Remote functionalization by tandem radical chain reactions

Abstract

Normal radical relay chlorination of cholestan-3α-ol directed by an attached m-iodobenzoate ester group affords a 9α-chloro steroid, but when the same reaction is conducted in the presence of an excess of CBr₄ the product is a 9α-bromo steroid. Similarly, when the same radical relay reaction is carried out in the presence of an excess of (SCN)₂ rather than CBr₄, the product is a 9α-thiocyano steroid. Several other examples of these reactions have been developed. These tandem remote functionalization reactions succeed because an intramolecular hydrogen abstraction by a complexed-chlorine atom generates a specific substrate radical in each case.