8-ENDO CYCLIZATION OF (ALKOXYCARBONYL)METHYL RADICALS - RADICAL WAYS FOR PREPARATION OF 8-MEMBERED-RING LACTONES

Cyclization of (alkoxycarbonyl)methyl radicals generated from bromoace tates proceeds in the 8-endo mode to generate heptanolactones. Three d istinct types of 8-endo/5-exo tandem radical cyclizations produce diff erent bicyclic heptanolactones. In certain cases, intramolecular free- radical attack on the heptanolactone carbonyl group initiates further skeletal rearrangement. Ab initio calculations indicate that the prefe rence of the 8-endo cyclization over the 5-exo mode originates from th e conformational bias of (allcoxycarbonyl)methyl radicals favoring the Z- over the E-conformation.