Method for producing optically active endo-2-norborneols

The present invention provides a method for producing optically active endo-2-norborneols, represented by the following formula: ##STR1## and the antipodes, it comprises asymmetrically hydrolysing racemic endo-2-acyloxynorbornane or endo-2-acyloxynorbornane, whose (R)-compound is in excess, having an optical purity of 50-95% ee with lipase derived from Candida genus. The optically active endo-2-norborneol useful for synthesizing intermediates of pharmaceutical preparations in large quantities.

BACKGROUND OF THE INVENTION 
The present invention relates to a method for producing efficiently 
optically active endo-2-norborneols useful as synthetic intermediates of 
pharmaceutical preparations and the like. 
In recent years, it has been earnestly required to synthesize physiological 
active materials of pharmaceutical preparations as optically active 
compounds. When these materials have optical isomers, several isomers have 
different active properties. One isomer exhibits strong activity and the 
other isomer exhibits little activity or exhibits undesirable toxicity. 
When physiologically active materials are synthesized for pharmaceutical 
preparations, therefore, it is desired to selectively prepare optical 
isomers having preferable steric configuration in view of a fully 
developed physiological activity and safety. 
Hitherto, to obtain optically active endo-2-norborneols, the following 
methods have been reported: (1) a method for optically resolving racemic 
endo-2-norborneols by a diastereomer method (Winstein et al., J. Am. Chem. 
Soc., 74, 1147 (1952)), (2) a method for optically resolving racemic 
2-norbornanone with a microorganism by stereoselective reduction (Nakazaki 
et al., J. Org. Chem., 45, 4432 (1980) ), (3) a method for 
stereoselectively reducing racemic 2-norbornanone with alcohol 
dehydrogenase of horse liver (Jones et al., J. Am. Chem. Soc., 98, 8476 
(1976)), (4) a method for stereoselectively acylating racemic 
endo-2-norborneol in the presence of lipase of pig spleen (Saccomano et 
al., Tetrahedron Lett., 33, 1201 (1992)), (5) a method of stereoselective 
transesterification of racemic endo-2-acetoxynorbornane in the presence of 
the lipase derived from Candida cylindracea (Macfarlane et al., J. Chem. 
Soc. Perkin. Trans., 1, 2287 (1993)), (6) a method for stereoselectively 
acylating racemic-endo-2-norborneol in the presence of lipase derived from 
Pseudomonas (Naemura et al., Bull. Chem. Soc. Jpn., 66, 573 (1993) ), (7) 
a method for stereoselectively hydrolyzing racemic 
endo-2-acetoxynorbornane in the presence of the lipase derived from 
Candida cylindracea (Brackenridge et al., J. Chem. Soc. Perkin. Trans., 1, 
1093 (1993) and the like. 
The method of (1) however is not efficient because recrystallization should 
be repeated to increase the optical purities of the resulting compounds. 
In the method of (2), further, it is difficult to obtain strains to be 
used and the substrate concentration is very low against much charge stock 
(0.075 w/v %). The method of (3) is impractical because it is difficult to 
obtain alcohol dehydrogenase and NAD of a coenzyme, and the yield is low 
in the asymmetric reaction. In the method of (4), a side reaction easily 
occurs and the water content of the reaction system should be decreased 
precisely to prevent the side reaction. Since the enzyme should be added 
several times during the reaction, the operation is very troublesome. The 
method is industrially disadvantageous because diethylether, which is very 
flammable as a reaction solvent, should be used. In the method of (5), it 
is difficult to obtain the ester of starting materials and the low optical 
purity of the product is insufficiently 32% ee. In the method of (6), the 
optical purity of the product is also very low and it is 63%. The water 
content of the reaction system should be decreased to prevent the side 
reaction. The stability of isopropenyl acetate of an acylation agent is 
not enough and it is difficult to obtain the compound in large quantities. 
Although the method of (7) is very close to the method of the present 
invention, it does not disclose enough the reaction conditions and the 
optical purity of the product is very low 60% ee. 
These conventional methods have been not perfect in practical use at the 
industrial level. 
SUMMARY OF THE INVENTION 
The present invention aims to overcome the problems of said conventional 
methods and to provide a method for efficiently producing in high optical 
purities and in large quantities optically active endo-2-norborneol of 
intermediates useful for synthesizing pharmaceutical preparations by a 
simple process. 
