Abstract
1α-Hydroxyvitamin D3 (20) was prepared from cholesterol (1) in ca. 1% overall yield. Hydroboration of the tetrahydropyranyl ether 2 followed by chromic acid oxidation and NaBH4 reduction gave the 6β-ol 5. Brief treatment of its acetate 6 with acid followed by Jones oxidation gave the 6β-acetoxy-3-one (7) in 55% yield from 1. Introduction of C-1 double bond (64%) was effected by bromination of 7, followed by dehydrobromination with CaCO3, yielding the 3-oxo-1-ene 9. Oxidation of 9 with alkaline H2O2 afforded the 1α, 2α-epoxide 10 (80%) and this was converted by successive 6 steps sequences to 1α-hydroxycholesterol 15 in 43% yield. The 5, 7-diene 19 was obtained from the acetate 16 in 40% yield by allylic bromination with N-bromosuccinimide, dehydrobromination with trimethyl phosphite and saponification. Ultraviolet irradiation of 19 in benzene solution followed by thermal isomerization, produced 1α-hydroxyvitamin D3 (20) (20%).
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