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.