Enzymatic desymmetrization of prochiral 2,3-bis(acetoxymethyl)bicyclo [2.2.1]hepta-2,5-diene and 2,3-bis(hydroxymethyl)bicyclo[2.2.1]hepta-2,5- diene

Article abstract of DOI:10.1016/S0957-4166(98)00027-5

Enzymatic desymmetrization of the title compound 1 is reported using various commercially available lipases in hydrolysis and alcoholysis reactions or ester synthesis. In this area, lipase Amano AK (Pseudomonas sp.) proved to be the best lipase whatever the experimental conditions used. The monoacetate product 2 is indifferently obtained with more than 95% enantiomeric excess (ee) as the levorotatory enantiomer 2a or the dextrorotatory one 2b.

Full text of DOI:10.1016/S0957-4166(98)00027-5

Pergamon
Tetrahedron: Asymmetry 9 (1998) 581–587
Enzymatic desymmetrization of prochiral
2
,3-bis(acetoxymethyl)bicyclo[2.2.1]hepta-2,5-diene and
2
,3-bis(hydroxymethyl)bicyclo[2.2.1]hepta-2,5-diene
a
a
b
a,∗
Magali Ranchoux, Jean-Michel Brunel, Gilles Iacazio and Gérard Buono
a
Ecole Nationale Supérieure de Synthèses, de Procédés et d’Ingénierie Chimiques d’Aix Marseille, UMR CNRS 6516, Faculté
de St Jérôme, Av. Escadrille Normandie Niemen, 13397 Marseille Cedex 20, France
Faculté des Sciences de St Jérôme, Laboratoire de Microbiologie, 13397 Marseille, Cedex 20, France
b
Received 14 November 1997; accepted 22 January 1998
Abstract
Enzymatic desymmetrization of the title compound 1 is reported using various commercially available lipases in
hydrolysis and alcoholysis reactions or ester synthesis. In this area, lipase Amano AK (Pseudomonas sp.) proved to
be the best lipase whatever the experimental conditions used. The monoacetate product 2 is indifferently obtained
with more than 95% enantiomeric excess (ee) as the levorotatory enantiomer 2a or the dextrorotatory one 2b.
©
1998 Published by Elsevier Science Ltd. All rights reserved.
1. Introduction
Recently, we reported the synthesis of a novel polyfunctional bicyclic compound, 2,3-bis-
(
acetoxymethyl)bicyclo[2.2.1]hepta-2,5-diene 1 and its use in palladium catalyzed elimination.1
Subtrate 1 may be considered as a useful substrate on the route toward natural products and compounds
of pharmaceutical interest such as prostaglandins of the PGF series.2,3 One of our aims was to use
prochiral synthon 1 in order to obtain the chiral bicyclic aldehyde 4, a potential precursor of β-santalene
(Fig. 1).
Fig. 1.
∗
Corresponding author. E-mail: buono@spi-chim.u-3rs.fr
0957-4166/98/$19.00 © 1998 Published by Elsevier Science Ltd. All rights reserved.
PII: S0957-4166(98)00027-5

