Preparation of an A-ring building block for the total synthesis of 1α,25-dihydroxy vitamin D3 and structurally related congeners: Lipase-catalyzed stereoselective esterification of a suitable epoxyalcohol

Article abstract of DOI:10.1016/S0957-4166(00)00239-1

An useful A-ring building block for the total synthesis of vitamin D₃ congeners, compound 7, has been prepared starting from vitamin D₂ by a chemo-enzymatic approach that relies on lipase-catalyzed acylation in an organic solvent for the stereoselective step. Copyright (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0957-4166(00)00239-1

Tetrahedron: Asymmetry 11 (2000) 2665±2668
Preparation of an A-ring building block for the total
synthesis of 1a,25-dihydroxy vitamin D3 and structurally
related congeners: lipase-catalyzed stereoselective
esteri®cation of a suitable epoxyalcohol
Patrizia Ferraboschi,a Shahrzad Reza-Elahi,a Luca Scottia and Enzo Santaniellob,*
a
Á
Dipartimento di Chimica e Biochimica Medica, Universita degli Studi di Milano, Milano, Italy
Dipartimento di Scienze Precliniche LITA Vialba, Universita degli Studi di Milano, Milano, Italy
b
Á
Received 2 June 2000; accepted 15 June 2000
Abstract
An useful A-ring building block for the total synthesis of vitamin D3 congeners, compound 7, has been
prepared starting from vitamin D2 by a chemo-enzymatic approach that relies on lipase-catalyzed acylation
in an organic solvent for the stereoselective step. # 2000 Elsevier Science Ltd. All rights reserved.
Vitamin D3 1a is a steroid-like vitamin that controls calcium homeostasis and bone
mineralization through its hormonally active form 1a,25-dihydroxycholecalciferol, calcitriol 1b,
that also shows many other important biological activities on cell di€erentiation and proliferation
or on immunological competent cells.1 Therefore, considerable e€ort has been made towards the
synthesis of structurally related congeners that show interesting pharmacological applications,
such as, for instance, calcipotriol 2, that has recently been introduced into clinics for the
treatment of psoriasis.2
* Corresponding author.
0957-4166/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved.
PII: S0957-4166(00)00239-1

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Synthetic approaches to vitamin D3 congeners include the elaboration of structurally related
steroid compounds or total synthesis starting from suitable cyclic precursors.3 A method for the
preparation of the two cyclic fragments corresponding to the A-ring and the de-AB-cholestane
derivative by oxidative cleavage, described for vitamin D3 1a,4 has been applied by us to vitamin
D2 3 that di€ers from vitamin D3 1a in the side chain (Scheme 1).5
Scheme 1. Reagents: (a) KMnO4, EtOH/H2O; (b) TBDMSCl/imidazole/DMF; (c) Pb(OAc)4, C6H6; Red-Al,
C6H5CH3
Both cyclic intermediates 4 and 5 are valuable cyclic building blocks for the total synthesis of
vitamin D3 analogues and have been isolated in 44 and 60% yield, respectively.6 The cleavage
of the double bond of compound 5 and further elaboration would permit the construction of
modi®ed side chains,7 whereas for the introduction of the C-1 hydroxy group it has been reported
that the epoxidation of the 5,6-double bond in compound 4 is needed in order to protect the 4-
position by undesired oxidation (Scheme 2).8 The stereochemical control of the C-1 hydroxylation
depends on the stereochemistry of the 5,6-epoxide, since the oxidation of the stereochemically
pure a-epoxide 6 leads mainly to the wrong C-1 a-hydroxylation product 8a, whereas only from
the b-epoxyalcohol 7, could the epoxytriol 8b be obtained with the required con®guration of the
Scheme 2.

