Chemoselectivity of the CuAAC/Ring Cleavage/Cyclization Reaction between Enaminones and α‑Acylketenimine

Ketenimines
represent an important class of reactive
species, useful
synthetic intermediates, and synthons. However, in general, ketenimines
preferentially undergoes nucleophilic addition reactions with hydroxyl
and amino groups, and carbon functional groups remain a less studied
subset of such systems. Herein, we develop a straightforward syntheses
of pyridin-4(1H)-imines that is achieved by cyclization
of a reacting enaminone unit with α-acylketenimine which is
generated from the reactions of sulfonyl azides and terminal ynones in situ (CuAAC/Ring cleavage reaction). The cascade process
preferentially starts with the nucleophilic α-C of the enaminone
unit instead of an amino group, attacking the electron-deficient central
carbon of ketenimine, and the chemoselectivity unconventional products
pyridin-4(1H)-imines were formed by intramolecular
cyclization.