Synthesis of pyrido[1,2-a]-1,3,5-triazines. Reactions of 2-pyridyl isothiocyanate with compounds containing a C=N bond

New pyrido[1,2-a]-1,3,5-triazine derivatives were prepared by the [4+2]cycloaddition reaction of 2-pyridyl isothiocyanates with isocyanate, aldimines, and N,N'-diphenylcarbodiimide. On the other hand, reaction of 2-pyridyl isothiocyanate with aliphatic carbodiimides afforded [2+2]cycloaddition products, 1,3-thiazetidines.