Asymmetric synthesis of 1α,25-dihydroxyvitamin D3 A-ring precursor starting with 5-tert-butyldimethylsiloxy-2-cyclohexenone

The A-ring precursor of 1α,25-dihydroxyvitamin D3 [(E)-4] has been prepared starting from the (5S)-tert-butyldimethylsiloxy-2-cyclohexenone [(S)-1] via eight steps in 19% overall yield, where a catalytic osmium dihydroxylation which sets the stereochemistry of the hydroxyl group at C1 and a regioselective protection of the hydroxy group as a TBS-ether are the key steps.