Pd-Catalyzed Carbocyclization−Negishi Cross-Coupling Cascade: A Novel Approach to 1α,25-Dihydroxyvitamin D3 and Analogues†

A mild palladium-catalyzed cascade has been used for the synthesis of the hormone 1α,25-dihydroxyvitamin D3 (calcitriol, 1a) and its analogues 1b and 1c. This one-pot process involves two consecutive transformations at room temperature:  An initial palladium-catalyzed 6-exo-cyclocarbopalladation of vinyl triflates followed by a Negishi cross-coupling reaction with an alkenyl zinc. This novel strategy opens new possibilities for the preparation of a variety of new vitamin D analogues of therapeutic potential, particularly with modifications at the triene and/or ring-A.