HETEROCYCLES STRUCTURALLY INFLUENCED BY A SIDE-CHAIN .10. 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), thoxycarbonyl)et hyl-1H-pyrido[2,3-b]-pyrazin-2-one (8), and 3-(1-benzoyl)ethyl-1H-quin oxalin-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 f orm to the imine form with elevated temperatures; however, 3-(carbonyl methylene) derivatives of 3,4-dihydro-1H-pyrido[2,3-b]pyrazin-2-one su ch as 10, 19 and 20 show little temperature effect. cylidene-1,2-dihyd ro-4H-pyrido[3,4-b]pyrazin-3-one (21) and cylidene-3,4-dihydro-1H-pyri do[3,4-b]pyrazin-2-one (22), which exist in the enamine form, show no temperature effect.