Asymmetric 1,3‐Dipolar Cycloadditions of <i>N</i>‐Benzyl and <i>N</i>‐Diphenylmethyl Nitrones and α,β‐Unsaturated Aldehydes Catalyzed by Bis‐Titanium Chiral Lewis Acids

Abstract

Bis-titanium chiral Lewis acids that contain two oxygen-bridged titanium centers were successfully applied to the asymmetric 1,3-dipolar cycloaddition of nitrones and alpha,beta-unsaturated aldehydes. The introduction of the diphenylmethyl group as the N substituent on the nitrones, with the aim of destabilizing the nitrone-Lewis acid complex, led to the drastic enhancement of not only the reactivity but also the enantioselectivity. By employing this approach, 1,3-dipolar cycloadditions of nitrones and the rather unreactive methacrolein were facilitated to give cycloadducts that bear one all-carbon quaternary center with unique regioselectivity and excellent stereoselectivity.