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‐1 H ‐pyrido[2,3‐ b ]pyrazin‐2‐one ( 7 ), 3‐(1‐ethoxycarbonyl)ethyl‐1 H ‐pyrido[2,3‐ b ]‐pyrazin‐2‐one ( 8 ), and 3‐(1‐benzoyl)ethyl‐1 H ‐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‐(β‐carbonylmethylene) derivatives of 1,2‐dihydro‐4 H ‐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‐1 H ‐pyrido[2,3‐ b ]pyrazin‐2‐one such as 10, 19 and 20 show little temperature effect. 2‐Phenacylidene‐1,2‐dihydro‐4 H ‐pyrido[3,4‐ b ]pyrazin‐3‐one ( 21 ) and 3‐phenacylidene‐3,4‐dihydro‐1 H ‐pyrido[3,4‐ b ]pyrazin‐2‐one ( 22 ), which exist in the enamine form, show no temperature effect.