The present invention has the following features to overcome the above 
problems. 
One of the features is a method for producing optically active 
endo-2-norborneols, characterized in that it comprises asymmetrically 
hydrolysing racemic endo-2-acyloxynorbornane represented by formula (1): 
##STR2## 
(wherein R is straight or branched alkyl of 1-22 carbon atoms, alkenyl of 
1-22 carbon atoms, cycloalkyl, aralkyl or aryl group) with lipase derived 
from Candida genus, obtaining a mixture of (R)-endo-2-norborneol 
represented by formula (2): 
##STR3## 
and (S)-endo-2-acyloxynorbornane represented by formula (3): 
##STR4## 
and isolating these compounds. 
Preferably, above (S)-endo-2-acyloxynorbornane represented by formula (3) 
is further hydrolyzed under acidic or basic conditions to obtain 
(S)-endo-2-norborneol represented by formula (4): 
##STR5## 
Another is a method for producing (R)-endo-2-norborneol having an optical 
purity of more than 95% ee and represented by formula (2): 
##STR6## 
by asymmetrically hydrolyzing endo-2-acyloxynorbornane whose (R)-compound 
is in excess, having an optical purity of 50-95% ee and represented by 
formula (5): 
##STR7## 
wherein R is straight or branched alkyl of 1-22 carbon atoms, alkenyl of 
1-22 carbon atoms, cycloalkyl, aralkyl or aryl group with lipase derived 
from Candida genus. By using this method, (R)-endo-2-norborneol having a 
high purity can be obtained. 
DETAILED DESCRIPTION OF THE INVENTION 
The above mentioned lipase is preferably derived from Candida antarctica. 
The lipase is preferably immobilized on a carrier. 
When the lipase is immobilized on the carrier, further, lipase derived from 
Candida antarctica is preferred. 
As the carrier for immobilizing the lipase, it is selected from the group 
consisting of porous polypropylene, porous acryl resin, polysaccharide gel 
modified with a formyl or epoxy group on the surface, acryl resin modified 
with a formyl or epoxy group on the surface, and polymer gel of a silicone 
type modified with a formyl or epoxy group. 
In the production method of the present invention, racemic 
endo-2-acyloxynorbornane represented by formula (1) or 
endo-2-acyloxynorbornane represented by formula (5) having (R)-compound in 
excess and having an optical purity of 50-95% ee is used as the starting 
material, and R in the formula is selected from straight or branched alkyl 
of 1-22 carbon atoms, alkenyl of 1-22 carbon atoms, cycloalkyl, aralkyl or 
aryl group. Most preferably, methyl, ethyl, n-propyl, n-butyl and n-pentyl 
can be exemplified as straight alkyl groups; i-propyl, sec-butyl, t-butyl 
and neopentyl can be exemplified as the branched alkyl groups; vinyl and 
2-methyl vinyl can be exemplified as the alkenyl groups; and cyclohexyl as 
the cycloalkyl group, phenyl as the aryl group and benzyl as the aralkyl 
can be exemplified, respectively. Most preferable ones are ethyl, n-pentyl 
or benzyl. 
The said racemic endo-2-acyloxynorbornane represented by formula (1) can be 
obtained by acylation of racemic endo-2-norbornanol by using a well-known 
acylation method, for example, in the presence of a base such as pyridine 
in a nonprotonic solvent with acyl chloride. 
The endo-2-acyloxynorbornane represented by formula (5) can be, further, 
obtained by a well-known acylation method: acylation of 
endo-2-norbornanol, which has an optical purity of 50-95% ee and 
(R)-compound in excess, with acyl chloride in nonprotic solvent in the 
presence of a base such as pyridine, or stereoselectively acylation of 
racemic endo-2-norbornanol in the presence of lipase (for example, 
Saccomano et al., Tetrahedron Lett., 33, 1201 (1992)). 
The production method of the present invention is conducted by mixing with 
stirring racemic endo-2-acyloxynorbornane represented by formula (1) or 
endo-2-acyloxynorbornane represented by formula (5) with lipase in a 
solvent for hydrolyzing reaction. 
The reaction temperature for hydrolyzing the compound is suitably from 
0.degree. C. to 100.degree. C. in general, preferably from 10.degree. C. 
to 50.degree. C. If the reaction temperature is 0.degree. C. and below, it 
becomes difficult for the reaction to proceed. If it is 100.degree. C. and 
over, a deactivation phenomenon of the lipase appears and the reaction is 
undesirably retarded. 