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M. Ranchoux et al. / Tetrahedron: Asymmetry 9 (1998) 581–587
Fig. 2.
The synthesis of chiral aldehyde 4 may be envisioned from precursor 2 resulting from a chiral
desymmetrization of prochiral compound 1. A method of choice for the desymmetrization of prochiral
alcohols or esters is the use of lipases and esterases. Numerous examples of such reactions have been
4
5
described in the literature, especially with substrates possessing a [2.2.1]-bicyclo heptane core. In this
work, we describe the enzymatic desymmetrization of substrate 1 using various commercially available
lipases in either hydrolytic, alcoholytic or ester synthesis mode.
2. Results
Pure 1 is easily obtained in 65% chemical yield, resulting from a Diels–Alder reaction, by heating a
mixture of dicyclopentadiene and 1,4-diacetoxy-2-butyne 2 (5:8 molar ratio) to 260°C for 1.5 h under 35
bar nitrogen pressure in a stainless steel autoclave, followed by two successive distillations.
2.1. Enzymatic hydrolysis of compound 1
Various lipases, known to have potential in the desymmetrization of prochiral compounds, have been
tested with substrate 1 (Fig. 2 and Table 1).
The reactions were performed at 25°C using a pH stat apparatus maintaining the pH of the borate buffer
(
1 mM) at pH 7.5 (except for entry 3 where the reaction was performed at pH 6.5) by addition of a 0.1
N NaOH solution. Lipase Amano AK appeared to be the best enzyme tested, since (−)-bicyclo[2.2.1]-
methyleneacetoxy,methylenehydroxy)-2,3-heptadiene-2,5 2a was obtained with more than 95% ee in
2% chemical yield (entry 13). Nevertheless, all other enzymes tested gave poor results in terms of
(
8
enantioselectivity. The influence of the experimental conditions has been investigated in detail in the
case of Mucor miehei lipase (MML) since it gives the best result in terms of enantioselectivity (entry
1
). In order to improve the enantiomeric excess, various water soluble cosolvents have been tested
9
such as acetone, acetonitrile and tetrahydrofuran. Acetone proved to be the best solvent since the
enantioselectivity increased from 46 to 75% ee.
2.2. Enzymatic alcoholysis of compound 1
When possible, it is preferable to perform enzymatic reactions in organic solvents. Thus, many
advantages are associated with such a procedure: better recovery of the products and enzymes by simple
filtration, possible reuse of the enzyme and sometimes better stability of the substrate/product of the
reaction in organic solvent versus water. For all these reasons, we have undertaken the study of the
alcoholysis of 1 with butan-1-ol (Fig. 3) and the same set of enzymes previously used (except PLE
and Aspergillus lipase). All the reactions were performed at 30°C and stopped after no evolution was
encountered in consumption of 1 (Table 2).
Once again, lipase Amano AK proved to be the best lipase (entries 9 and 10), leading to (−)-2a with an
enantiomeric excess of up to 95% ee, whatever the solvent used (n-hexane or THF). The isolated yields
of (−)-2a are also excellent, varying respectively from 85% to 90%. The reaction performed with lipase

M. Ranchoux et al. / Tetrahedron: Asymmetry 9 (1998) 581–587
583
Table 1
Enzymatic hydrolysis of diacetate 1
Fig. 3.
MML in a more polar solvent than hexane allowed the ee to be improved from 44% (entry 1) to 57%
entry 4). Subsequent addition of water increased the ee to 61% probing the influence of the polarity of
(
the solvent used on the enantioselectivity. Except for MMLS leading to 75% ee (entry 5), the other tested
lipases proved to be poor enantioselective catalysts for this desymmetrization.
2.3. Esterification of compound 3
In either hydrolytic or alcoholytic mode, lipase Amano AK afforded the levorotatory enantiomer of
compound 2a. In order to get access to the dextrorotatory enantiomer 2b, we turned to the ester synthesis
mode of lipase action. Thus, due to the microreversibility principle, the dextrorotatory enantiomer 2b
should be obtained from diol 3 (Fig. 4). Various enzymes have been used in various solvents and different
irreversible acetyl donor compounds have also been tested (Table 3).

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M. Ranchoux et al. / Tetrahedron: Asymmetry 9 (1998) 581–587
Table 2
Enzymatic alcoholysis of diacetate 1
Fig. 4.
Generally speaking, in ester synthesis mode the ees are lower than in the case of hydrolysis or
alcoholysis mode, whatever the tested lipase and the experimental conditions, except for lipase Amano
AK which led exclusively to the formation of compound 2b as the dextrorotatory enantiomer in >95%
ee (entry 11) and 84% isolated yield.
3. Conclusion
In this study we have shown that the two enantiomers of the monoacetate 2 could be easily obtained
through enzymatic desymmetrization of prochiral synthons 1 or 3. Of all the enzymes tested, lipase
Amano AK proved to be an outstanding catalyst. Thus, the two enantiomers 2a and 2b were obtained with
enantiomeric excesses of up to 95% ee, by a hydrolysis or alcoholysis reaction for 2a and an irreversible
ester synthesis for 2b, respectively, and in all cases with high chemical yields.

