published by Elsevier Science Ltd.
1. Introduction
the carbonyl group. Thus, with (phenylsulfonyl)-
alkanones of the type 1 (see Table 1), the best reported
result was obtained when the R substituent was a
methyl group,6 but with the larger pentyl6b and phenyl7
substituents, the e.e.s of the resulting b-hydroxy sul-
fones were very low (10 and 15%, respectively).
Optically active b-hydroxy sulfones are of great utility
in organic synthesis; they have been used in the prepa-
ration of many compounds, but find particular use in
the synthesis of chiral, non-racemic lactones,1 2,5-di-
substituted tetrahydrofurans,2 and other b-hydroxy-a-
substituted sulfones.3 Recently, a compound of this
class has proved its efficiency as a chiral controller in
asymmetric Diels–Alder and alkylation reactions.4
Taking into account the importance of b-hydroxy sul-
fones, the limitations of the baker’s yeast reduction and
the availability of the starting b-keto sulfones, herein
we investigate an alternative biocatalytic method
for the enantioselective reduction of these compounds
using whole cells of Curvularia lunata CECT 2130.8
Our own results in the reduction of other prochiral
ketones with this microorganism supported its selec-
tion.9
One of the most useful strategies to access chiral, non-
racemic b-hydroxy sulfones has been the baker’s yeast
mediated asymmetric reduction of b-keto sulfones.5
However, a major factor in the enantioselectivity of
these processes is the size of the substituents attached to
Table 1. Enantioselective reduction of b-keto sulfones
Entry
Substratea
R
tR (h)
Product
Yieldb (%)
E.e. (%)
1
2
3
4
5
1a
1b
1c
2-Furyl
Ph
n-C5H11
2-Furyl
2-Furyl
7
10
7
8
24
(S)-2a
(S)-2b
(S)-2c
(S)-2a
(S)-2a
91
90
92
91
89
97
87
92
97
97
1ac
1a
a 75–78 mg is always used, except in entry 5 (225 mg).
b Yield of crude, substantially pure material.
c Cells recycled from entry 1 reaction (without re-suspension in AcOEt; see experimental).
* Corresponding author.
0957-4166/01/$ - see front matter © 2001 Published by Elsevier Science Ltd.
PII: S0957-4166(01)00048-9

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V. Gotor et al. / Tetrahedron: Asymmetry 12 (2001) 513–515
2. Results and discussion
2-furyl and phenyl substituents shield the methoxy
group in the u-diastereomers, but not in the l-
diastereomers. In addition, in the (R)-MTPA ester
derived from the product 2a obtained with C. lunata,
the methoxy signal appears at 3.38 ppm. Consequently,
we deduced that the actual metabolite was (S)-2a. In
the (R)-MTPA esters derived from the metabolite (S)-
2b, the methoxy group resonates at 3.42 ppm, corrobo-
rating the assignation of 2a. From these results it is
evident that these C. lunata reductions obey Prelog’s
rule, the largest substituent being the phenylsulfonyl-
methyl in all cases.
Reduction of the b-keto sulfones 1a–1c10 was carried
out with resting cells of C. lunata as previously
described,9b which involved suspending the mycelia in
phosphate buffer (0.20 M, pH 6.0) in the presence of
methanol (10 mL/mL of buffer).11 As indicated in Table
1 (entries 1–3), the corresponding b-hydroxy sulfones
2a–2c were obtained with high e.e.s, the best substrate
being 1-(2-furyl)-2-(phenylsulfonyl)ethan-1-one 1a,
from which alcohol 2a was obtained with an e.e. of
97%. In order to check if the mycelia retain their
catalytic activity after the biotransformation, recovered
cells from the reaction with 1a were suspended in fresh
phosphate buffer, and the same amount of 1a added.
After stirring for 8 hours, 2a was obtained in the same
yield and e.e. (entry 4). We have also demonstrated that
these processes can be carried out with a higher sub-
strate/biocatalyst ratio than other baker’s yeast or C.
lunata biotransformations. When the amount of 1a is
trebled (entry 5) a longer reaction time is necessary to
complete the reduction, but 2a is obtained in very good
yield and with the same e.e. No efforts have previously
been made to determine the optimum tolerance ratio
(substrate mass/wet fungal biomass).
