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

-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.