PHOTOCHEMISTRY OF N-(PYRIMIDIN-2-ONE-4-YL)PYRIDINIUM DERIVATIVES - THE RING CONTRACTION OF PYRIMIDINONE INTO IMIDAZOLINONE

Photochemical reactions (lambda > 300 nm) of N-(1-methylpyrimidin-2-on e)- and N-(1,5-dimethylpyrimidin-2-one)pyridinium chlorides were studi ed in deoxygenated aqueous solution at various pH's. Only the former c ompound was found to be reactive under these conditions to give pyrimi dine ring contraction photoproducts 1-methyl-4-imidazolin-2-one and 1- methyl-4-imidazolin-2-one-5-carboxyaldehyde, with pH-dependent chemica l yields. The photochemical pyrimidine ring contraction reaction does not occur for other photochemically reactive pyrimidin-2-ones bearing 3-methylimidazolium-1,1,2,4-triazol-1-yl, or imidazol-1-yl as substitu ents at the C-4 position. The suggested mechanism of the reaction invo lves the addition of water to the pyrimidinone part of the N-(1-methyl pyrimidin-2-one)pyridinium salt in the excited triplet state as the pr imary photochemical step. Addition of alcohol to the pyridinium ring w as found to be the major reaction under irradiation of N-(1-methylpyri midin-2-one-4-yl)pyridinium chloride in methanol.