Photochemical ring contraction of 4-heteroaryl-substituted 1,4(or 3,4)-dihydropyrimidines into imidazoles

Photolysis of 4-R-1,4(or 3,4)-dihydropyrimidines causes ring contraction into imidazoles, provided that the substituent R is capable of sufficiently stabilizing an anionic centre (R = 2-thiazolyl, 2-pyridyl, 4-pyridyl). Chemical evidence is presented to show that the ring contraction of 6-phenyl-4-(2-pyridyl)1,4(or 3,4)-dihydropyrimidine occurs via heterolytic fission of the C(1)-C(6) bond of intermediate 1-phenyl-6-(2-pyridyl)-2,4-diazabicyclo[3.1.0]hex-2(or 3)-ene. The anion-stabilizing effect of R is correlated with the acid strength (pKa) of R–CH3. A pKa value around 30 probably determines the border-line between ring contraction into an imidazole and formation of an isomeric 5-R-1,2(or 2,3)-dihydropyrimidine.