The reaction time is suitably one to 1000 hours in general, preferably one 
to 200 hours. The reaction time may be changed by the kind of substrates 
and the reaction temperature. If the reaction time is too short, the 
conversion may be insufficient. If the reaction time is too long, it is 
undesired from the reaction efficiency. 
The amount of lipase is suitably 0.1 to 500% by weight of the substrate in 
general, preferably one to 100% by weight. If the amount is 0.1% by weight 
and below, it is difficult for the reaction to proceed. If the amount is 
500% and over, the stirring operation becomes difficult by increasing the 
viscosity of the reaction liquid. In addition to operation problems such 
as difficulty of isolation of the object products, the lipase is used too 
much to catalyze the reaction, and such problems are economically 
undesirable. 
As the kind of lipase, the lipase derived from Candida is suitable for 
catalyzing the hydrolytic reaction to act on racemic 
endo-2-acyloxynorbornane, and the lipase derived from Candida antarctica 
is more preferred. An embodiment, the lipase commercially available in the 
trade name of Novozym 435 or SP 525 (both are manufactured by Novo 
Nordisc) can be exemplified. 
After using the lipase, the lipase may be used again. For such reuse, 
immobilized lipase is efficiently used. 
Immobilized lipase commercially available is used as it is, or lipase not 
immobilized can be used after it is immobilized by the following method. 
Namely, the latter lipase is dispersed in ion exchange water, distilled 
water or a buffer solution and then a carrier is added. When the carrier 
is an adsorption type, the lipase is adsorbed and immobilized on it. When 
the carrier is a covalent linkage type, the lipase is reacted with a 
functional group on the carrier, if necessary, stabilized by a method such 
as reduction of the reaction part, and immobilized on the carrier. 
As the above carrier, powder or granules of polymer materials such as 
polypropylene, acryl resins, polysaccharide gel or silicone resin can be 
exemplified. The carrier of an adsorption type may have multi-cellular 
structure, and the carrier of a covalent bond type may have the surface 
activated with cyanogen bromide or modified with formyl, epoxy, carboxyl, 
amino or the like. 
The carrier of an adsorption type, especially, may be preferably produced 
with polymer materials such as polypropylene and acryl resins. The carrier 
of a covalent bond type, particularly, acryl resin, polysaccharide gel and 
silicone resin having the surface modified with formyl or epoxy can be 
preferably obtained. An embodiment of the polysaccharide gel modified with 
formyl is Formyl-cellulofine (Trade name, manufactured by CHISSO 
CORPORATION). 
After the hydrolysis reaction, the immobilized lipase is recovered by 
filtration, decantation or the like from the reaction system and the 
lipase may be used as it is in the next reaction. 
Ion exchange water, distilled water or buffer solution is suitably used as 
solvent. An organic solvent miscible with the above solvent such as 
acetone, N,N-dimethylformamide, dimethylsulfoxide and alcohol can be used 
as compatible solvent. The amount of the solvent is 0.1 to 100 times by 
weight of racemic endo-2-acyloxynorbornane (1) which is a substrate, 
preferably one to 10 times by weight when the reaction efficiency is 
considered. 
Since organic acids are liberated from the mixture with the reaction, it is 
necessary to maintain an optimum pH of lipase in the reaction system. For 
that purpose, a buffer solution adjusted to optimum pH may be used, or the 
optimum pH in the system may be maintained by dropping a solution of 
sodium hydroxide. 
After the hydrolysis reaction, the resulting mixture of 
(R)-endo-2-norborneol and (S)-endo-2-acyloxynorbornane is extracted with 
organic solvent, the extract solution is concentrated, and the concentrate 
is treated by distillation or column chromatography to isolate 
(R)-endo-2-norborneol and (S)-endo-2-acyloxynorbornane, respectively. The 
resulting (S)-endo-2-acyloxynorbornane is treated by hydrolysis under 
acidic or basic conditions to obtain optically active 
(S)-endo-2-norborneol. 
The optically active endo-2-norborneol obtained in the present invention is 
useful as an intermediate for synthesizing medical supplies. 
The resulting (R)-(+)-endo-2-norborneol (2) can be led, for example, via 
the following steps to a pyrimidone derivative (9) useful as medicines for 
treating asthma, bronchitis, dermatitis and the like. 
