NHC-Catalyzed Formal [2+2] Annulations of Allenoates for the Synthesis of Substituted Oxetanes

An N-heterocyclic carbene (NHC)-catalyzed reaction of gamma-substituted allenoates for the synthesis of substituted oxetanes has been developed. The method provides an approach to access substituted oxetanes in a single step and is the first example of an NHC-catalyzed formal [2+2]-annulation employing gamma-substituted allenoates with trifluoromethyl ketones. Mechanistic and modeling studies provide a rationale for the divergence in reactivity observed compared to the analogous reaction using unsubstituted allenoates and inform a hypothesis to explain the observed diastereoselectivity under different reaction conditions.