The use of vinyl esters significantly enhanced enantioselectivities and reaction rates in lipase-catalyzed resolutions of arylaliphatic carboxylic acids

Full text of DOI:10.1021/jo981780l

J . Org. Chem. 1999, 64, 1709-1712
1709
Sch em e 1
Th e Use of Vin yl Ester s Sign ifica n tly
En h a n ced En a n tioselectivities a n d
Rea ction Ra tes in Lip a se-Ca ta lyzed
Resolu tion s of Ar yla lip h a tic Ca r boxylic
Acid s
Hong Yang, Erik Henke, and Uwe T. Bornscheuer*,†
Institute for Technical Biochemistry, University of Stuttgart,
Allmandring 31, D-70569 Stuttgart, Germany
Sch em e 2
Received September 1, 1998
In tr od u ction
Lipases and esterases accept a wide range of sub-
strates, which are usually converted with high enantio-
selectivity. These enzymes also exhibit high stability in
nonaqueous solvents.1 In most cases, optically pure
alcohols were prepared starting from racemic or proste-
reogenic precursors and reactions are often performed via
transesterification in organic solvents. To increase the
reaction rate and to shift the equilibrium toward product
synthesis, activated esters such as vinyl acetate are
routinely employed.2 This takes advantages of the tau-
tomerization of the vinyl alcohol generated during the
esterification to the highly volatile acetaldehyde; thus,
the undesired back reaction is suppressed.
For comparison of enantioselectivities and reaction
rates, the corresponding arylaliphatic carboxylic acid
ethyl esters (()-4-6 (Scheme 1) were synthesized and
transesterified with n-hexanol in toluene to afford n-hexyl
esters (R)-(-)-7-9 (Scheme 2) or subjected to hydrolysis
in sodium phosphate buffer leading to acids (R)-(-)-10-
12 (Schemes 1 and 3).
In contrast, the enzymatic synthesis of optically pure
carboxylic acids is normally performed as hydrolysis of
simple esters, e.g., methyl or ethyl esters. Unfortunately,
hydrolysis cannot be performed with water-labile sub-
strates and a partial racemization in aqueous media can
lead to a decrease in the optical purity. The alternative
transesterification using nonactivated esters is hampered
by an unfavored equilibrium causing extremely long
reaction times. To circumvent this problem, we investi-
gated the transesterification of vinyl esters of racemic
carboxylic acids. As model compounds three arylaliphatic
carboxylic acids were chosen, which can be used as chiral
building blocks3 or as chiral derivatizating agents.4 The
corresponding vinyl esters (()-1-3 (Scheme 1) were
synthesized using a known procedure2b and then sub-
jected to lipase-catalyzed transesterifications with n-
hexanol in toluene yielding n-hexyl esters (R)-(-)-7-9
(Schemes 1 and 2).
Resu lts a n d Discu ssion
Vinyl esters of arylaliphatic carboxylic acids were
prepared in acceptable yields around 50% and subjected
to hydrolase-catalyzed transesterification with n-hexanol
in toluene. A preliminary study5 revealed, that only lipase
B from Candida antarctica exhibited sufficient rates and
enantioselectivities (Table 1).
The most interesting observation was the increase in
enantioselectivity found when using vinyl esters instead
of ethyl esters. The highest enantioselectivity was de-
termined for the transesterification of 2-phenylbutyric
acid vinyl esters (()-1 (E >100), whereas in reactions
using ethyl ester (()-4, only E ) 6.5 (transesterification
with n-hexanol in toluene) or E ) 3.7 (hydrolysis in
sodium phosphate buffer) were achieved. Figure 1 shows
the time course of the conversion of vinyl ester (()-1
compared to the reaction of ethyl ester (()-4 at 60 °C. It
is obvious that the reaction employing the vinyl ester
proceeded with much higher enantioselectivity and reac-
tion rate and after ca. 50 h the enantiomeric excess of
remaining (S)-(+)-1 was >99%ee. Enantioselectivities
were also significantly increased in the resolution of
2-phenylpropionic acid vinyl ester (()-2 (E ) 26) com-
pared to the reactions using ethyl ester (()-5. For
3-phenylbutyric acid vinyl ester (()-3, the highest value
† Tel.: +49 711 685 4523.Fax: +49 711 685 3196.email: bornscheuer@
po.uni-stuttgart.de.