##STR8## 
Namely, (R)-(+)-endo-2-norborneol (2) in tetrahydrofuran is reacted under 
reflux with 3-hydroxy-4-methoxybenzaldehyde in the presence of diethyl 
azodicarboxlate and triphenyl phosphine to obtain 
3-[(2S)-exo-bicyclo[2.2.1]hepto-2-yloxy]-4-methoxybenzaldehyde (6), then 
via three steps (as shown in EPO 428 313 A), and the pyrimidone derivative 
(9) is derived. 
The above-mentioned (R)-(+)-endo-2-norborneol (2) can be led to thromboxane 
A2 receptor anthagonist (14) useful for an anticoagulant via the following 
steps. 
##STR9## 
(R)-(+)-endo-2-norborneol (2) in acetone is reacted with a Jones reagent 
previously prepared from sulfuric acid and chromium trioxide under cooling 
conditions to obtain (R)-(+)-2-norbornanone (10), then via several steps 
(as shown in a method of Narisada et al., J. Med. Chem., 31, 1847 (1988)), 
and the desired thromboxane A2 receptor anthagonist (14) is derived. 
The advantages of the invention are as follows: 
(1) Optically active endo-2-norborneol can be obtained at high optical 
purities (more than 76% ee). 
(2) When the lipase is immobilized, it can be recovered and reused. 
(3) The production method of the present invention is based on hydrolysis 
reaction so that it can be conducted in an open system without considering 
contamination of moisture. 
(4) The reaction process can be conducted at moderate temperatures (about 
room temperature).

DESCRIPTION OF PREFERRED EMBODIMENTS 
The following examples and comparative examples illustrate the present 
invention more specifically, but these will not always be precise in 
practical applications. 
Optical purities of optically active endo-2-norborneol in these examples 
are determined by analysis of capillary gas chromatography (Trade name: 
.beta.-DEX (Trademark) 120; manufactured by Sperco). 
EXAMPLE 1 
Racemic endo-2-propionyloxy norbornane 1.7 g (10 mmol), lipase (SP 435, 
origin: Candida antarctica, manufactured by Navo Nordisc Company) 0.25 g 
and a 0.7M phosphoric buffer solution (pH 7.0) 30 ml were mixed with 
stirring for 160 hours at 35.degree. C. Then, the conversion of the 
racemic compound was 37%. The lipase was filtered from the reaction 
solution. The filtrate was extracted with ethyl acetate, the extract was 
dried over magnesium sulfate and the solvent was distilled off under 
reduced pressure. The residue was subjected to a chromatograph over silica 
gel to obtain (R)-endo-2-norborneol (0.39 mg, 3.5 mmol, 35%). After the 
compound was subjected to the said chromatograph, the optical purity was 
93% ee. 
EXAMPLE 2 
The same procedure as in Example 1 was repeated and a mixture was prepared 
except that SP 525 (origin: Candida antarctica, manufactured by Novo 
Nordisc Company) was used instead of Novozym 435, and the mixture was 
stirred for 110 hours. Then, the conversion was 47%. The same steps as in 
Example 1 were conducted from the filtration of the lipase to the 
chromatography of the product to obtain (R)-endo-2-norborneol (0.50 g, 4.3 
mmol, 43%). After the compound was subjected to the said chromatograph, 
the optical purity was 88% ee. 
EXAMPLE 3 
Racetalc endo-2-norbornane phenyl acetate 2.3 g (10 mmol ), lipase (Trade 
name: type VII, origin: Candida cylindracea, manufactured by Sigma 
Company) 0.25 g and a 0.7M phosphoric buffer solution (pH 7.0) 30 ml were 
mixed with stirring for 264 hours at 35.degree. C. Then, the conversion of 
the racemic compound was 16%. The lipase was filtered off. The filtrate 
was extracted with ethyl acetate, the extract was dried over magnesium 
sulfate and the solvent was distilled off under reduced pressure. The 
residue was subjected to a chromatograph over silica gel to obtain 
(R)-endo-2-norborneol (0.1 3 g, 1.2 mmol, 12%). After the compound was 
subjected to the said chromatograph, the optical purity was 82% ee. 