M. Ranchoux et al. / Tetrahedron: Asymmetry 9 (1998) 581–587
585
Table 3
Esterification of diol 3
4. Experimental
THF was distilled from potassium–benzophenone ketyl and degassed thoroughly with dry nitrogen
1
13
directly before use. H and C NMR spectra were recorded in C D solution at 100.00 MHz and 25.18
6
6
MHz on a Bruker AC100 instrument, respectively, when not indicated otherwise (the usual abreviations
are used: s=singlet, d=doublet, t=triplet, q=quadruplet, m=multiplet). The positive chemical shift values
are given in ppm. Gas chromatographic analyses were run on a BP 20 capillary column (25 m×0.32
mm; gas vector He, 1.0 bar). Porcine pancreatic lipase (PPL, type II) and Pig liver esterase (PLE) were
purchased from Sigma. Mucor miehei lipase (MML) was from Biocatalysts. Aspergillus niger lipase was
from Röhm. Candida cylindracea lipase (CCL, Candida cylindracea), Pseudomonas fluorescens lipase
(PFL, Amano P) and Pseudomonas sp. lipase (PL, Amano AK) were from Amano pharmaceuticals.
Mucor miehei supported lipase (MMLS) was from Novo industries.
4.1. Preparation of 2,3-bis(hydroxymethyl)bicyclo[2.2.1]hepta-2,5-diene 3
In a two-necked flask equipped with a dropping funnel and a condenser was placed 3.1 g (0.055 mol)
of potassium hydroxide in anhydrous methanol (40 mL). A solution of 1 (6.2 g, 0.026 mol) in anhydrous
methanol (15 mL) was slowly added at 0°C with vigorous stirring. The mixture was stirred overnight.
Removal of the solvent in vacuo left an oily residue which was distilled to afford 3.5 g (89% yield) of
1
3
as a colorless liquid: bp 120°C/0.07 torr; H NMR δ 1.98 (m, 2H), 3.47 (t, 2H), 4.25 (s, 4H), 4.63 (s,

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M. Ranchoux et al. / Tetrahedron: Asymmetry 9 (1998) 581–587
H), 6.83 (t, 2H); 13C NMR δ 53.2, 59.3, 71.7, 142.8, 149.2; IR (cm ) 3320 (w), 3060 (s), 2975 (m),
−1
2
2
932 (m), 2861 (m), 1556 (s), 1289 (sh), 988 (m).
4.2. General procedure for enzymatic hydrolysis of 1
To 1.7 mmole of 1 (401 mg) placed in a beaker were added 20 mL of a 1 mM borate buffer solution,
pH 7.5 and 100 mg of various commercial lipases. The evolution of the reaction was monitored by
addition of 0.1 N NaOH solution using a pH stat apparatus (Methröm AG), the stirring being set at
250 rpm. At approximately 80% conversion, the reaction was stopped and the aqueous phase extracted
three times with diethyl ether. The organic phases were dried over MgSO , filtered and evaporated under
4
reduced pressure. The monoacetate was then purified by flash chromatography on silica gel (eluent ethyl
1
acetate:petroleum ether=1:4) to afford the desired product as an oil. H NMR δ 1.94 (m, 2H), 2.02 (s,
1
3
3
5
H), 3.18 (s, 1H), 3.49 (m, 2H), 4.28 (s, 2H), 4.74 (s, 2H), 6.75 (m, 2H); C NMR δ 19.1, 51.7, 51.4,
−1
8.1, 59.4, 70.3, 142.3, 142.5, 152.4, 169.7; IR (cm ) 3320 (w), 3080 (s), 2980 (m), 2870 (m), 1750
(m), 1380 (s), 1225 (sh), 960 (m).
4.3. General procedure for enzymatic alcoholysis of 1
2
mmoles of 1, 30 mmoles of butan-1-ol and 9 mL of dried n-hexane, THF or MTBE were placed in a
screw-cap tube. When using the latter two solvents, 250 µL of water were added at that time, followed by
00 mg of various lipases. The tube was placed in a reciprocal shaker. The evolution of the reaction was
monitored by TLC (CH Cl :diethyl ether=9:1). When the formation of diol 3 was detected, the reaction
2
2
2
was stopped by filtration of the enzyme on a plug of Celite. The solvents were evaporated under reduced
pressure and the monoacetate was purified as described above.
4.4. Enzymatic acylation of 3
In a screw-cap tube were placed 2.5 mmole of 3, 10 mmole of the acylating agent dissolved in different
solvent systems and 100 mg of various lipases. The tubes were placed in a reciprocal shaker. After a
classical treatment the monoacetate was purified as described above.
Acknowledgements
We are indebted to Amano Pharmaceutical Co. Ltd for the kind supply of lipases Amano AK and
Amano P.
References
1. Muchow, G.; Brunel, J. M.; Maffei, M.; Buono, G. J. Org. Chem. 1995, 60, 852.
2. (a) Mitra, A. The Synthesis of Prostaglandins; John Wiley: New York, 1977. (b) Roberts, S. M.; Scheinmann, F. New
Synthetic Routes to Prostaglandins and Thromboxanes; Academic Press: London, 1982.
3
4
. Apsimon, J. The Total Synthesis of Natural Products; John Wiley: New York, 1983, Vol. 5, p. 249.
. (a) Jones, J. B. Tetrahedron 1986, 42, 3351. (b) Santaniello, E.; Ferraboschi, P.; Grisenti, P.; Manzocchi, A. Chem. Rev.
1992, 92, 1071. (c) Azerad, R. Bull. Soc. Chim. Fr. 1995, 132, 17.
5. (a) Ito, Y.; Shibata, T.; Arita, M.; Sawai, H.; Ohno, M. J. Am. Chem. Soc. 1981, 103, 6739. (b) Arita, M.; Adachi, K.; Ito, Y.;
Sawai, H.; Ohno, M. J. Am. Chem. Soc. 1983, 105, 4049. (c) Ohno, M.; Ito, Y.; Arita, M.; Shibata, T.; Adachi, K.; Sawai,