(1) For reviews and books, see: (a) Schoffers, E.; Golebiowski, A.;
J ohnson, C. R. Tetrahedron 1996, 52, 3769. (b) Kazlauskas, R. J .;
Bornscheuer, U. T. In Biotechnology-Series; Rehm, H. J ., Reed, G.,
Pu¨hler, A., Stadler, P. J . W., Kelly, D. R., Eds.; VCH: Weinheim, 1998;
Vol. 8, p 37. (c) Schmid, R. D.; Verger, R. Angew. Chem., Int. Ed. Engl.
1998, 37, 1608. (d) Wong, C.-H.; Whitesides, G. M. Enzymes in
Synthetic Organic Chemistry; Pergamon Press: Oxford, 1994. (e) Faber,
K. Biotransformations in Organic Chemistry, 3rd ed.; Springer: Berlin,
1997.
(2) For first publications on the use of vinyl esters, see: (a) Degueil-
Castaing, M.; de J eso, B.; Drouillard, S.; Maillard, B. Tetrahedron Lett.
1987, 28, 953. (b) Wang, Y. F.; Lalonde, J . J .; Momongan, M.;
Bergbreiter, D. E.; Wong, C. H. J . Am. Chem. Soc. 1988, 110, 7200. (c)
Laumen, K.; Breitgoff, D.; Schneider, M. P. J . Chem. Soc., Chem.
Commun. 1988, 1459.
(5) The following enzymes were used: recombinant esterase from
Pseudomonas fluorescens (Krebsfa¨nger, N.; Schierholz, K.; Born-
scheuer, U. T. J . Biotechnol. 1998, 60, 105); recombinant esterase from
Bacillus stearothermophilus (Amaki, Y.; Nakano, H.; Yamane, T. Appl.
Microbiol. Biotechnol. 1994, 40, 664); pig liver esterase (Sigma,
Deisenhofen, Germany); lipase B from C. antarctica (SP435, Boehringer
Mannheim, Penzberg, Germany); lipase from Pseudomonas cepacia
(PS, Amano Pharmaceutical Co., Nagoya, J apan).
(3) Kogure, T.; Eliel, E. L. J . Org. Chem. 1984, 49, 576.
(4) (a) Helmchen, G.; Vo¨lter, H.; Schu¨hle, W. Tetrahedron Lett. 1977,
1417. (b) Bravo, P.; Piovosi, E.; Resnati, G.; Fronza, G. J . Org. Chem.
1989, 54, 5171.
10.1021/jo981780l CCC: $18.00 © 1999 American Chemical Society
Published on Web 02/17/1999

1710 J . Org. Chem., Vol. 64, No. 5, 1999
Notes
Sch em e 3
variation of the substrate structure.8 The latter approach
was based on changes in the size of substituents8a or
protecting group strategies.8b In contrast, in the examples
shown here, the only structural change is from an ethyl
to a vinyl group, which corresponds only to a neglectible
change in the size of the substrates. One explanation
could be that the increase in enantioselectivity is due to
an electronic effect, presumably based on the interaction
of the π-electrons of the vinyl double bond with amino
acid residues in the active site of CAL-B. A related
example was found in the literature, where the resolution
of 3-nonanol with CAL-B proceeded with high enantio-
selectivity (E > 300)8c but the resolution of 1-bromo-2-
octanol with low enantioselectivity (E ) 7.6)8d under
similar conditions. Both an ethyl and a CH2Br group are
similar in size, so the difference in E suggests that an
electronic effect lowered the enantioselectivity there too.
One might also speculate that the use of vinyl esters
promotes acylation of the active-site serine and deacy-
lation becomes the rate-determining step. However, this
implies that the rate of deacylation must be different for
each enantiomer.