EXAMPLE 4 
The same procedure as in Example 3 was repeated and a mixture was prepared 
except that racemic endo-2-propionyloxy norbornane 1.7 g (10 mmol) was 
used instead of racemic endo-2-norbornyl phenylacetate 2.3 g (10 mmol) as 
a substrate, and the mixture was stirred for 29 hours. Then, the 
conversion was 67%. The same steps as in Example 3 were conducted from the 
filtration of the lipase to the chromatography of the product to obtain 
(S)-endo-2-propionyloxy norbornane (0.51 g, 3.0 mmol, 30%). The resulting 
compound was dissolved in ethanol 10 ml, and 1N sodium hydroxide 10 ml was 
added to it and stirred for one night at room temperature. The reaction 
solution obtained was neutralized with 1N hydrochloric acid and extracted 
with ethyl acetate, the extract was dried over magnesium sulfate, and the 
solvent was distilled off under reduced pressure to obtain 
(S)-endo-2-norborneol (0.17 g, 2.6 mmol, 26%). After the compound was 
subjected to the said chromatograph, the optical purity was 76% ee. 
Comparative examples 1-8 
Several kinds of enzymes 0.25 g as shown in Table 1, respectively, are 
mixed with racemic endo-2-propionyloxy norbornane 1.7 g (10 mmol) and a 
0.7M phosphoric buffer solution (pH 7.0) 30 ml, and the mixture was 
stirred at 35.degree. C. for the fixed time. The results are shown in 
Table 1. 
TABLE 1 
______________________________________ 
Optical 
Comparative 
Enzyme Stirring Conversion 
purity 
example (Trade name) 
(hr.) (%) (% ee) 
______________________________________ 
1 Pancreatic 94 0.3 -- 
trypsin.sup.1 
2 Lecitase.sup.2 
67 0 -- 
3 Paratase A.sup.3 
95 0 -- 
4 SP 523.sup.4 
142 45 50 
5 Lipase 70 1.0 -- 
type II.sup.5 
6 Protease 43 0 -- 
type I.sup.6 
7 Liver acetone 
77 42 15 
powder.sup.7 
8 Pancreatin F.sup.8 
94 0.3 -- 
______________________________________ 
.sup.1) Origin: Porcin pancreas, manufactured by Novo Nordisc Co., Ltd. 
.sup.2) Origin: Porcin pancreas, manufactured by Novo Nordisc Co., Ltd. 
.sup.3) Origin: Aspergillus niger, manufactured by Novo Nordisc Co., Ltd. 
.sup.4) Origin: Humicola sp., manufactured by Novo Nordisc Co., Ltd. 
.sup.5) Origin: Porcin pancreas, manufactured by Sigma Co., Ltd. 
.sup.6) Origin: Bovine pancreas, manufactured by Sigma Co., Ltd. 
.sup.7) Origin: Porcin liver, manufactured by Sigma Co., Ltd. 
.sup.8) Origin: Porcin pancreas, manufactured by Amano Pharmaceutical Co. 
Ltd. 
As shown in Table 1, enzymes, which are not contained in the present 
invention, are used in each comparative example. Theses results show the 
reactivity (conversion) and stereoselectivity (optical purity) of 
comparative examples to be very inferior to those of examples. Even though 
the reactivity is good (as shown in comparative examples 4 and 7), the 
stereoselectivity is inferior to that of examples. On the other hand, 
enzyme derived from Candida genus in the above Examples 1-4 of the present 
invention, shows that the conversion is 16-76%, and the optical purity is 
76-93% ee. From examples 1 and 2, in which the enzyme derived from Candida 
antarctica is used, it is particularly known that the conversion is 
37-47%, the optical purity is 88-93% ee and both reactivity and 
stereoselectivity are excellent. 
Reference example 1 
Recemic endo-2-norborneol 700 g (6.42 mol), 2, 2, 2-trichloroethanol 770 g 
(3.11 mol), lipase (type II, origin: Porcine pancrease, manufactured by 
Sigma Co., Ltd.) 560 g and diisopropylether 700 ml were mixed and the 
mixture was stirred for 290 hours at 25.degree. C. Then, the conversion of 
the above racemic compound was 38%. The lipase was filtered from the 
reaction solution. The filtrate was concentrated under reduced pressure, 
and distilled under reduced pressure to obtain 
(R)-endo-2-caproyloxynorbornane (500 g, 2.38 mol, 37%). The resulting 
compound was subjected to a gas chromatograph and the optical purity of 
83% ee was determined. 
