Synthesis and unusual photochemistry of a highly reactive pyrimidinedione

The herein obtained 3-methyl-5,5-diphenylpyrimidine-2,4-(3H,5H)-dione forms upon photolysis an unusual bicyclic product, namely, 6-methyl-3,3,8,8-tetraphenyl-8,8a-dihydroimidazo[1.5-c]pyrimidine-5,7(3H,6H)-dione in 21% yield. The transformation starts with elimination of carbon monoxide and methyl isocyanate from the molecule of substrate via a ‘Type A’ photochemical process giving 3,3-diphenylazirine. Then 1,3-dipole species generated from the latter is trapped by the starting compound via a [3+2] cycloaddition.