Acylation of O-alkylbenzohydroxamic acids; configurational assignment, interconversion, and rearrangement of the E- and Z-isomers of a new group of O-acyl isoamides

Abstract

Acylation of the silver salts of O-alkylbenzohydroxamic acids (1) gives Z-acetic benzoalkoximic anhydrides (3) which do not rearrange to the isomeric N-acyl-N-benzoyl-O-alkylhydroxylamines (5) on heating. U.v. irradiation of the Z-isomers (3) in hexane or benzene induces a photostationary equilibrium of the E- and Z-isomers (4) and (3), which were separated by chromatographic, methods. On heating at reflux for 30 min in carbon tetrachloride the E-isomers (4) rearrange to (5). by an O → N [1,3] acyl migration. The ¹H n.m.r. chemical shifts and the u.v. extinction coefficients for isomers (3) and (4) were found to vary in the same relative way in the six pairs of compounds examined. Diazotization of O-(n-propyl)benzamidoxime in acetic acid yields only the Z-isomer (3), possibly via stereospecific reaction of acetate with an intermediate N-alkoxynitrilium ion (13). Consistent with this is the observation that only the Z-O-alkylbenzohydroximoyl chloride (9a) is formed on reaction of (13) with Cl^– under similar conditions.