Enzymatic resolution of (±)-conduritol-B, a key intermediate for the synthesis of glycosidase inhibitors

Article abstract of DOI:10.1016/S0957-4166(99)00344-4

Lipases from porcine pancreas, Candida cylindracea and Mucor miehei (adsorbed on support, Lipozyme IM) catalysed in t-butylmethylether the alcoholysis of rac-conduritol-B peracetate, (±)-1, by n-butanol to give enantiopure (2S,3S)-diacetoxy-(1R,4R)-dihydroxycyclohex-5-ene, (-)-3, and (1S,2R,3R,4S)-tetraacetoxy-cyclohex-5-ene, (+)-1. The enantioforms (+)- and (-)-conduritol-B, obtained after chemical hydrolysis of (-)-3 and (+)-1, respectively, may be employed to prepare both the enantiomers of conduritol-B epoxide and cyclophellitol, powerful inhibitors of glycosidases.

Full text of DOI:10.1016/S0957-4166(99)00344-4

Pergamon
Tetrahedron: Asymmetry 10 (1999) 3273–3276
Enzymatic resolution of (ꢀ)-conduritol-B, a key intermediate for
the synthesis of glycosidase inhibitors
Claudia Sanfilippo, Angela Patti and Giovanni Nicolosi ∗
Istituto CNR per lo Studio delle Sostanze Naturali di Interesse Alimentare e Chimico-Farmaceutico, Via del Santuario 110,
I-95028 Valverde CT, Italy
Received 15 July 1999; accepted 10 August 1999
Abstract
Lipases from porcine pancreas, Candida cylindracea and Mucor miehei (adsorbed on support, Lipozyme® IM)
catalysed in t-butylmethylether the alcoholysis of rac-conduritol-B peracetate, (ꢀ)-1, by n-butanol to give enan-
tiopure (2S,3S)-diacetoxy-(1R,4R)-dihydroxycyclohex-5-ene, (−)-3, and (1S,2R,3R,4S)-tetraacetoxy-cyclohex-5-
ene, (+)-1. The enantioforms (+)- and (−)-conduritol-B, obtained after chemical hydrolysis of (−)-3 and (+)-
1, respectively, may be employed to prepare both the enantiomers of conduritol-B epoxide and cyclophellitol,
powerful inhibitors of glycosidases. © 1999 Elsevier Science Ltd. All rights reserved.
A remarkable interest in the preparation of chiral conduritols (5-cyclohexen-1,2,3,4-tetrols) exists
today because these polyalcohols may be used as starting material in the preparation of densely-
functionalised molecules, including inositol derivatives and pseudosugars, which exhibit important
biological activity.1
The presence of four stereogenic carbon atoms in the framework of conduritols allows them to exist
in 10 different stereoforms, two of which are meso-compounds, conduritol-A and -D, while the others
constitute four enantiomeric couples, conduritol-B, -C, -E and -F, respectively. Due to their synthetic
importance, the preparation of these tetrols in enantiopure form and the selective transformation of the
OH functions present on the cyclohexene skeleton is particularly useful but not always readily performed.
In this context biocatalysed procedures can afford easy and stereocontrolled access to enantiopure
conduritols. Thus, an efficient desymmetrisation by esterification in the presence of Pseudomonas sp.
lipase of meso-conduritol-A 1,4-diester, previously transformed into the corresponding 2,3-ketal, has
been reported by Johnson et al.,2 whereas in our laboratory a highly enantioselective asymmetrisation of
conduritol-D tetraester has been obtained through transesterification mediated by Mucor miehei lipase.3
The same lipase catalysed the enantiomeric alcoholysis of conduritol-E tetraacetate or 1,2-diacetate
producing, at the same time, selective partially protected derivatives, profitable for synthetic purposes.4
∗
Corresponding author. E-mail: nicolosi@issn.ct.cnr.it
0957-4166/99/$ - see front matter © 1999 Elsevier Science Ltd. All rights reserved.
PII: S0957-4166(99)00344-4

