The syntheses of the previously unknown, sterically hindered geometric isomers 3 and 4 of the steroid hormone 1 alpha,25-dihydroxyvitamin D-3 (1,1,25) have been achieved for the first time. The stereoselective synthesis of the vinylallene precursor 23a was achieved by the Inanaga method, Pd(0)Sm(II)-iPrOH reduction of propargyl benzoate 33. The latter observation reveals that the Inanaga propargyl ester to allene transformation is in fact stereoselective (similar to 10:1), involving a formal anti-S(N)2’ displacement of benzoate by hydrogen. Highly stereoselective (50:1, 85% yield) (1,5)-hydrogen shift of vinylallene 23a to the sterically hindered 7,8-cis isomer of the hormone 1, namely, 3, was achieved by the Shibasaki method using (naphthalene)tricarbonylchromium (22, (np)(CO)(3)Cr). By examining this chromium(0)-mediated isomerization on all four diastereomeric vinylallene analogue systems 6a,b and 7a,b, all of which resulted highly selectively (50:1) in hindered 7,8-cis geometric isomers, new stereomechanistic information concerning the Shibasaki type 1,5-shift has also emerged. By cheleotropic addition-extrusion of sulfur dioxide on 3, there was obtained the final unknown geometric isomer of 1,5,6-trans-7,8-cis-1,25 4; this result parallels the known transformation of 1 to 2. Although the vinylallene 23a can be thermally rearranged via a [1,5]-sigmatropic hydrogen shift to the natural hormone 1, the yield and selectivity are modest. By contrast, one-way triplet photosensitized isomerism of the now readily available 7,8-cis-isomer 3 results primarily in the natural hormone 1 in good yield and selectivity. Taken collectively, these observations reveal that the synthesis of all four geometric isomers 1-4 can be achieved from a single precursor, the vinylallene 23a. Comparative biochemical evaluation, in vitro, of the four geometric isomers in terms of their ability to bind to the chick intestinal receptor reveals that 7,8-cis isomerism (3 and 4) significantly suppresses their ability to bind receptor (<2% each versus 100% for the hormone 1,25).