Synthesis of Spiropyrrolines via One-Pot Tf 2 O-Mediated Amide Activation/Formal [3 + 2]-Cycloaddition of α-Formylamino Ketones.

An efficient method for the synthesis of spiropyrrolines from readily accessible α-formylamino ketones is reported. The method involves amide activation using Tf 2 O, followed by a formal [3 + 2]-cycloaddition of the resulting enolic nitrilium intermediate with Michael acceptors, ultimately affording spiropyrrolines. Mechanistic insights were gained through NMR studies, elucidating the precise role of the base additive and suggesting the formation of an enolic nitrilium intermediate.