Conformational effects on lipase-mediated acylations of 2-substituted cyclohexanols

Article abstract of DOI:10.1016/S0040-4039(03)01514-4

Lipase-mediated acetylations of trans- and cis-2-substituted cyclohexanols gave the corresponding (1R)-cyclohexyl acetates and (1S)-cyclohexanols in high yields and ee, but c-4-tert-butyl-c-2-ethenyl-r-1-cyclohexanol was unreactive owing to the steric interaction between the axial OH group and the axial H atoms at the 3- and 5-positions. In the cis-isomer the OH group occupies an equatorial position to bind to the lipase, and less bulky axial alkenyl and alkynyl groups might not so much prevent acetylations than an alkyl group.

Full text of DOI:10.1016/S0040-4039(03)01514-4

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 44 (2003) 6781–6783
Conformational effects on lipase-mediated acylations of
2-substituted cyclohexanols
Rikuhei Tanikaga,* Yoshimasa Matsumoto, Maki Sakaguchi, Yohei Koyama and Kentaro Ono
Department of Bioscience and Biotechnology, Faculty of Science and Engineering, Ritsumeikan University, Nojihigashi 1-1-1,
Kusatsu, Shiga 525-8577, Japan
Received 9 May 2003; revised 9 June 2003; accepted 13 June 2003
Abstract—Lipase-mediated acetylations of trans- and cis-2-substituted cyclohexanols gave the corresponding (1R)-cyclohexyl
acetates and (1S)-cyclohexanols in high yields and ee, but c-4-tert-butyl-c-2-ethenyl-r-1-cyclohexanol was unreactive owing to the
steric interaction between the axial OH group and the axial H atoms at the 3- and 5-positions. In the cis-isomer the OH group
occupies an equatorial position to bind to the lipase, and less bulky axial alkenyl and alkynyl groups might not so much prevent
acetylations than an alkyl group.
© 2003 Elsevier Ltd. All rights reserved.
Lipase PS (lipase from Pseudomonas cepacia) is one of
the most frequently utilized enzymes in organic synthe-
sis because it is inexpensive, and it has high enantio-
selectivity and a broad substrate specificity.1–4 Some
lipases including lipase PS are well known to react with
(R)-alcohols faster to give the corresponding acetates
for secondary alcohols.5,6 To comprehend the mecha-
nistic aspects, many active site models were reported, 6–10
and hence it seems to be favorable for a binding model
to use a compound having a rigid conformation.
Although some examples of lipase-mediated resolutions
of cyclohexanols have been reported,6,10–13 the relation-
ship between reactivities and conformations including
ring inversion had not been discussed. Here, we report
conformational effects on the lipase-mediated acetyla-
tions of 2-substituted cyclohexanols 1–5 as a reasonable
model.
chromatography on silica gel afforded diastereoisomers
3b–5b, but no c-4-tert-butyl-t-2-ethenyl-r-1-cyclohex-
anol was obtained.
The structures of these diastereoisomers were deduced
by the synthetic mechanism, and also confirmed by a
combination of H–C COSY measurements and the
detections of large coupling constants J between an
axial H(1) and a neighboring axial H(2) in the NMR
spectra of 1a–e, 3b.14
In a typical experiment, a suspension of 2-substituted
cyclohexanols 1–5 (2.0 mmol), lipase PS (Amano) (0.5
,
g), vinyl acetate (2.0 mmol), and molecular sieves 4 A
(2.0 g) in toluene (20 ml) was stirred at 30°C. In order
to learn the conversion, samples were withdrawn at
intervals, and the mixture was kept stirred until about
50% conversion. Thereafter the enzyme was filtered off,
and the filtrate was found to contain only cyclohexyl
acetates 6–10 and 1–5. After concentration in vacuo the
residue was purified by chromatography on silica gel to
afford (1R)-cyclohexyl acetates R-6–10 and (1S)-alco-
hols S-1–5. Treatments of S-1–5 with acetic anhydride
(2 equiv.), 4-(dimethylamino)pyridine (2 equiv.) in ace-
tonitrile (10 ml) provided the corresponding (1S)-
acetates S-6–10 nearly quantitatively.
The trans-2-substituted cyclohexanols 1 were prepared
by the ring-opening reaction of cyclohexene oxide using
RLi or RMgBr, while the cis-isomers 2 were synthe-
sized by the reduction of the corresponding cyclohex-
anones with diisobutylaluminium hydride, followed by
purification with silica gel column. The reduction of
4-tert-butyl-2-ethenylcyclohexanone with NaBH4 gave
a diastereoisomeric mixture of 4-tert-butyl-2-ethenylcy-
clohexanols, and the subsequent separation by column
The ee values of S-1–5 and S-6–10 were determined by
HPLC equipped with a chiral column (Daicel chiralcel
OD-H) or capillary GC with Chrompack WCOT. The
absolute structures of the products were assigned
according to the empirical rule,5,6 and were further
Keywords: lipase; acylation; 2-alkenylcyclohexanol; 2-alkynylcyclo-
hexanol.
* Corresponding author: Tel.: +81-077-561-2766; fax: +81-077-561-
2659; e-mail: tanikaga@se.ritsumei.ac.jp
0040-4039/$ - see front matter © 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/S0040-4039(03)01514-4

