Abstract: The syntheses of the previously unknown, sterically hindered geometric isomers 3 and 4 of the steroid hormone la, 25-dihydroxyvitamin D3 (1, 1, 25) have been achieved for the first time. The stereoselectivesynthesis of thevinylallene precursor 23a was achieved by the Inanaga method, Pd (O) Sm (II)-iPrOH reduction of propargyl benzoate 33. The latter observation reveals that the Inanaga propargyl ester to allene transformation is in fact ...