4β,5β-Epoxy-4α-methylcholestane undergoes rearrangement with boron trifluoride to give mainly 5-methyl-5α-cholestan-4-one. Under similar conditions the 4α,5α-epoxide gives the 5β-methyl-A-nor-B-homo-6-ketone (VI), which rearranges on prolonged treatment with boron trifluoride to give 5-methyl-5β-cholestan-4-one.