Ta ble 1. Resolu tion of Ar yla lip h a tic Ca r boxylic Acid s
(()-1-6 w ith Lip a se B fr om C. a n ta r ctica
enantiomeric
excess
reactn convn
compd reactna timeb [h] [%] [% eeS]d [% eeP]e
Ec
(()-1
(()-4
(()-4
(()-2
(()-5
(()-5
(()-3
(()-6
(()-6
T
T
H
T
T
H
T
T
H
68
304
1
1.3
44
0.3
68
720
3
43
18
43
47
44
47
56
19
56
74 (1)f 99 (7) >100
14 (4)f 66 (7)
34 (4)f 22 (10)
75 (2)f 53 (8)
41 (5) 68 (8)
38 (5) 28 (11)
86 (3) 31 (9)
22 (6) 60 (9)
93 (6) 13 (12)
6.5 (0.37)g
3.7
26
17 (0.79)g
3.5
13 (23)h
13 (0.53)g
9
a
c
T, transesterification; H, hydrolysis. bAt 40 °C. Calculated
according to ref 9 using the equation E ) [ln{(1 - c)(1 - eeS)}]/
[ln{(1 - c)(1 + eeS)}]. dAll remaining substrates had (S)-(+)
e
f
configuration. All products had (R)-(-) configuration. Determined
by GC using a chiral column. gValues in parentheses correspond
to the equilibrium constant calculated according to ref 10 using
the computer program available at http://bendik.mnfak.unit.no.
hValues in parentheses corresponds to reaction at room temper-
ature.
was E ) 13 (E ) 23 when the reaction was performed at
room temperature) and slightly higher compared to
values determined for the hydrolysis of ethyl ester (()-6
(Table 1).
Although our method requires the extra synthesis of
vinyl esters of racemic carboxylic acids, it might be
especially useful for the resolution of (i) acids which are
only converted with low enantioselectivity in hydrolysis
reactions, (ii) water-labile compounds, and/or (iii) reac-
tions where the exclusive use of organic solvents fits best
into a synthetic route.
Furthermore, the transesterification of the activated
vinyl esters (()-1-3 proceeded with significantly higher
reaction rates. For instance, the resolution of 2-phenyl-
butyric acid vinyl ester (()-1 gave 43% conversion after
68 h, but only 18% conversion after more than 300 h
when using ethyl ester (()-4. With the other ethyl esters,
maximum conversions were less than half and reaction
times were significantly prolonged, making this approach
synthetically useless. However, hydrolysis in sodium
phosphate buffer proceeded faster in all cases studied.
Exp er im en ta l Section
1
Gen er a l Meth od s. H NMR spectra were recorded at 250.1
and 500.1 MHz and 13C NMR spectra at 62.9 and 125.7 MHz,
in CDCl3 with tetramethylsilane as internal standard. Sub-
strates and products were also synthesized in optically pure form
by chemical synthesis from commercially available optically pure
carboxylic acids 10-12 to determine the enantiomeric excess
(based on optical rotation or chiral GC, see below) and to assign
the absolute configurations of biotransformation products. In
cases where both racemic and enantiomerically pure compounds
were synthesized, the racemates were used for full characteriza-
tion including IR spectra (data not shown). Purity of optically
pure compounds was verified by NMR and GC analysis and
matched that of racemates. Gas chromatographic analyses were
conducted using an Optima 5 column (25 m × 0.25 mm;
Macherey & Nagel, Du¨ren, Germany) for the determination of
conversion and purity and using a heptakis(2,3-di-O-acetyl-6-
O-TBDMS)-â-cyclodextrin column (25 m × 0.25 mm, Prof. W.
Changes in the enantioselectivity in lipase-catalyzed
biotransformations have been published already, and E
was influenced by temperature,6 solvent,7 enzyme,8a and
(6) Sakai, T.; Kawabata, I.; Kishimoto, T.; Ema, T.; Utaka, M. J .
Org. Chem. 1997, 62, 4906.
(7) See, for instance: (a) Orrenius, C.; Norin, T.; Hult, K.; Carrea,
G. Tetrahedron: Asymmetry 1995, 6, 3023. (b) Bornscheuer, U.; Herar,
A.; Kreye, L.; Wendel, V.; Capewell, A.; Meyer, H. H.; Scheper, T.;
Kolisis, F. N. Tetrahedron: Asymmetry 1993, 4, 1007. Even reverse
stereoselectivities were found, for the most drastic example, see: (b)
Hirose, Y.; Kariya, K.; Sasaki, I.; Kurono, Y.; Ebiike, H.; Achiwa, K.
Tetrahedron Lett. 1992, 33, 7157.
(8) (a) Numerous examples can be found in Kazlauskas, R. J .;
Weissfloch, A. N. E.; Rappaport, A. T.; Cuccia, L. A. J . Org. Chem.
