Selectivity of Candida rugosa lipase in simultaneous separation of skeletal isomers, desymmetrization, and kinetic racemate cleavage of 9-oxabicyclononanediols

Article abstract of DOI:10.1016/S0040-4039(02)02876-9

The diols 2 and 3, available in one step from cycloocta-1,5-diene, are selectively acetylated at the (R)-centers using Candida rugosa lipase to give the corresponding enantiopure compounds. In contrast, the (S,S)-enantiomer of 11 is transformed under iden

Full text of DOI:10.1016/S0040-4039(02)02876-9

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 44 (2003) 2225–2229
Selectivity of Candida rugosa lipase in simultaneous separation
of skeletal isomers, desymmetrization, and kinetic racemate
cleavage of 9-oxabicyclononanediols
Klaus Hegemann,a Holger Schimanski,a Udo Ho¨welerb and Gu¨nter Haufea,*
aOrganisch-Chemisches Institut, Westfa¨lische Wilhelms-Universita¨t Mu¨nster, Corrensstraße 40, D-48149 Mu¨nster, Germany
bCHEOPS, Zur Quelle 6, D-48341 Altenberge, Germany
Received 29 November 2002; accepted 11 December 2002
Abstract—The diols 2 and 3, available in one step from cycloocta-1,5-diene, are selectively acetylated at the (R)-centers using
Candida rugosa lipase to give the corresponding enantiopure compounds. In contrast, the (S,S)-enantiomer of 11 is transformed
under identical conditions. The stereoselectivity of the lipase has been investigated by modeling of the transition state geometries
at the active site of the enzyme. © 2003 Published by Elsevier Science Ltd.
Enantiopure diols, particularly C2-symmetric ones, gain
much interest as auxiliaries or sources for ligands of metal
catalysts for asymmetric synthesis.1 In several cases such
compounds have been synthesized with high stereoselec-
tivity by asymmetric alkylation of primary 1,n-alkanedi-
ols,2 or by catalytic asymmetric3 respective microbio-
logical reduction4 of the corresponding diketones. In such
reactions, in addition to the desired enantiopure diols,
frequently significant amounts of the meso-diastereomers
have been formed. An alternative pathway is the differ-
entiation of the stereoisomers by hydrolases. Applying
this protocol the separation of several meso- and racemic
secondary 1,n-diols was successful.5 In numerous cases
the selectivity of the applied enzymes was quite low or
very different for the meso- and the racemic diols. In these
cases the meso-compounds had to be separated prior to
cleavage of the racemate,6 or two subsequent enzyme-cat-
alyzed separations have been necessary.7
Scheme 1.
Keywords: bicyclic diols; desymmetrization; enantiospecificity; lipase-catalyzed acetylation; molecular modeling; racemate cleavage.
* Corresponding author. Tel.: +49-251-83-33281; fax: +49-251-83-39772; e-mail: haufe@uni-muenster.de
0040-4039/03/$ - see front matter © 2003 Published by Elsevier Science Ltd.
doi:10.1016/S0040-4039(02)02876-9