P. Ferraboschi et al. / Tetrahedron: Asymmetry 11 (2000) 2665±2668
2667
C-1 and C-3 hydroxy groups.8 However, depending on the epoxidation procedure, the
diastereomerically pure a-epoxide 6 (m-chloroperbenzoic acid) or prevalent b-epoxyalcohol 7
(vanadyl acetylacetonate/tert-butylhydroperoxide, 7:6, 6.3:1 ratio) can be prepared.8 In our
hands, the vanadyl acetylacetonate/tert-butylhydroperoxide epoxidation of compound 4 a€orded
a 3:1 (b:a) diastereomeric ratio of the epoxides9 and only a careful puri®cation by column
chromatography a€orded the required epoxyalcohol 7.10
We therefore decided to investigate the stereoselectivity of a lipase-catalyzed acetylation of the
above mixture.11 A few lipases did not work successfully or were not selective for preparative
purposes.12 Only Candida antarctica lipase13 catalyzed the required acetylation and, using vinyl
acetate as solvent and reagent, in 0.75 h from the above a/b-epoxides 6 and 7 mixture the pure
unreacted b-epoxyalcohol 7 and a 75:25 mixture of a- and b-epoxyacetates 9 was obtained
(Scheme 3).14
Scheme 3.
We have also veri®ed the enzymatic preference for the a-stereoisomer in the above
transesteri®cation, preparing the a-epoxyalcohol 6 that was quantitatively converted to the
corresponding acetate in 0.75 h. In conclusion, taking advantage of the stereopreference of the
CAL-catalyzed acetylation of a cyclic epoxyalcohol, the synthesis of diastereomerically pure
b-epoxyalcohol 7 can be presented as an advantageous method for the preparation of a crucial
synthon for the total synthesis of vitamin D3.15
Acknowledgements
This work has been ®nancially supported by the Italian National Council for Research (CNR,
Target Project in Biotechnology) and Ministero dell'Universõta e della Ricerca Scienti®ca e
Technologica (Fondi ex-MURST 60%).
References
1. Hamada, K.; Shinomiya, H. Drugs of the Future 1993, 18, 1057.
2. Zeelen, F. J. Adv. Drug Res. 1992, 22, 149.
3. Zhu, G.-D.; Okamura, W. H. Chem. Rev. 1995, 95, 1877.
4. Toh, H. T.; Okamura, W. H. J. Org. Chem. 1983, 48, 1414.
5. The experimental procedure was essentially as described in Ref. 4. One main modi®cation concerns the
permanganate oxidation (step a). Filtration on Celite of the ®nal aqueous ethanol solution should be preferred
to the described use of silica gel, which made the recovery more tedious and troublesome. Silylation (step b) and

