Chemoselective access to substituted butenolides via a radical cyclization pathway: mechanistic study, limits and application

We developed a new approach to gamma-lactols and methylene-gamma-lactols based upon the radical cyclization of aluminium acetals obtained by reduction of alpha-bromoesters with DIBAL-H. The cyclic aluminium acetals resulting from the cyclization process could engage in situ in further functionalization, as illustrated by the Oppenauer-type oxidation to give the corresponding lactones and gamma-butenolides. The preparation of butenolides using this strategy compared favourably with the direct, tin-mediated cyclization of alpha-bromoesters, for which side reactions such as epimerization via [1,5]-HAT processes have been observed