1991, 56, 2656. (b) Lampe, T. F. J .; Hoffmann, H. M. R.; Bornscheuer,
U. T. Tetrahedron: Asymmetry 1996, 7, 2889. (c) Orrenius, C.; O¨ hrner,
N.; Rotticci, D.; Mattson, A.; Hult, K.; Norin, T. Tetrahedron: Asym-
metry 1995, 6, 1217. (d) Kim, M. J .; Choi, G. B.; Kim, J . Y.; Kim, H. J .
Tetrahedron Lett. 1995, 36, 6253.
(9) Chen, C. S.; Fujimoto, Y.; Girdaukas, G.; Sih, C. J . J . Am. Chem.
Soc. 1982, 104, 7294.
(10) Anthonsen, H. W.; Hoff, B. H.; Anthonsen, T. Tetrahedron:
Asymmetry 1996, 7, 2633. Note that the example used there is based
on the resolution of a chiral alcohol using an excess of an achiral acyl
donor, whereas we used a chiral acyl donor and an excess of an achiral
alcohol.

Notes
J . Org. Chem., Vol. 64, No. 5, 1999 1711
F igu r e 1. Time course of the resolution of vinyl ester (()-1 and ethyl ester (()-4 by transesterification with n-hexanol in toluene
catalyzed by lipase B from C. antarctica at 60 °C.
A. Ko¨nig, University of Hamburg, Germany) for the determi-
nation of enantiomeric excesses in the case of compounds 1, 2,
and 4. Immobilized lipase B from C. antarctica (Chirazyme L-2,
c.-f., C2; 5000 U/g) was donated by Boehringer Mannheim,
Penzberg, Germany.
1H), 7.13-7.27 (m, 6H); 13C NMR (62.9 MHz; CDCl3) δ 21.82,
36.27, 42.60, 97.76, 126.63, 126.78, 128.66, 141.18, 145.41,
169.48. Anal. Found: C, 75.63; H, 7.68. Calcd for C12H14O2: C,
75.76; H, 7.42.
(R)-(-)-3-P h en ylbu tyr ic a cid vin yl ester (R)-(-)-3: 120
Gen er a l P r oced u r e for P r ep a r a tion of Vin yl Ester s 1-3.
Vinyl esters were synthesized according to a known method.2b
Carboxylic acid (500 mg) and HgOAc2 (70 mg, 0.22 mmol) were
dissolved in 10 mL of vinyl acetate. After stirring the mixture
for 30 min at room temperature, 0.1 mL of concentrated H2SO4
was added and the solution was refluxed for 6 h. Then the
mixture was allowed to cool to room temperature, and NaOAc
(400 mg) was added to quench the catalyst. The solution was
filtered and concentrated. The crude products were purified by
silica gel column chromatography (petroleum ether:Et2O 20:1).
All vinyl esters were obtained as colorless liquids needing no
further purification. Optically pure vinyl esters were synthesized
in smaller amounts using about 60-120 mg of carboxylic acid.
(()-2-P h en ylbu tyr ic Acid Vin yl Ester (()-1. Preparation
following the general procedure starting with 500 mg of (()-10
(3.05 mmol) yielded 290 mg of (()-1 (1.52 mmol, 50%): 1H NMR
(500.1 MHz; CDCl3) δ 0.91 (t, J ) 7.4, 3H), 1.80-2.16 (m, 2H),
3.51 (t, J ) 7.7, 1H), 4.54 (dd, J ) 6.32, 1.6, 1H), 4.85 (dd, J )
14.0, 1.7, 1H), 7.23-7.34 (m, 6H); 13C NMR (125.8 MHz; CDCl3)
δ 12.08, 26.62, 53.17, 97.90, 127.41, 128.00, 128.67, 138.28,
141.32, 171.12. Anal. Found: C, 75.80; H, 7.41. Calcd for
C12H14O2: C, 75.76; H, 7.42;
µL of (R)-(-)-12 (128.3 mg, 0.78 mmol) yielded 29 mg of (R)-(-
)-3 (0.15 mmol, 19%); [R]22 ) -21.2 (c 1.543, 1,4-dioxane).
D
Gen er a l P r oced u r e for P r ep a r a tion of Eth yl Ester s 4-6.