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P. Ferraboschi et al. / Tetrahedron: Asymmetry 11 (2000) 2665±2668
following steps (Pb(OAc)4 and Red-Al) were performed as described and compounds 4 and 5 were isolated in 44
and 60%, respectively, from vitamin D2 3.
6. 1H NMR (500 MHz, Bruker 500 AM) spectra in CDCl3 are in agreement with the structures of the intermediates
in the oxidation procedure. Vitamin D2 5,6-diol (step a): ꢀ 0.77±0.84 (overlapped d+s, 9H, CH3-18 and CH3-
26,27), 0.89 (d, 3H, CH3-21), 0.96 (d, 3H, CH3-28), 3.72±3.84 (m, 1H, CH-3), 4.84±4.90 (overlapped d+s, 2H, CH-7
and CH-19), 4.98 (s, 1H, CH-19), 5.08±5.24 (m, 2H, CH-22,23), 5.54 (s, 1H, CH-6). 3b-Silyl ether (step b): ꢀ 0.00
(s, 6H, CH3 Si), 0.78±0.82 (overlapped d+s, 9H, CH3-18 and CH3-26,27), 0.88±0.92 (overlapped d+s, 12H, CH3-21
and (CH3)3C), 0.96 (d, 3H, CH3-28), 3.66±3.76 (m, 1H, CH-3), 4.86 (s, 1H, CH-19), 4.90 (d, 1H, CH-7), 4.96 (s,
1H, CH-19), 5.08±5.24 (m, 2H, CH-22,23), 5.50 (d, 1H, CH-6). Compound 4: ꢀ 0.00 (s, 6H, CH3Si), 0.90 (s, 9H,
(CH3)3C), 1,45±1.61 (m, 1H, CH-2), 1.61±1.79 (m, 1H, OH), 1.79±1.90 (m, 1H, CH-2), 1.99±2.08 (m, 1H, CH-1),
2.12±2.20 (m, 1H, CH-4), 2.32±2.44 (m, 2H, CH-1,4), 3.78±3.84 (m, 1H, CH-3), 4.11±4.25 (m, 2H, CH2-7), 4.60 (s,
1H, CH-19), 4.92 (s, 1H, CH-19), 5.41 (t, 1H, CH-6).
7. For instance, ozonolysis/reduction of double bond in compound 5 a€ords the so-called Inho€en-Lythgoe diol
(75%), that has been used as intermediate for the synthesis of several vitamin D3 congeners. Among many
available examples, see: (a) Sardina, F. J.; Mourino, A.; Castedo, L. J. Org. Chem. 1986, 51, 1264.
(b) Hatakeyama, S.; Ikeda, T.; Irie, H.; Izumi, C.; Mori, H.; Uenoyama, K.; Yamada, H.; Nishizawa, M.
J. Chem. Soc., Chem. Commun. 1995, 1959.
8. Kiegel, J.; Wovkulich, P. M.; Uskokovic, M. R. Tetrahedron Lett. 1991, 32, 6057.
9. NMR spectra of epoxides 6 and 7 have been reported in detail in Ref. 8; typically, our sample from
VO(acac)2/t-BuOOH epoxidation showed the most signi®cant resonances at 3.10 (t, 0.24H, CH, 6a-isomer) and
3.16 (dd, 0.76H, CH 6b-isomer) ppm corresponding to a 3:1 ratio of b:a-epimers.
10. Puri®cation of the epoxyalcohol mixture was carried out by column chromatography (neutral aluminum oxide III:
crude mixture, 10:1). Elution with hexane:ethyl acetate (8:2) a€orded pure b-epoxyalcohol 7 (30% yields).
11. For a recent review on the preparation of chiral synthons by enzymatic acylation and esteri®cation reactions, see:
Santaniello, E.; Reza-Elahi, S.; Ferraboschi, P. In Stereoselective Biocatalysis; Patel, R. N., Ed.; M. Dekker: New
York, 2000; pp. 415±460.
12. Negative results were obtained with PCL (Pseudomonas cepacia lipase) by transesteri®cation with vinyl acetate in
CHCl3 or by alcoholysis of the acetates of the b/a-epoxide mixture with methanol in tert-butyldimethylether.
Candida cylindracea lipase (CCL) transesteri®cation with vinyl acetate in CHCl3 is not selective, since the
b/a-acetate mixture 9 is in the same ratio as the starting material.
13. Candida antarctica lipase (CAL B, Novozym 435) was a gift from Novo Nordisk (Italy).
14. To a solution of a/b epoxide mixture (0.154 g, 0.54 mmol) in vinylacetate (19 ml) CAL (0.310 g) was added. The
reaction mixture was kept at 30ꢀC (0.75h) monitoring the reaction progress by GLC (HP-5 WB oven temperature
200ꢀC). The retention times TR (min) were as follows: a-epoxide 6: 7.99; b-epoxide 7: 7.68; a-epoxide acetate: 9.85;
b-epoxide acetate: 10.58. Silica gel column chromatography (10:1) a€orded epoxyacetates (a:b, 75:25, 30 mg) by
elution with hexane:ethylacetate 9:1 and pure b-epoxide 7 (75 mg) by elution with hexane:ethylacetate 7:3. NMR
resonances were as follows: b-epoxide 7: 3.16 ppm (dd, 1H, CH-6); a/b-epoxide acetate mixture 9: 3.08 ( t, 0.76H,
CH, 6a-isomer) and 3.16 (dd, 0.24H, CH, 6b-isomer). For a comparison, the NMR resonances of the acetate
mixture prepared (Py/Ac2O) from the a/b-epoxide 6:7 (1:3 ratio): 3.08 (t, 0.25H, CH, 6a-isomer) and 3.16 (dd,
0.75H, CH, 6b-isomer) ppm.
15. For an extensive review on the application of biocatalysis to steroids and, in particular, to the preparation of
vitamin D synthons, see: Ferrero, M.; Gotor, V. In Stereoselective Biocatalysis; Patel, R. N., Ed.; M. Dekker: New
York, 2000; pp. 579±631.