M. Ranchoux et al. / Tetrahedron: Asymmetry 9 (1998) 581–587
587
H. Tetrahedron 1984, 40, 145. (d) Bloch, R.; Guibe-Jampel, E.; Girard, C. Tetrahedron Lett. 1985, 26, 4087. (e) Andreu,
C.; Marco, J. A.; Asensio, G. J. Chem. Soc., Perkin Trans. 1 1990, 3209. (f) Asensio, G.; Andreu, C.; Marco, J. A. Chem.
Ber. 1992, 125, 2233.
6
. The determination of the enantiomeric excess of 2a or 2b has been realized by 19F NMR spectroscopy after derivatization
3
using (R)-(+)-α-methoxy trifluoromethylphenyl acetic acid chloride as derivatizing agent. In the presence of Pr(fod) ,
the spectra show two well-separated singlets at −73.2 and −71.2 ppm. Thus, the enantiomeric purity can be accurately
measured and no kinetic resolution has been observed.
7
. A racemic mixture of monoacetate 2 submitted to a hydrogenolysis reaction catalyzed by Pd(PPh
3 4
) in the presence of an
excess of NEt and formic acid led to the formation of compound 5. Subsequent treatment of 5 by Mosher reagent led to
3
1
9
the formation of a mixture of diastereomers 6a and 6b. By F NMR spectroscopic analysis and according to the results
8
previously described by Oppolzer et al., it appears that the signals observed at −71.6 ppm and −71.8 ppm correspond to
diastereomers 6b and 6a, respectively. Moreover, according to this procedure it appears that treatment of a pure solution
of levogyre 2 led to the obtention of diastereomer 6a possessing an (S) absolute configuration at the carbon bearing the
CH
2
OMosher group. A similar study on dextrogyre 2 led to the opposite conclusion (compound 6b possesses an (R) absolute
OMosher group).
configuration at the carbon bearing the CH
2
8. Oppolzer, W.; Chapuis, C.; Dupuis, D. Helv. Chim. Acta 1985, 68, 2100.
9. Chinsky, N.; Margolin, A. L.; Klibanov, A. M. J. Am. Chem. Soc. 1989, 111, 386.