6782
R. Tanikaga et al. / Tetrahedron Letters 44 (2003) 6781–6783
confirmed on the basis of the octant rule in the CD
spectra15 of the corresponding cyclohexanones, which
were obtained by Swern oxidation of a OH group and
the reduction of CꢀC and CꢁC group using NaBH4–
CoCl3.16 These results are given in Table 1.
in 57:43 ratio. The compound numbers 2-eq and 2-ax
mean that the compound 2 has the conformers contain-
ing an equatorial and an axial OH group, respectively,
and the conformational inversion of 2-eq and 2-ax
rapidly occurs at a room temperature. The NMR spec-
trum showed one singlet proton peak in the area of a
ꢁCH group at a room temperature, but at −99°C the
spectrum revealed two distinct peaks in that area. The
peak at the lower field is characteristic of a ꢁCH proton
at an equatorial position and the relative area of those
two peaks was 62:38 in favor of 2c-eq. The other
2-eq/2-ax ratios predicted by MM calculations are as
follows; 2a; 9:91, 2b; 17:83, 2d; 5:95, and 2e; 61:39
(69:31 found in NMR spectrum at −99°C).
The trans-cyclohexanols 1 and 3b were found to be
more reactive than the cis-isomers 2 and 5b, (Run 1–5,
11, 6–10, and 13), and the substituent effects in 1a–c are
opposite to those in 2a–c (Run 1“3 and 6“8). Inter-
estingly, the cis-isomer 4b containing an axial OH
group was unreactive even after 600 h, because a bind-
ing of the axial OH group to the enzyme might be
sterically hindered by the presence of two axial H
atoms at the 3 and 5 positions (Run 12). On the other
hand, in the other cis-isomer 5b with an equatorial OH
group an acetylation took place, though not so
efficiently probably owing to a bulky 4-tert-butyl group
(Run 13).
Cyclohexanol 4b is unreactive to LPS because a 4-tert-
butyl group ensures that the equilibrium lies heavily to
the side having the tert-butyl group equatorial, that is,
a hydroxy group axial. This finding strongly suggests
that the conformer 2-ax cannot react with the enzyme.
In the cis isomers 2 a hydroxy group occupies an
equatorial position to bind to the enzyme, and one
enantiomer of racemic 2-eq undergoes the acetylation
Molecular mechanics calculation of 2c indicates that
the conformer 2c-eq having the OH group at the equa-
torial position is more stable than the other one 2c-ax
Table 1. Lipase-mediated acylations of cyclohexanols 1–5a
Run
Substrates
Reaction time (h)
Conversion (%)
e.e. (yield%)b
E
R-6–10
S-1–5
1
2
3
4
5
6
7
8
1a
1b
1c
1d
1e
2a
2b
2c
2d
2e
3b
4b
5b
7
9
12
8
13
46
22
6
51
104
48
600
102
43
42
42
40
44
43
44
42
43
42
14
0c
99 (40)
99 (40)
99 (37)
99 (37)
94 (42)
96 (39)
99 (38)
99 (40)
99 (39)
97 (32)
82 (11)
–
75 (49)
72 (48)
72 (49)
78 (56)
79 (56)
74 (49)
78 (45)
73 (48)
79 (50)
70 (64)
13 (64)
–
>200
>200
>200
>200
>200
111
>200
>200
>200
141
11
–
9
10
11
12
13
33
99 (26)
48 (64)
50
a Vinyl acetate, LPS, molecular sieves 4A, toluene, 30°C.
b Isolated yield.
c No reaction.

R. Tanikaga et al. / Tetrahedron Letters 44 (2003) 6781–6783
6783
Scheme 1. Acylations of cis-2-substituted cyclohexanols 2.
Scheme 2. Reagents and conditions: (a) NaOH, EtOH–H2O, rt (98%); (b) N-iodosuccinimide, MeCN, rt (52+44%); (c) 1,1%-thiocar-
bonydiimidazole, THF, reflux (73%); (d) Et3B, Bu3SnH, toluene, −78°C, then aq. HCl (78%).
to give R-7-eq as shown in Scheme 1. Less bulky axial
2-alkenyl and 2-alkynyl groups in 2-eq do not much
prevent acetylations than an alkyl group (Run 7 and 8).
This explains the tendency for the reactivities of 2a, 2b,
and 2c, though completely enough to exclude the stabil-
ity by p-electrons.
6. Cygler, M.; Grochulski, P.; Kazlaukas, R. J.; Schrag, J.
D.; Bouthillier, F.; Rubin, B.; Serreqi, A. N.; Gupta, A.
K. J. Am. Chem. Soc. 1994, 116, 3180–3186.
7. Toone, E. J.; Werth, M. J.; Jones, J. B. J. Am. Chem.
Soc. 1990, 112, 4946–4952.
8. Naemura, K.; Fukuda, R.; Murata, M.; Konishi, M.;
Hirose, K.; Tobe, Y. Tetrahedron: Asymmetry 1995, 6,
2385–2394.
9. Nishizawa, K.; Ohgami, Y.; Matsuo, N.; Kishida, H.;
Hirohara, H. J. Chem. Soc., Perkin Trans. 2 1997, 1293–
1298.
10. Ema, T.; Kobayashi, J.; Maeno, S.; Sakai, T.; Utaka, M.
Bull. Chem. Soc. Jpn. 1998, 71, 443–453.
These nonracemic chiral 2-alkenyl- and 2-alkynyl-cyclo-
hexanols can be utilized in a wide variety of synthetic
reactions, for example, formation of oxygen heterocy-
cles.17 18 The examples of ionic and radical cyclizations
using R-6f and R-6g obtained by the above method
(99% ee, E=>200) are given in Scheme 2.
11. Whitesell, J. K.; Chen, H.-H.; Lawrence, R. M. J. Org.
Chem. 1985, 50, 4663–4665.
12. Esser, P.; Buschmann, H.; Meyer-Stork, M.; Scharf, H.-
D. Angew. Chem., Int. Ed. Engl. 1992, 31, 1190–1192.
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Asymmetry 1996, 7, 1649–1658.
Further details including the stereochemistry of these
reactions will be reported in the near future.
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