It should be pointed out that the Dl values for esters 3a
(R=2-furyl; +0.16 ppm) and 3b (R=Ph; +0.18 ppm)
are high when compared with those observed for the
corresponding Mosher’s esters derived from alkyl (het-
ero)aryl carbinols (ca. 0.09 ppm).14 For esters 3c [R=n-
C5H11 in place of (hetero)Ar; i.e. no shielding effect acts
on the methoxy group in the u diastereomer], the Dl
value is only 0.09 ppm. From this one can conclude
that the phenylsulfonylmethyl group exhibits
a
deshielding effect of about 0.09 ppm, which, acting in
concert with the shielding one of the (hetero)aryl ring,
leads to the high Dl values observed for 3a and 3b.
This phenylsulfonylmethyl group deshielding effect can
be useful for the future assignment of other structurally
related disubstituted carbinols.
Enantiomeric excesses of (S)-2a–2c were determined by
reaction with (S)-3,3,3-trifluoro-2-methoxy-2-phenyl-
propanoyl chloride (MTPA-Cl),12 with analysis of the
resulting MTPA esters by 1H and 19F NMR. In all
cases, two resolved signals were observed in the 19F
NMR spectra of the MTPA esters derived from ( )-2a–
2c. In the case of (S)-2a obtained in the reactions of
entries 1, 4 and 5, the 19F NMR spectra indicated the
presence of only one peak. A more accurate analysis by
HPLC using a Chiralcel OD column showed an e.e. of
97%.13
3. Conclusion
In summary, we have developed a simple and efficient
method for the enantioselective reduction of b-keto
sulfones. The furan derivative is of special interest
because, conveniently functionalised, it can be used for
intramolecular Diels–Alder reactions.16 Application of
this methodology to the reduction of other b-keto
sulfones and sulfoxides is currently in progress.
The (S)-configuration for 2b and 2c was assigned by
comparison of the signs of their specific rotations with
those previously reported.1d The (S)-configuration for
2a was assigned by means of Kelly’s empirical method14
as follows. In Table 2 the usual working models for the
(R)-MTPA esters l- and u-3a–3c derived from ( )-2a–
2c can be seen, as well as the l values (1H NMR) for
their methoxy groups, and also their corresponding Dl
(l−u) values. The l values for both diastereomers 3a
and 3b were assigned taking into account that the
4. Experimental
The typical procedure used for these bioreductions is as
follows. A loop of solid culture of C. lunata, from an
agar plate, was sowed on 75 mL of a sterilised liquid
medium15 [composed of corn steep (Merck, 11928; 5 g),
D
-glucose (30 g), KH2PO4 (1 g), K2HPO4 (2 g), NaNO3
(2 g), KCl (0.5 g), MgSO4·7H2O (0.5 g), FeSO4·7H2O
(0.02 g) and distilled water (1 L)], in a 250 mL Erlen-
meyer flask. After incubating for 72 hours (rotary
shaker, 200 rpm, 28°C), cells (black filamentous myce-
lia) were harvested by filtration (3.5 g wet biomass),
washed with 0.20 M phosphate buffer pH 6.0 (2×20
mL), and suspended in the same, fresh buffer (75 mL).
To this cell suspension, compound 1 (75–78 mg) and
methanol (750 mL) were added. Incubation (same con-
ditions as above) was then carried out until the reaction
Table 2. (R)-MTPA esters 3 derived from (9)-2a–2c and
their methoxy signals (1H NMR)
R
Ester
OMe, l
(ppm)
l-3
OMe, l
(ppm)
u-3
Dl (l−u)
was
complete
(TLC
monitoring,
chloro-
form:hexane:diethyl ether:methanol, 3:2:1:0.25), the
cells were filtered again and washed with aqueous 0.8%
NaCl (2×20 mL). The combined aqueous phases were
continuously extracted with ethyl acetate (12–15 h).
2-Furyl
Ph
n-C5H11
3a
3b
3c
3.54
3.60
3.59
3.38
3.42
3.50
+0.16
+0.18
+0.09

V. Gotor et al. / Tetrahedron: Asymmetry 12 (2001) 513–515
515
3. Tanikaga, R.; Hosoya, K.; Kaji, A. J. Chem. Soc., Perkin
Trans. 1 1988, 2397–2402.