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Table 1
Biocatalysed alcoholysis of conduritol-B tetraacetate, (ꢀ)-1a
Within our research project that foresees the development of enzymatic procedures to prepare enantio-
pure bioactive cyclitols, we have taken into consideration the preparation of both (+)- and (−)-conduritol-
B by lipase mediated kinetic resolution of the corresponding racemic peracetate, (ꢀ)-1, readily available
in high yield from benzoquinone.5
In a preliminary investigation tetraester (ꢀ)-1, dissolved in t-BME, has been treated with n-butanol
in the presence of three different lipases. Lipase from porcine pancreas catalysed the alcoholysis of
tetraacetate (ꢀ)-1 with a moderate rate to give, after 4 h, a triacetate (−)-2 as the main product, whose
1H NMR spectrum was consistent with the presence of a free OH group in the allylic position of
the molecule. GC analysis on a chiral column of this compound, as its peracetate (−)-1 obtained by
conventional acetylation, evidenced its enantiopure nature;6 conversely its chemical hydrolysis gave (−)-
conduritol-B, (−)-4, allowing assignment of the 1R,2S,3S,4R configuration to triester (−)-2 (Table 1).
In a reaction prolonged for 24 h, in addition to (−)-2, diester (−)-3 in 30% yield and enantiopure form
was observed. The formation of this diester in the alcoholysis process is possible because triester (−)-2
possesses a second function at C-4 of adapted configuration so becoming itself a new substrate for the
lipase.
The reaction catalysed by lipase from Candida cylindracea gave comparable results in terms of
products formed and enantioselectivity but showed a very low reaction rate. Conversely, when lipase
from Mucor miehei adsorbed on support (Lipozyme® IM) was employed, after a 4 h reaction, (ꢀ)-1
suffered 30% of conversion to give homochiral enantiopure (−)-2 and (−)-3 in equal amounts.7
All the lipases utilised possess R stereopreference. It is noteworthy that ester groups located on C-2 and
C-3 in the enantiomer (+)-1, although having the suitable configuration, do not undergo any alcoholysis.
This unreactivity can be interpreted in terms of steric hindrance exercised by the vicinal allylic ester
group.
Since Lipozyme catalysed the alcoholysis with the best rate, it was selected to perform the alcoholysis
of conduritol (ꢀ)-1 on a two gram scale. After 24 h the reaction mixture contained (−)-3 and the

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unreacted (+)-1 in a 1:1 ratio. Chromatographic purification on Silica-gel column (eluting with ethyl ace-
tate:hexane, 6:4) furnished (−)-3 (44% yield, >98% ee)8 and (+)-1 (47% yield, >98% ee).9,10 Treatment
of diester (−)-3 and tetraester (+)-1 with MeOH:NH4OH, 9:1, afforded, quantitatively, (1R,2S,3S,4R)-
1,2,3,4-tetrahydroxy-5-cyclohexene, (−)-4, and (1S,2R,3R,4S)-1,2,3,4-tetrahydroxy-5-cyclohexene, (+)-
4, respectively.11
From a synthetic point of view, the C2 symmetry of conduritol-B allows the exploitation of (+)- and
(−)-4 to prepare, in a highly diastereospecific manner, two different cyclitols (see Scheme 1), already
proven as having inhibitory activity versus glycosidase. Their direct oxidation with 3-chloroperbenzoic
acid can give both the enantiomers of conduritol-B epoxide.12 The selective oxidation with PtO2 of one
of the allylic OH functions present in conduritol-B enantiomers, followed by exhaustive benzylation,
leads to the corresponding enones (+)- and (−)-5,13 of which the former has been employed as the chiral
key-intermediate in an elegant synthesis for (+)-cyclophellitol, recently reported by Letellier et al.14
Scheme 1.
Further investigations are being carried out in our laboratory to prepare regioselective partial enantio-
pure esters of conduritol-B, to be used as starting material in the preparation of new chiral cyclitols.
Acknowledgements
Financial support from the CNR Target Project on ‘Biotechnology’ is gratefully acknowledged. Thanks
are also due to Novo Nordisk for kindly providing lipase from Mucor miehei.
References
1. Balci, M. Pure Appl. Chem. 1997, 69, 97–104.
2. Johnson, C. R.; Plè, P. A.; Adams, J. P. J. Chem. Soc., Chem. Commun. 1991, 1006–1007.
3. Patti, A.; Sanfilippo, C.; Piattelli, M.; Nicolosi, G. Tetrahedron: Asymmetry 1996, 7, 2665–2670.
4. (a) Sanfilippo, C.; Patti, A.; Piattelli, M.; Nicolosi, G. Tetrahedron: Asymmetry 1997, 8, 1569–1573. (b) Sanfilippo, C.;
Patti, A.; Piattelli, M.; Nicolosi, G. Tetrahedron: Asymmetry 1998, 9, 2809–2817.
5. Guo, Z.-X.; Haines, A. H.; Pyke, S. M.; Pyke, S. G.; Taylor, R. J. K. Carbohydrate Res. 1994, 264, 147–153.
6. Enantiomeric excesses (ee) were determined by GC analysis on chiral capillary column (Megadex DMP β,
dimethylpenthyl β-CDX, OV1701 0.25 µm×0.25 mm×25 m). The ee of tetracetylconduritol-B was determined by direct
injection. After conventional acetylation, the same method was utilised for excess determinations of (−)-2 and (−)-3.

