Asymmetric Synthesis of Vitamin D3 Analogues: Organocatalytic Desymmetrization Approach toward the A-Ring Precursor of Calcifediol.

A novel asymmetric synthesis has been developed for the construction of the A-ring of a chiral precursor to calcifediol. The highlights of this synthesis include (i) the introduction of the stereochemistry at the C5-position of the A-ring through the organocatalytic enantioselective desymmetrization of a prochiral cyclic anhydride using a bifunctional urea catalyst and (ii) the introduction of the exo-cyclic (Z)-dienol side chain by a tandem Claisen rearrangement/sulfoxide thermolysis of an allylic alcohol.