The carboxylic acid was dissolved in a mixture of EtOH (15 mL)
and toluene (100 mL) and 0.5 mL of concentrated H2SO4 was
added. Then the flask was equipped with a condenser and a
Dean-Stark trap and the mixture was refluxed until the
formation of water stopped. The solution was allowed to cool to
room temperature and was washed with ice water (100 mL), a
saturated solution of Na2CO3 (100 mL), and 100 mL of H2O
successively. After drying the organic layer over Na2SO4 and
removal of solvent, the crude product was purified by silica gel
column chromatography (petroleum ether:Et2O 15:1). Optically
pure vinyl esters were synthesized in smaller amounts using
about 60-120 mg of carboxylic acid.
(()-2-P h en ylbu tyr ic Acid Eth yl Ester (()-4. Following the
general procedure 5.0 g of (()-10 (30.5 mmol) yielded 3.1 g of
(()-4 (16.1 mmol, 53%) as a colorless oil: 1H NMR (250.1 MHz;
CDCl3) δ 0.82 (t, J ) 7.4, 3H), 1.13 (t, J ) 7.1, 3H), 1.66-2.08
(m, 2H), 3.36 (t, J ) 7.7, 1H), 4.04 (m, 2H), 7.23 (m, 5H); 13C
NMR (62.9 MHz; CDCl3) δ 12.24, 14.22, 26.88, 53.63, 60.66,
127.16, 128.02, 128.58, 139.34, 174.14. Anal. Found: C, 74.96;
H, 8.54. Calcd for C12H16O2: C, 74.97; H, 8.39.
(R)-(-)-2-P h en ylbu tyr ic a cid vin yl ester (R)-(-)-1: 120
µL of (R)-(-)-10 (126.6 mg, 0.77 mmol) yielded 44 mg (R)-(-)-1
(R)-(-)-2-P h en ylbu tyr ic a cid eth yl ester (R)-(-)-4: 120
(0.23 mmol, 30%); [R]22 ) -23.9 (c 0.664, CHCl3).
µL of (R)-(-)-10 (126.6 mg, 0.77 mmol) yielded 80 mg of (R)-(-
D
(()-2-P h en ylp r op ion ic a cid vin yl ester (()-2: 420 µL of
(()-11 (460.7 mg, 3.07 mmol) yielded 210 mg of (()-2 (1.19 mmol,
39%): 1H NMR (500.1 MHz; CDCl3) δ 1.53 (d, J ) 7.2, 3H), 3.79
(q, J ) 7.1, 1H), 4.54 (d, J ) 6.18, 1H), 4.77 (d, J ) 14.0, 1H),
7.23-7.35 (m, 6H); 13C NMR (125.7 MHz; CDCl3) δ 18.41, 45.26,
97.92, 127.36, 127.52, 128.73, 139.71, 141.36, 171.59. Anal.
Found: C, 74.73; H, 6.93. Calcd for C11H12O2: C, 74.98; H, 6.86.
(R)-(-)-2-P h en ylp r op ion ic a cid vin yl ester (R)-(-)-2: 60
µL of (R)-(-)-11 (65.8 mg, 0.44 mmol) yielded 18 mg of (R)-(-
)-4 (0.42 mmol, 55%); [R]22 ) -65.7 (c 1.508, Et2O).
D
(()-2-P h en ylp r op ion ic a cid eth yl ester (()-5: 2.0 g of (()-
11 (13.3 mmol) yielded 1.1 g of (()-5 (6.2 mmol, 47%) as a
colorless oil: 1H NMR (250.1 MHz; CDCl3) δ 1.21 (t, J ) 7.1,
3H), 1.51 (d, J ) 7.2, 3H), 3.72 (q, J ) 7.2, 1H), 4.13 (m, 2H),
7.23-7.34 (m, 5H); 1C NMR (62.9 MHz; CDCl3; Me4Si) δ 14.19,
18.68, 45.64, 60.79, 127.13, 127.54, 128.65, 140.78, 174.63. Anal.
Found: C, 74.08; H, 8.01. Calcd for C11H14O2: C, 74.13; H, 7.92.
