Diazomethane, reactions with tautomeric heterocycles

Reviews dealing with a specific reaction or property from the heterocyclic point of view have been rarer—tautomerism (continued from Volume 1), free radical substitution, metal catalysts and pyri-dines, acid-catalyzed polymerization of pyrroles, and diazomethane reactions have been covered in this volume. 

The imidazole derivative (217) underwent a series of reactions with diazomethane which resulted in the formation of products (222) and (223). Thus, AT-methylation of compound (217) gave compound (219), which participated in a 1,3-dipolar cycloaddition giving cycloadduct (220). This cycloadduct then eliminated nitrous acid and the resulting product tautomerized giving heterocycle (221), which was further methylated giving a mixture of the products (222) and (223) <82IJC(B)997>. Thiazole derivative (218) underwent a similar reaction. 

Identification of the product(s) resulting from the reaction of heterocyclic compounds with diazomethane has been used in attempts to elucidate their tautomeric composition (for summaries, see references 7 and 41). This work was based on the assumption that if a compound which is capable of existing in both an —NH and an —OH form produced only the =NMe derivative when it w as treated with diazomethane, it existed entirely in the =NH form. On the other hand, formation of the —OMe derivative was interpreted to mean that a finite amount of the compound existed in the —OH form. In some cases the tautomer present in the solid state w as concluded to be different from that present in solution for example, 41 42 gave a higher proportion of the 3,4-dimethoxy derivative when ethereal diaz-... 

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