Synthesis of Fused 2-amino-4-oxo-1,3,5-triazines via Microwave-Assisted Ring Closure Carbonylation of Azahetarylguanidines

The microwave-assisted ring closure of 2-benzoxazolylguanidines (1a-c), 2-benzothiazolylguanidines (3a-c) and 2-benzimidazolylguanidine (5) using phenyl isocyanate as a carbonylating reagent afforded 2-amino-4-oxo-[1,3,5]triazino[2,1-b][1,3]benzoxazoles (2a-c), 2-amino-4-oxo-[1,3,5]triazino[2,1-b][1,3]benzothiazole (4a-c) and 2-amino-4-oxo-3H-[1,3,5]triazino[1,2-a]benzimidazole (6), respectively. The tautomerism in the prepared compounds was investigated. Compounds 2a-c and 4a-c were found to exist in DMSO solution as 2-imino-4-oxo- tautomers. 2-Amino-4-oxo-3H- and 2-amino-4-hydroxy- forms of 6 predominated in tautomeric equilibrium at the same condition.