Photochemical reactions and the chemical evolution of purines and nicotinamide derivatives.

The photochemical rearrangement of a tetramer of hydrogen cyanide to 4-aminoimidazole-5-carbonitrile, a critical step in the proposed prebiotic synthesis of purines, proceeds in high yield in the absence of oxygen without photodestruction of the reaction product. The mechanism of the rearrangement involves the conversion of the excited singlet of the hydrogen cyanide tetramer to 2-imino-3-cyano-4-aminoazetine, which then rearranges to the imidazole product. The photolysis of the vinylogous enaminonitrile 1,3-dicyano4-amino-1,3-butadiene yields 6-aminonicotinonitrile. The latter reaction affords an efficient route to a nicotinamide derivative from cyanoacetylene.