Diels–Alder reactions of 2-carbomethoxy-4,4-dimethyl-2-cyclopenten-1-one

Diels–Alder additions to keto ester 2 proceed in moderate to high yields with a variety of dienes under selected Lewis acid and thermal conditions. Thermal conditions provide the highest total yields for non-functionalized dienes, but Lewis acid catalysts, particularly stannic chloride, provide highly selective additions in greater net yields. A study of several functionalized dienes shows that zinc chloride is particularly effective in catalyzing additions to 2. No difference in stereoselectivity is observed between zinc chloride and thermal reactions for highly functionalized dienes. The significance of the regiochemical and stereochemical results of the Diels–Alder reactions is discussed.