olestane (Ia) undergoes a “backbonerearrangement” with BF3etherate to give the (enantio) 5β,14β-dimethyl-18,19-bisnorcholest-13(17)-ene derivative (IIa). One of the products obtained from reaction of 3α-acetoxy-4α,5-epoxy-5α-cholestane (Ib) with a Grignard reagent is now shown to be the related 13(17)-olefin (IIb). The epoxide Ib also undergoes a backbonerearrangement on treatment with BF3-etherate