Two Approaches to Diverting the Course of a Free-Radical Cyclization: Application of Cyclopropylcarbinyl Radical Fragmentations and Allenes as Radical Acceptors

Free radical cyclization of 4 and 7 gave the expected cyclization-reduction products (5 and 8) along with considerable amounts of products derived from a cyclization−atom transfer−secondary cyclization process (6 and 9). Two approaches to avoiding these unexpected products were explored. Use of a cyclopropylcarbinyl fragmentation avoided the secondary cyclization reaction (25 or 43 → 26 or 44), whereas use of an allene as a radical acceptor avoided the atom-transfer reaction altogether (49 → 52).