Precursors of the A-ring of vitamin D and method and intermediates for the preparation thereof

A method of preparing a compound of formula (I):

• in which:
• A is a group —CH2OH, —CH2—OCOR′, —COR″, —CSR″ or an ethynyl;
• R is hydrogen or a (C1-C6)alkyl;
• R1 is hydrogen, a (C1-C6)alkyl or a group —(CH2)n—OP;
• R2 is hydrogen or a group —OP;
• R′ is a (C1-C6)alkyl or a phenyl;
• R″ is hydrogen, a hydroxyl, a (C1-C6)alkyl, a (C1-C6)alkoxy, a (C1-C6)alkylthio, or a di(C1-C3)alkylamino;
• P is hydrogen;
• a (C1-C6)alkanoyl;
• a benzoyl in which the phenyl is optionally substituted by a (C1-C4)alkyl, a halogen or a nitro;
• a (C1-C6)alkoxycarbonyl;
• a group —Si(R3)3 in which each R3 independently represents a (C1-C6)alkyl or a phenyl;
• a mono- or di-(C1-C6)alkoxy(C1-C6)alkyl;
• a tetrahydrofuranyl;
• or a tetrahydropyranyl;
• n is 0, 1, 2, 3 or 4,
• which method comprises the steps of
• (i) reacting a compound of formula 1
• in which A is a (C1-C6)alkoxycarbonyl or a di(C1-C3)alkylaminocarbonyl and R is as defined above, with a lipase in a vinylalkanoate or an acid anhydride, and
• (ii) converting the resulting compound of formula 2 or 2′
• in which Z is an alkyl, to the corresponding compound of formula (I).