6β-Bromocholest-4-en-3-one (I) was
converted to 6β-bromo-4α,5-epoxy-5α-cholestan-3-one (II) with
alkaline hydrogen peroxide. 5,6β-Dibromo-5α-cholestane- 3β,4β-diol
(X) with alkali gave 6β-bromo-4β,5-epoxy-5β-cholestan-3β-ol
(IXa), which was oxidized to non-crystalline 6β-bromo-4β,5-epoxy-5β-cholestan-3-one
(XIII), characterized as the crystalline 3,3-dimethyl ketal (XIIa). Cleavage of
(II) with hydrogen bromide in acetic acid gave 4,6β-dibromocholest-4-en-3-one
(V) via the isomeric bromohydrins (IVa) and (IVb). Compound (XIII) yielded (V)
directly.
Sodium borohydride reduction of (I) gave cholesterol
in high yield, while reduction with lithium tri-t-butoxyaluminium hydride
afforded cholest-4-en-3-one (52%), cholesterol (30%), and cholest-4-en-6β-ol-3-one
(7%).