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K. Hegemann et al. / Tetrahedron Letters 44 (2003) 2225–2229
Figure 1. Time dependent conversion and development of the enantiomeric excesses of the products of CRL catalyzed acetylation
of meso-2 and rac-3.
In this letter, the first example of a lipase catalyzed
simultaneous separation of skeletal isomeric diols, with
kinetic resolution of the one and desymmetrization of
the other constitutional isomer is reported. After
screening of several lipases and esterases,8 Candida
rugosa lipase (CRL, Lipase Amano AY) has proved the
most efficient enzyme for the separation of the isomeric
diols 2 and 3. The selectivity of this enzyme was
investigated by means of molecular modeling of the
transition state geometries using the X-ray structure of
CRL, which has been determined some years ago.9
in hydrolysis of endo,endo-2,6-diacetoxybicyclo[3.3.1]-
nonane.16
Furthermore, it was most interesting to prove the enan-
tioselectivity of CRL in the acetylation of the
diastereomeric and also C2-symmetric exo,exo-9-oxabi-
cyclo[3.3.1]nonane-2,6-diol (11). This compound was
prepared using a principally known17 pathway. We
started the five-step synthesis from endo,endo-2,6-
diiodo-9-oxabicyclo[3.3.1]nonane (9)18 using a reductive
desulfuration of 2-oxa-6-thiatricyclo[3.3.1.13,7]decane-
4,8-diol (10) as the final step (Scheme 2).
The acetylation10 with vinyl acetate and CRL as the
biocatalyst of a 38:62 mixture of meso-2 and rac-3,
which were prepared according to known procedures11
by transannular O-heterocyclization of 1 with per-
formic acid, reached 67% conversion after 6 hours.
That means that the meso-compound 2 was trans-
formed almost quantitatively, while the conversion of
the racemate 3 stopped at 50% conversion. After sepa-
ration of the enzyme, (1S,2S,5S,6S)-(−)-312,13 (14%
yield, >98% ee), (1R,2R,5R,6R)-(+)-412,13 (10% yield,
>98% ee, 6% of meso-7 as an impurity) and 40% of a
68:32 mixture of (1R,2R,5S,6S)-(+)-512,13 (>98% ee) and
(1R,2R,5R,6R)-(+)-612,13 (80% ee) were isolated
(Scheme 1).
Using CRL and vinyl acetate, compound rac-10 has
been acetylated also (R)-selectively. The remaining diol
(1S,3S,4S,5S,7S,8S)-(+)-1013 showed >98% ee, while
the monoacetate (1R,3R,4R,5R,7R,8R)-1213 had 34%
ee. For the diacetate (1R,3R,4R,5R,7R,8R)-(−)-13, 92%
ee was detected.13 Thus, this tricyclic dihetero adaman-
tane derivative was also acetylated according to
Kazlauskas’ rule. The CH2-CH-S- moiety is larger com-
pared to the CH2-CH-O- group and has also the higher
priority in terms of the CIP rule (Scheme 2).
Contrary to all reactions mentioned above, the CRL
catalyzed acetylation of rac-11 was faster at the OH-
groups attached to (S)-centers. The enantiomeric excess
of the remaining diol, (1S,2R,5S,6R)-(+)-11,12,13 was
determined to 94% ee, while the monoacetate
(1R,2S,5R,6S)-(−)-1413 had only 30% ee and the di-
acetate (1R,2S,5R,6S)-(−)-1513 showed 92% ee (Scheme
3).
The conversion of 2 and 3 and the time dependent
development of the enantiomeric excesses of the prod-
ucts are shown in Figure 1. For this experiment a
slightly less active, but equally selective lipase prepara-
tion was used in order to guarantee the timely work up
and analysis of the ratio of enantiomers.
As predicted by Kazlauskas’ rule14 in transformation of
secondary alcohols or their derivatives with CRL, the
enantiomer shown in Figure 2 was transferred pre-
ferred, which is the (R)-enantiomer in case that the
larger substituent has also the higher priority according
to the Cahn–Ingold–Prelog (CIP) rule.15 This is true for
the acetylation of 2 and 3, and was observed earlier also
Figure 2. Enantiomer preferably transferred by lipases
according to Kazlauskas’ rule (adapted to the diols).14b

