Organocatalytic Asymmetric Ring Expansion of Cyclobutanones

Significance

A secondary amine catalyzed ring expansion of substituted cyclobutanones with ­nitrosobenzene is reported. Surprisingly, none of the expected α-aminoxylation products was isolated and the ring-expanded products were obtained in reasonable yields, diastereo- and enan­tioselectivities. The proposed mechanism involves an enamine formation, followed by a formal nitroso aldol reaction (1). However, intermediate 2 cyclizes, presumably directed by the catalyst, and leads to bicyclic intermediate 3, which undergoes a rearrangement and hydrolysis, thus liberating the ­catalyst and product.