Two efficient enantioselective syntheses of an A-ring precursor to 1 a
lpha,25-dihydroxyvitamin D-3 have been developed. The key step in thes
e sequences involves the stereoselective cyclopropanation of a chiral
gamma-alkoxy-alpha-diazo-beta-keto ester. In one sequence, the 1 alpha
-hydroxy group was introduced in the first step by a diastereoselectiv
e ene reaction and in the other sequence by use of (S)-malic acid as s
tarting material. Both routes eliminate the need for a SeO2 oxidation.