(R)-(-)-2-P h en ylp r op ion ic a cid eth yl ester (R)-(-)-5: 60
µL of (R)-(-)-11 (65.8 mg, 0.44 mmol) yielded 38 mg of (R)-
)-2 (0.10 mmol, 23%); [R]22 ) -34.6 (c 0.9, EtOH).
D
(()-3-P h en ylbu tyr ic a cid vin yl ester (()-3: 500 mg of (()-
12 (3.05 mmol) yielded 300 mg of (()-3 (1.58 mmol, 52%): 1H
NMR (250.1 MHz; CDCl3) δ 1.25 (d, J ) 7.0, 3H), 2.59 (m, 2H),
3.24 (m, 1H), 4.47 (dd, J ) 6.3, 1.5, 1H), 4.77 (dd, J ) 14.0, 1.5,
(-)-5 (0.21 mmol, 48%); [R]22 ) -60.0 (c 1.63, CHCl3).
D
(()-3-P h en ylbu tyr ic a cid eth yl ester (()-6: 5.0 g of (()-
12 (30.5 mmol) yielded 3.1 g of (()-6 (16.1 mmol, 53%) as a
colorless oil: 1H NMR (250.1 MHz; CDCl3) δ 1.19 (t, J ) 7.1,

1712 J . Org. Chem., Vol. 64, No. 5, 1999
Notes
3H), 1.32 (d, J ) 7.0, 3H), 2.58 (m, 2H), 3.30 (m, 1H), 4.09 (q, J
) 7.1, 2H), 7.17-7.36 (m, 5H); 13C NMR (62.9 MHz; CDCl3) δ
14.24, 21.88, 36.60, 43.08, 60.31, 126.45, 126.84, 128.54, 145.83,
172.46. Anal. Found: C, 74.89; H, 8.52. Calcd for C12H16O2: C,
74.97; H, 8.39.
Tr a n sester ifica tion of (()-1: 98.9 mg of (()-1 (0.52 mmol)
yielded after 68 h 49.7 mg of (R)-(-)-7 (0.20 mmol, 38%, 99% ee
([R]22 ) -29.0 (c 0.78, CHCl3))) and 43.8 mg of (S)-(+)-1 (0.23
D
mmol, 45%, 74% ee (GC)).
Tr a n sester ifica tion of (()-4: 100 mg of (()-4 (0.52 mmol)
yielded after 304 h 19.4 mg of (R)-(-)-7 (0.08 mmol, 15%, 66%
(S)-(+)-3-P h en ylbu tyr ic a cid eth yl ester (S)-(+)-6: 120 µL
ee ([R]22 ) -19.4 (c 0.680, CHCl3))) and 73.1 mg of (S)-(+)-4
of (S)-(+)-12 (128.3 mg, 0.78 mmol) yielded 60 mg of (S)-(+)-6
D
(0.31 mmol, 40%); [R]22 ) +21.4 (c 1.538, Et2O).
D
(0.38 mmol, 74%, 14% ee (GC)).
Gen er a l P r oced u r e for Syn th esis of Hexyl Ester s 7-9.
The carboxylic acid and toluene-4-sulfonic acid (20 mg) were
dissolved in a mixture of n-hexanol (3.5 mL) and toluene (50
mL) in a round-bottom flask. The flask was equipped with a
Dean-Stark trap and a condenser before the mixture was
refluxed. Reaction was continued until formation of water
stopped. The mixture was washed with ice water (30 mL), a
saturated Na2CO3 solution (30 mL), and water (30 mL). After
drying over Na2SO4 and evaporation of the solvent, the crude
product was purified by silica gel column chromatography
(petroleum ether:Et2O 40:1) and obtained as a colorless oil.
(()-2-P h en ylbu tyr ic a cid h exyl ester (()-7: 500 mg of (()-
10 (3.05 mmol) yielded 510 mg of (()-7 (2.05 mmol, 67%); 1H
NMR (250.1 MHz; CDCl3) δ 0.76-0.85 (m, 6H), 1.17 (m, 6H),
1.49 (t, J ) 6.7, 2H), 1.66-2.09 (m, 2H), 3.36 (t, J ) 7.7, 1H),
3.98 (dt, J ) 6.6, 1.8, 2H), 7.16-7.25 (m, 5H);13C NMR (62.9
MHz; CDCl3) δ 12.25, 14.02, 22.53, 25.52, 26.73, 28.59, 31.40,
53.70, 64.82, 127.15, 128.02, 128.56, 139.36, 141.32, 174.20. Anal.
