Direct Asymmetric Aldol Reaction of 5H-Oxazol-4-ones with Aldehydes Catalyzed by Chiral Guanidines

A new chiral bicyclic guanidine-catalyzed direct catalytic aldol reaction of 5H-oxazol-4-ones with aldehydes has been developed. The present aldol reaction proceeds smoothly with high enantioselectivity using bicyclic guanidines bearing a hydroxy group at the appropriate position, and various combinations of 5H-oxazol-4-ones and aldehydes are applicable. The method provides synthetically useful alpha,beta-dihydroxycarboxylates bearing a chiral quaternary stereogenic center at the alpha-carbon atom.