Heterocycles structurally influenced by a side chain.X. Effect of temperature and side chain on the imine-enamine tautomerism in the quinoxalinone and pyridopyrazinone systems

3-(1-Benzoyl)ethyl-1H-pyrido[2,3-b]pyrazin-2-one (7), 3-(1-ethoxycarbonyl)ethyl-1H-pyrido[2,3-b]-pyrazin-2-one (8), and 3-(1-benzoyl)ethyl-1H-quinoxalin-2-one (9) exist only in the imine form due to the steric effect of the methyl substituent. As regards the imine-enamine tautomerism, 3-(beta-carbonylmethylene) derivatives of 1,2-dihydro-4H-pyrido[2,3-b]pyrazin-3-one such as 12 and 15-18 gradually change from the enamine form to the imine form with elevated temperatures; however, 3-(carbonylmethylene) derivatives of 3,4-dihydro-1H-pyrido[2,3-b]pyrazin-2-one such as 10, 19 and 20 show little temperature effect. 2-Phenacylidene-1,2-dihydro-4H-pyrido[3,4-b]pyrazin-3-one (21) and 3-phenacylidene-3,4-dihydro-1H-pyrido[3,4-b]pyrazin-2-one (22), which exist in the enamine form, show no temperature effect.