Found: C, 77.54; H, 9.78. Calcd for C16H24O2: C, 77.38; H, 9.74.
(R)-(-)-2-P h en ylbu tyr ic a cid h exyl ester (R)-(-)-7: 120
µL of (R)-(-)-10 (126.6 mg, 0.77 mmol) yielded 61 mg of (R)-(-
Tr a n sester ifica tion of (()-2: 91.6 mg of (()-2 (0.52 mmol)
yielded after 1.3 h 51.7 mg of (R)-(-)-8 (0.21 mmol, 40%, 53%
ee ([R]22 ) -19.0 (c 0.435, CHCl3))) and 40.3 mg of (S)-(+)-2
D
(0.23 mmol, 44%, 75%ee (GC)).
Tr a n sester ifica tion of (()-5: 92.7 mg of (()-5 (0.52 mmol)
yielded after 44 h 36.6 mg of (R)-(-)-8 (0.16 mmol, 30%, 68% ee
([R]22 ) -24.2 (c 1.3, CHCl3))) and 44.5 mg of (S)-(+)-5 (0.25
D
mmol, 48%, 41% ee ([R]22 ) +24.6 (c 0.35, CHCl3)).
D
Tr a n sester ifica tion of (()-3: 98.9 mg of (()-3 (0.52 mmol)
yielded after 68 h 65.1 mg of (R)-(-)-9 (0.26 mmol, 50%, 31% ee
([R]22 ) -6.6 (c 2.39, CHCl3))) and 34.6 mg of (S)-(+)-3 (0.18
D
mmol, 35%, 86% ee ([R]22 ) +18.2 (c 0.372, CHCl3)).
D
Tr a n sester ifica tion of (()-6: 100 mg of (()-6 (0.52 mmol)
yielded after 720 h 13.0 mg of (R)-(-)-9 (0.05 mmol, 10%, 60%
ee ([R]22 ) -13.0 (c 0.680, CHCl3))) and 71.0 mg of (S)-(+)-6
D
(0.37 mmol, 71%, 22% ee ([R]22 ) +4.7 (c 0.458, CHCl3)).
D
Hyd r olysis of Eth yl Ester s (()-4-6. Hydrolysis reactions
were performed in a pH-stat system. In general 1 mmol of
substrate ((()-4-6) was added to 20 mL of sodium phosphate
buffer (50 mM, pH 7.5) at 40 °C. Then 200 mg (1000 U) of CAL-B
was added to start the reaction and the pH was kept constant
automatically with 0.1 N NaOH. After consumption of base
indicated the desired conversion, nonreacted substrate was
extracted from the mixture with heptane. Subsequently the
aqueous layer was saturated with NaCl and the pH was adjusted
to 3 by adding H2SO4 before it was extracted again three times
with EtOAc to obtain the free carboxylic acids (10-12). The
organic layers were dried over Na2SO4, and the enantiomeric
excess was determined by optical rotation and by chiral column
GC for compound (()-4.
)-7 (0.25 mmol, 32%); [R]22 ) -29.3 (c 1.438, CHCl3).
D
(()-2-P h en ylp r op ion ic a cid h exyl ester (()-8: 100 µL of
(()-11 (109.7 mg, 0.73 mmol) yielded 91 mg of (()-8 (0.39 mmol,
53%); 1H NMR (250.1 MHz; CDCl3) δ 0.78 (t, J ) 6.6, 3H), 1.14-
1.22 (m, 6H), 1.42 (d, J ) 7.2, 3H), 1.48 (t, J ) 6.8, 2H), 3.63 (q,
J ) 7.2, 1H), 3.97 (t, J ) 6.7, 2H), 7.15-7.25 (m, 5H); 13C NMR
(62.9 MHz; CDCl3) 14.00, 21.88, 22.52, 25.54, 28.55, 31.41, 36.56,
43.01, 64.50, 126.37, 126.75, 128.47, 145.75, 172.50. Anal.
Found: C, 76.93; H, 9.56. Calcd for C15H22O2: C, 76.88; H, 9.46.