K. Hegemann et al. / Tetrahedron Letters 44 (2003) 2225–2229
2227
Scheme 2. Synthesis and lipase-catalyzed cleavage of racemic 2-oxa-6-thiaadamantane-4,8-diol (10) and synthesis of exo,exo-9-
oxabicyclo[3.3.1]nonane-2,6-diol (11).
Scheme 3.
Experimental
Closer examination of the most stable conformers cal-
culated semiempirically (AM1)19 of the five diols,
makes their very similar geometry obvious (Fig. 3).
Apparently, one enantiomer of these compounds can fit
perfectly into the active site of the enzyme, even when
the absolute configuration of the OH-bearing centers of
diastereomeric compounds is different (cases of
(1R,2R,5R,6R)-3 and (1R,2S,5R,6S)-11, respectively).
A 38:62 mixture of compounds 2 and 3 (10.0 g, 63.3
mmol) in vinyl acetate (250 ml) was treated with CRL
(2.5 g) and stirred at 20°C for 6 h (67% conversion).
The lipase was filtered off using a short column with
silica gel and vinyl acetate was removed in vacuo. The
residue was dissolved in ethyl acetate (60 mL) and
successively treated with n-pentane until no more of the
non-reacted diol (−)-3 precipitated. After separation of
(−)-3 the solvent was evaporated and the residue was
The known X-ray crystal structures of CRL9 with and
without (1R)- and (1S)-menthylhexyl phosphonate as
transition state analogues have been edited for
modeling20–22 and used for the construction of the
respective structures of the acetylation of 2 and of the
enantiomers of 3.23,24
Using the energetically most stable geometries of each
pair of enantiomers (or the enantiotopic groups of 2)
relative energies have been calculated. The energy dif-
ference for the model substrate menthol amounts 3.3
kcal/mol, while for the acetylation of the first OH
group of endo,endo-2 2.6 kcal/mol and of endo,endo-3
0.9 kcal/mol were calculated in favor of the respective
(R)-enantiomers (Fig. 4).
These force field calculations using a quite simple
model of the active site of the enzyme (isolated protein,
space-restricted optimization, no solvent), are therefore
suitable for the determination of the absolute configu-
ration of the preferably acetylated isomers of the model
substrate menthol9 as well as for the polycyclic diols 2
and 3.
Figure 3. Most stable conformers of diol enantiomers trans-
formed by Candida rugosa lipase. (a) (+)-3; (b) (−)-10); (c)
(1S,2R,5S,6R)-(+)-bicyclo[3.3.1]nonane-2,6-diol; (d) (−)-11;
(e) meso-2.

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K. Hegemann et al. / Tetrahedron Letters 44 (2003) 2225–2229
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Figure 4. Arrangement of transition state analogous phos-
phate of (1R,2R,5R,6R)-(+)-3 at the active site of Candida
rugosa lipase.
10. For an excellent review on enzyme-catalyzed syntheses in
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2226–2254.
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chromatographed (silica gel, cyclohexane/ethyl acetate,
1:1) in order to separate compounds (+)-5 and (+)-6
from (+)-4. The diols rac-10 and rac-11 were treated
analogously.
12. The determination of the absolute configuration by X-ray
crystal structure analysis of these compounds will be
reported in detail elsewhere (Hegemann, K.; Fro¨hlich, R.;
Haufe, G., in preparation).
Acknowledgements
The generous support by the ‘Deutsche Forschungs-
gemeinschaft’ and the ‘Fonds der Chemischen Indus-
trie’ is gratefully acknowledged.
13. The spectroscopic data of the isolated optically active
compounds agreed with those observed for the racemic
ones 3,11c 4,11c 5,11c 6,11c 10,17b 11,11c 1317b and 15.11c The
enantiomeric excesses, if not stated otherwise, have been
detemined, by chiral GC using a chiral b-Dex™ 120
phase, 170°C, isothermic. (1S,2S,5S,6S)-(−)-3: >98% ee,
[h]2D0=−45.8 (c 1, ethanol). (1R,2R,5S,6S)-(+)-5: >98% ee
(1H shift spectrum with 100 mol% of Eu(hfc)3), mixture
with (1R,2R,5R,6R)-(+)-6: 80% ee (1H shift spectrum
with 100 mol% of Eu(hfc)3). [h]2D0 of the 68:32 mixture=
+31.4 (c 1, ethyl acetate). (1S,2S,5S,6S)-(−)-4: Prepared
from (1S,2S,5S,6S)-(−)-3 by esterification with acetic
anhydride in pyridine; >98% ee (GC, after hydrolysis),
[h]2D0=−53.7 (c 1, ethyl acetate). (1S,3S,4S,5S,7S,8S)-(+)-
10: >98% ee (GC, 190°C), [h]2D0=+22.5 (c 1, ethanol).
(1S,2R,5S,6R)-(+)-11: 94% ee, [h]2D0=+22.5 (c 1, ethanol).
(1R,3R,4R,5R,7R,8R)-(−)-12: mp 144°C (ethyl acetate),
34% ee (GC, 190°C, after hydrolysis), [h]2D0=−17.6 (c 1,
ethyl acetate). (1R,3R,4R,5R,7R,8R)-(−)-13: 92% ee (GC,
190°C, after hydrolysis), [h]2D0=−55.9 (c 1, ethyl acetate).
(1R,2S,5R,6S)-(−)-14: mp 70–71°C (ethyl acetate), 30% ee
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