Formation and thermal reaction of O-(N-acetylbenzimidoyl)benzamidoxime: comparison with the formation of 3,5-disubstituted 1,2,4-oxadiazoles from O-acetylarylamidoximes and O-aroylacetamidoximes

Abstract

Salts of O-(benzimidoyl)benzamidoxime have been obtained by the action of N-chloro- or N-bromo-succinimide, or the halogens, on benzamidoxime. Acetylation of the free base gave the title compound, which underwent a thermal cyclisation, with loss of acetamide, to give 3,5-diphenyl-1,2,4-oxadiazole. The mechanism of this reaction, in diphenyl ether, closely paralleled the thermal cyclisation of O-acetylarylamidoximes and O-aroylacetamidoximes, and is thought to involve a polar cyclisation step followed by rate-determining proton transfer. ¹³C N.m.r. spectra for 32 oxime, amidoxime, and oxadiazole derivatives are recorded, and substituent chemical shifts for the oxime, amidoxime, O-acetylamidoxime, 5-methyl-1,2,4-oxadiazol-3-yl, and 3-methyl-1,2,4-oxadiazol-5-yl groups on the phenyl ring are calculated.