Pyrazine photochemical transformations

Photochemical transformations of pyrazines into imidazoles are well known (see CHEC-I). Although diphenyl- and methylphenyl-substituted azidopyrazines give only resins when photolyzed,... 

A number of thermal and photochemical transformations of the 1,2-diazine system are noteworthy. Thermolysis isomerizes pyridazines into pyrimidines and/or pyrazines, as has been demonstrated for a series of perfluoro and perfluoroalkyl derivatives (see p 288). Pyridazine is converted into pyrimidine at 300°C. A process of valence isomerism is thought to be responsible for the thermal isomerization of pyridazines via diazabenzvalenes (simplified as 3 and 4). 

The presence of a 2-substitutent in 3-phenylazirines (17, R —H in Scheme 21) modifies the mode of reaction with molybdenum carbonyl.47 In contrast to pyrazine formation for (17, R =H see Section V,C,2), the alkenyl azirine (18, Scheme 22) is transformed in excellent yield into 2-phenyl-5-carboxy-methylpyrrole. This product probably arises by intramolecular cyclization within an intermediate dienylnitrene intermediate, and related reactions have been devised to synthesize isoxazoles (see Section IV,E,2) and pyrazoles (see Section IV,D,1).47 The molybdenum carbonyl-promoted formation of 2,5-disubstituted pyrroles47 has analogy in uncatalyzed thermal, but not photochemical decomposition of 3-phenyl-2//-azirine 2-acrylate.49... 

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