EXAMPLE 5 
The (R)-endo-2-caproyloxynorbornane 5.0 g (23.8 mmol) having optical purity 
of 83% ee, lipase (Novozyme 435, origin: Candida antarctica, manufactured 
by Novo Nordisc) 2.0 g and water 50 ml were mixed. A 1N-NaOH aqueous 
solution was added dropwise to the mixture to adjust to pH 7 and stirred 
for 105 hours at 35.degree. C. Then, the conversion of the above compound 
was 91%. The lipase was filtered from the reaction solution. The filtrate 
was extracted with ethyl acetate, the extract was dried over magnesium 
sulfate, and the solvent was distilled off under reduced pressure. The 
residue was subjected to a column chromatograph over silicagel to obtain 
(R)-endo-2-norborneol (2.33 g, 20.8 mmol, 87%). The resulting compound was 
subjected to the said gas chromatograph and the optical purity of 96% ee 
was determined. 
EXAMPLE 6 
Using the same procedure as in Example 5 except that pH of the reaction 
solution was 8, a mixture was prepared with stirring for 24 hours. Then, 
the conversion was 89%. The steps from the filteration of the lipase to 
silica gel chromatography were conducted as in Example 5, and 
(R)-endo-2-norborneol (2.36 g, 21.0 mmol, 89%) was obtained. The resulting 
product was subjected to the said gas chromatograph and the optical purity 
of 99% was determined. 
EXAMPLE 7 
The same lipase (SP 525) 0.25 g as used in Example 2 was dissolved in a 
0.2M phosphoric buffer solution (pH 7.0) 40 ml containing 0.1M NaCl. The 
mixture and washed carrier (Trade name: Formylcellulofine, manufactured by 
CHISSO CORPORATION) 20 g were mixed and stirred for 30 minutes at room 
temperature. After adding sodium cyano borohydride 0.14 g, the mixture was 
stirred for 12 hours at room temperature. The supernatant fraction was 
filtered off, the residue was washed with a 0.2M tris-hydrochloric buffer 
solution (pH 7.2) 300 ml, a 0.2M tris-hydrochloric buffer solution (pH 
7.2) 4 0 ml containing ethanol amine 0.24 g and sodium cyano borohydride 
0.1 g was added, and the mixture was stirred for three hours at room 
temperature. The supernatant fraction was filtered off, the residue was 
washed with a 0.1M phosphoric buffer solution (pH 8.0) 150 ml and with a 
0.07M phosphoric buffer solution (pH 7.0) 10 ml, and the immobilized 
lipase was obtained. 
A mixture of the resulting immobilized lipase 20 g, racemic 
endo-2-propionyloxy norbornane 1.7 g (10 mmol ) and a 0.7M phosphonic 
buffer solution (pH7.0) 30 ml was stirred for 50 hours at 35.degree. C. 
Then, the conversion was 47%. The immobilized lipase was filtered and 
recovered from the reaction solution. The filtrate was extracted with 
ethyl acetate, the extract was dried over magnesium sulfate and the 
solvent was distilled under reduced pressure to obtain 
(R)-endo-2-norborneol (0.50 g, 4.5 mmol, 45%). After the residue was 
subjected to a gas chromatograph, the optical purity was 88% ee. 
EXAMPLE 8 
Using the immobilized lipase which was recovered in Example 7, racemic 
endo-2-propionyloxy norbornane was hydrolyzed as shown in Example (the 
second time), and the following operation was repeated up to 11 times. The 
results are shown in Table 2. 
TABLE 2 
______________________________________ 
Reaction Conversion 
Optical purity 
time (hr.) 
(%) (% ee) 
______________________________________ 
2nd time 94 48 88 
3rd time 45 46 89 
4th time 51 47 89 
5th time 62 49 87 
6th time 68 50 87 
7th time 52 45 89 
8th time 60 46 89 
9th time 60 45 89 
10th time 91 51 85 
11th time 91 50 85 
______________________________________ 
As shown in Table 2, by using the immobilized lipase which was used in 
Example 7 and recovered in Example 8, the stereoselectively is good even 
if the lipase is repeatedly used, and the activity is difficult to lower. 
According to the method of the present invention, since expensive lipase 
can be repeatedly used, it is possible to efficiently prepare optically 
active endo-2-norborneols at low cost and in large quantities.