Synthesis of Pyrrolidine-2-ylidenes from Isoxazolines and Allenes

An addition and rearrangement reaction has been developed for the synthesis of pyrrolidine-2-ylidenes from NH-isoxazolines and electron-deficient allenes. This method generates pyrrolidine-2-ylidenes with a gem-dione at the 4-position via the rearrangement of a proposed N-alkenylisoxazoline intermediate. Reaction optimization and substrate scope are described, in addition to studies comparing the reactivity of the gem-dione and enaminone groups of the products. This method overcomes functional group tolerance limitations of alternative approaches and expands the scope of accessible pyrrolidine-2-ylidenes.