Unusual 1,2‐Acyl Rearrangement Accompanying Ring Expansion Reaction of Alkaloid Cotarnine under the Action of α‐Haloketones: a New Route to 5‐Acyl‐1,2‐dihydro‐3‐benzazepine Moiety

Abstract

AbstractThe ring expansion reaction of the isoquinoline alkaloid cotarnine with α‐haloketones which normally leads to the formation of 4‐acyl‐7,8‐metylenedioxy‐1,2‐dihydro‐3‐benzazepines, in some cases, occurs with an unusual, non‐acid‐catalytic 1,2‐acyl rearrangement. As a result, corresponding 5‐acyl isomers are formed. A carbocation mechanism of the rearrangement was proposed. It involves formation of 4‐acyl‐5‐hydroxy‐7,8‐metylenedioxy‐1,2,3,4‐tetrahydro‐3‐benzazepines and their dissociation (as leuko bases) into corresponding benzazepine carbocations. As it follows from DFT calculations, these carbocations are prone to almost barrierless intracationic 1,2‐acyl group migration with conversion into corresponding isomeric carbocations. At the last stage of the reaction, these cations are deprotonated into final rearranged compounds.