O‐Methyl‐α‐ketophenylacetohydroximoyl chloride (1) was prepared by the reaction of O‐methyl‐α‐methoxyphenylacetohydroximoyl chloride (5) with N‐bromosuccinimide and concentrated hydrobromic acid. Reaction of 1 with ethylenediamine gave 3‐phenyl‐5,6‐dihydro‐2(1H)‐pyrazinone‐O‐methyloxime (6). 3‐Phenyl‐5,6‐cyclohexano‐5,6‐dihydro‐2(1H)‐pyrazininone‐O‐methyloxime (7) was prepared by reaction of 1 with trans‐1,2‐diaminocyclohexane. The X‐ray structure of 6 has been determined. The crystals are orthorhombic, space group Pbca with a = 10.264(3), b = 18.262(4), c = 23.530(4)Å, V = 4411(2)Å3, and Z = 16. The structure, which was refined to R = 0.038 using 1652 observed reflections, shows the amidoxime moiety to be the Z configuration. Reaction of benzohydroximoyl chloride with aziridine gave (Z)‐aziridinylbenzaldoxime (16a). Ultraviolet irradiation of a benzene solution of 16a gave a mixture of the Z and E isomers 16a and 16b. The E isomer 16b underwent thermal isomerization to 16a at 100°. Reaction of 16a with dimethyl sulfate in sodium hydroxide solution gave (Z)‐O‐methylaziridinylbenzaldoxime (17a). Photoisomerization of a hexane solution of 17a gave a mixture of the Z and E isomers 17a and 17b which were separated by preparative glc. The isomers 17a and 17b are resistant to thermal Z = E isomerization. The mechanisms of thermal isomerization of benzamidoximes are discussed.

α-Methoxyphenylacetonitrile (I), prepared from benzaldehyde, reacts with hydroxylamine to give the amidoxime (II) which is methylated and then converted to the chlorooxime (IV). Subsequent oxidation yields the benzoylchlorooxime (V). This cyclizes with the diamines (VI) or (VIII) to form the pyrazine derivatives (VII) or (IX). The chlorooxime (X) reacts with aziridine (XI), producing the aziridinylbenzaldoxime Z-(XII) which isomerizes reversibly to give E-(XII). The methyl ether Z-(XIII) is irreversibly transformed into the stable isomer E-(XIII).