Stereoselective Preparationof Functionalized Unsaturated Lactones and Esters via FunctionalizedMagnesium Carbenoids

The reaction of β,β-dibromo or diiodo unsaturated esters with i-PrMgCl (1 equivalent) in diethyl ether allows the generation of functionalized alkenylmagnesium carbenoids which react with retention of configuration with various electrophiles providing polyfunctionalized unsaturated esters and lactones. By using two equivalents of i-PrMgCl, a 1,2-migration with retention of configuration occurs in diethyl ether allowing a new synthesis of tetrasubstituted esters and lactones.