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7. For comparison of the alcoholysis of the conduritol-E tetraacetate see: Ref. 4a.
8. [α]D −90.8 (c 0.3, CH3Cl); 1H NMR (CDCl3): δ 2.15 (6H, s), 2.53 (2H, d, J=5.5 Hz), 4.44 (2H, ddd, J=2.3, 5.0 and 5.5
Hz), 5.04 (2H, dd, J=2.3 and 5.0 Hz), 5.76 (2H, s); 13C NMR (CDCl3): δ 22.2, 72.2, 76.8, 130.4, 172.7.
9. [α]D +170.6 (c 1.2, CHCl3; lit.10 [α]D −172.4, c 1.1, CHCl3 for the R enantiomer).
10. Le Drian, C.; Vionnet, J.-P.; Vogel, P. Helv. Chim. Acta 1990, 73, 161–168.
11. The monoalcohol (−)-2, which, having a single free allylic hydroxyl group is of considerable synthetic interest, is recovered
in low yield by all the above alcoholysis processes. In order to prepare triester (−)-2 in higher yields an alternative synthetic
way can be the direct esterification of diester (−)-3 with vinyl acetate in the presence of Lipozyme. Although hydroxyl
functions in C-1 and C-4 are both located on an R-configuration centre and (−)-3 has C2 symmetry, the concentration
of desired (−)-2 reaches 70% yield before significant concentration of the total esterified product appears in the reaction
1
medium. (−)-(1R,2S,3S,4R)-1-Hydroxy-2,3,4-triacetyloxycyclohex-5-ene, (−)-2: [α]D −111.8, (c 0.1 CHCl3); H NMR
(CDCl3): δ 2.07 (3H, s), 2.08 (3H, s), 2.14 (3H, s), 2.59 (1H, d, J=6.2 Hz), 4.46 (1H, ddd, J=8.0, 6.2 and 2.3 Hz), 5.07
(1H, dd, J=10.8 and 8.0 Hz), 5.33 (1H, dd, J=10.8 and 8.0 Hz), 5.63 (2H, m), 5.85 (1H, dd, J=10.2 and 2.0); 13C NMR
(CDCl3): δ 21.9, 22.1, 22.2, 72.0, 72.5, 73.1, 76.9, 126.8, 132.4, 171.4, 171.6, 172.7.
12. Lee, K. J.; Boyd, S. A.; Radin, N. S. Carbohydr. Res. 1985, 144, 148–154.
13. Heyns, K.; Gottschalck, H.; Paulsen, H. Chem. Ber. 1962, 95, 2660–2668.
14. Letellier, P.; Railainairina, D.; Beaupere, D.; Uzan, R. Synthesis 1997, 925–930.