A concise enantioselective synthesis of a key A-ring synthon for 1 alpha-hydroxyvitamin D3 compounds.

[figure: see text] This report describes a concise enantioselective synthesis of the A-ring synthon for the synthesis of 1 alpha-hydroxyvitamin D3 compounds. The synthesis involves two notable transformations: (I) stereoselective construction of the enol triflate from the vinyl ketone by Michael addition of Ph2P(O)Li followed by in situ triflation of the resulting enolate and (II) palladium-catalyzed Heck type cyclization of the enol triflate.