Physicochemical Properties of Isomeric Azines of 3-Acetyl-4-hydroxy-2-methoxy-4-phenylcrotonic Acid Lactones

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X-Ray crystallographic analyses of two isomeric azines of 3-acetyl-4-(2-chlorophenyl)-4-hydroxy-2-methoxycrotonic acid lactones (5b and 6b), which were pepared by reaction of 1b with hydrazine dihydrochloride, were carried out. The stereochemical difference between 5b (red needles) and 6b (yellow needles) is mainly in the =N-N= bonding mode ; this group takes the completely planar conformation in 5b, and the twisted conformation in 6b. From the results of energy calculations, thermal analyses, and epimerization reactions, it could be concluded that the yellow crystals (6) are structurally more stable than the red crystals (5).