Neighbouring group participation in the cleavage of some steroidal hydroxy epoxides
Cleavage of steroidal 6β-hydroxy 4α,5α,-epoxides by the action of perchloric acid, acetic acid, formic acid, and boron trifluoride–diethyl ether is described. It is shown that, when attack at C-4 by an external nucleophile is inhibited by the inductive effect of a 3-methoxy group, epoxide migration occurs, involving intramolecular nucleophilic attack at C-5 by the adjacent hydroxy group. The product isolated then arises either by rearrangement or by diaxial cleavage of the intermediate isomeric epoxide thus formed. The latter process affords a product derived by overall cis-opening of the original oxirane ring.