A Concise Enantioselective Synthesis of a Key A-Ring Synthon for 1α-Hydroxyvitamin D₃ Compounds

This report describes a concise enantioselective synthesis of the A-ring synthon for the synthesis of 1α-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.