Alcohol-assisted phosphine catalysis: one-step syntheses of dihydropyrones from aldehydes and allenoates.

This paper describes the phosphine-catalyzed annulation of methyl allenoate with various aromatic aldehydes to form 6-aryl-4-methoxy-5,6-dihydro-2-pyrones. In this reaction, the addition of an alcohol was necessary to induce dihydropyrone formation, with the optimal agent being methanol. Moreover, the addition of n-butyllithium suppressed the formation of the noncyclized product, leading to the exclusive isolation of the dihydropyrone. This method provides an efficient, one-step route toward disubstituted dihydropyrones from simple, stable starting materials.