(R)-(-)-2-P h en ylp r op ion ic a cid h exyl ester (R)-(-)-8: 60
µL of (R)-(-)-11 (65.8 mg, 0.44 mmol) yielded 39 mg of (R)-(-
Hyd r olysis of (()-4: 192.3 mg of (()-4 (1.00 mmol) yielded
after 1 h 55.8 mg of (R)-(-)-10 (0.34 mmol, 34%, 22% ee ([R]22
)-8 (0.17 mmol, 39%); [R]22 ) -35.8 (c 1.58, CHCl3).
D
D
) -20.5 (c 1.005, toluene; lit. [R]22 ) +93.0 (c 0.900, toluene)))
D
(()-3-P h en ylbu tyr ic a cid h exyl ester (()-9: 130 mg of (()-
12 (0.79 mmol) yielded 98 mg of (()-9 (0.39 mmol, 49%); 1H NMR
(250.1 MHz; CDCl3) δ 0.81 (t, J ) 6.7, 3H), 1.18-1.26 (m, 9H),
1.46 (t, J ) 6.8, 2H), 2.51 (m, 2H), 5.80 (m, 1H), 3.93 (t, J ) 6.7,
2H), 7.11-7.25 (m, 5H); 13C NMR (62.9 MHz; CDCl3) δ 14.00,
21.88, 22.52, 25.54, 28.55, 31.41, 36.56, 43.01, 64.50, 126.37,
126.75, 128.47, 145.75, 172.50. Anal. Found: C, 77.42; H, 9.80.
Calcd for C16H24O2: C, 77.38; H, 9.74.
and 76.9 mg of (S)-(+)-4 (0.40 mmol, 40%, 34% ee (GC)).
Hyd r olysis of (()-5: 178.2 mg of (()-5 (1.00 mmol) yielded
after 0.3 h 63.1 mg of (R)-(-)-11 (0.42 mmol, 42%, 28% ee ([R]22
D
) -20.4 (c 2.39, CHCl3; lit. [R]22D ) -72 (c 1.6; CHCl3)) and 92.7
mg of (S)-(+)-5 (0.52 mmol, 52%, 38% ee ([R]22D ) +22.8 (c 0.54,
CHCl3)).
Hyd r olysis of (()-6: 192.1 mg of (()-6 (1.00 mmol) yielded
after 3 h 83.7 mg of (R)-(-)-12 (0.51 mmol, 51%, 13% ee ([R]22
(R)-(-)-3-P h en yl-bu tyr ic Acid Hexyl Ester (R)-(-)-9: 120
D
) -9.2 (c 3.74, C6H6; lit. [R]22D ) -57 (c 1.0; C6H6)) and 57.7 mg
µL of (R)-(-)-12 (128.3 mg, 0.78 mmol) gave 78 mg of (R)-(-)-9
of (S)-(+)-6 (0.30 mmol, 30%, 93% ee ([R]22 ) +20.0 (c 0.5,
(0.31 mmol, 40%); [R]22 ) -21.6 (c 2.73, CHCl3).
D
D
Et2O)).
Lip a se-Ca ta lyzed Tr a n sester ifica tion . A total of 0.52
mmol of the substrate ((()-1-6) was dissolved in 5 mL of
toluene, 560 µL of n-hexanol (4.49 mmol) was added, and the
reaction was started by addition of 100 mg (500 U) of CAL-B.
The reaction mixture was stirred, and the temperature was
adjusted by use of an oil bath. Samples withdrawn from the
solution were diluted with toluene, lipase was removed by
centrifugation, and conversion was determined by GC using the
Optima 5 column. After termination of the reaction by filtration,
product and nonconverted substrate were separated by flash
column chromatography. Enantiomeric purities were calculated
from optical rotation values. In case of substrates (()-1, (()-2,
and (()-4 determination of optical purity was also feasible by
means of chiral column GC.
Ack n ow led gm en t. The authors are grateful for a
grant by the German Research Foundation (DFG, Bonn,
Germany) and a postdoctoral stipend for Dr. Hong Yang
provided by the Alexander v. Humboldt Foundation
(Bonn, Germany). We thank Prof. W. A. Ko¨nig (Uni-
versity of Hamburg, Germany) for providing the column
for chiral analysis by gas chromatography and Prof. R.
D. Schmid (Institute for Technical Biochemistry, Uni-
versity of Stuttgart, Germany) for useful discussions.
J O981780L