Metal-free anomalous [5+1] cycloaddition reactions of donor-acceptor aziridines for the synthesis of 2H-1,4-oxazines.

A new type of metal-free [5+1] cycloaddition reaction of donor-acceptor aziridines with 2-(2-isocyanoethyl)indoles is reported herein. This method exhibits broad substrate scope and atom-economy. A series of 2H-1,4-oxazines containing an indole heterocycle skeleton were obtained in up to 92% yield under mild reaction conditions. Control experiments revealed that free indole N-H is crucial for the above transformations. The theoretical calculation studies provided guidance on the in-depth insight into the reaction mechanism and the hydrogen-bond between the free indole N-H and carbonyl group was identified to lower the free energy barrier in the transition states.