Abstract 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.