Evaporation of the organic phase yields the corre-
sponding, essentially pure compound 2. A second frac-
tion of product 2 (in some cases, up to 18% of the total
yield) was obtained when the washed cells were re-sus-
pended in ethyl acetate (50 mL) for 1–2 hours (incuba-
tion as above). Compound 2a was purified by column
chromatography (hexane:ethyl acetate, 5:2): mp 72–
4. Sarakinos, G.; Corey, E. J. Org. Lett. 1999, 1, 1741–1744.
5. Reviews including examples of b-keto sulfones reduction
with baker’s yeast: Servi, S. Synthesis 1990, 1–25; Csuk,
R.; Gla¨nzer, B. I. Chem. Rev. 1991, 91, 49–97; Roberts, S.
M. J. Chem. Soc., Perkin Trans. 1 1999, 1–21.
6. (a) Iriuchijima, S.; Kojima, N. Agric. Biol. Chem. 1978,
42, 451–455; (b) Crumbie, R. L.; Deol, B. S.; Nemorin, J.
E.; Ridley, D. D. Aust. J. Chem. 1978, 31, 1965–1980.
7. However, using the much less available sake yeast, 1-
phenyl-2-(phenylsulfonyl)ethan-1-one 1b is reduced to the
corresponding alcohol (S)-2b with 92% e.e.: Nakamura,
K.; Ushio, K.; Oka, S.; Ohno, A. Tetrahedron Lett. 1984,
25, 3979–3982.
8. C. lunata CECT 2130=NRRL 2380 (CECT=Coleccio´n
Espan˜ola de Cultivos Tipo, Universidad de Valencia,
Edificio de Investigacio´n, 46100 Burjasot, Valencia,
Spain. E-mail: cect@uv.es).
9. (a) Gotor, V.; Dehli, J. R.; Rebolledo, F. J. Chem. Soc.,
Perkin Trans. 1 2000, 307–309; (b) Dehli, J. R.; Gotor, V.
Tetrahedron: Asymmetry 2000, 11, 3693–3700.
10. Compound 1b is available from Aldrich, and ketones 1a
and 1c are synthesised as described: Thomsen, M. W.;
Handwerker, B. M.; Katz, S. A.; Belser, R. B. J. Org.
Chem. 1988, 53, 906–907.
1
73°C, [h]2D2 +13.0 (c 0.98, CHCl3) e.e.=97%. H NMR
(200 MHz, CDCl3) l 7.95 (m, 2H), 7.75–7.50 (m, 3H),
7.30 (m, 1H), 6.32 (s, 2H), 5.30 (dt, J=3.1 and 9.2 Hz,
1H), 3.70 (dd, J=9.2 and 14.4 Hz, 1H), 3.55 (dd,
J=3.1 and 14.4 Hz, 1H), 3.50 (d, J=3.1 Hz, 1H, OH)
ppm. 13C NMR (50 MHz, CDCl3) l 152.5 (C), 142.5
(CH), 139.0 (C), 134.0 (CH), 129.3 (CH), 127.9 (CH),
110.3 (CH), 107.3 (CH), 62.5 (CH), 60.4 (CH2) ppm.
MS (EI) m/z (%): 252 (M+, <2), 234 (20), 110 (100).
Compound (S)-2b1d was purified by column chro-
matography (hexane:diethyl ether:dichloromethane,
3.5:1:1): [h]2D0 +28.2 (c 0.83, CHCl3) e.e.=87%. Com-
pound (S)-2c1d was purified by column chromatogra-
phy (hexane:ethyl acetate, 8:1): [h]2D2 +13.0 (c 0.93,
CHCl3) e.e.=92%.
Acknowledgements
11. The presence of methanol precludes a competitive a-alky-
lation reaction when b-keto nitriles are used as substrates
(see Ref. 9a). However, our present results with b-keto
sulfones are essentially identical in the presence of the
same amount of ethanol, or even with no alcoholic
additive, but no reaction was observed when ethanol
(10% v/v, 7.5 mL) was added.
Financial support of this work by CICYT (Project BIO
98-0770) is gratefully acknowledged.
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