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

An N-heterocyclic
carbene (NHC)-catalyzed reaction
of γ-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 γ-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.