Photochemical syn–anti isomerization reactions in N4-methoxycytosines: A matrix isolation study

Photochemical syn–anti isomerizations were studied for N4-methoxycytosine and 1-methyl-N4-methoxycytosine isolated in argon or nitrogen low-temperature matrices. Upon UV (λ>295 nm) irradiation photoconversion of the syn isomers into the corresponding anti forms was observed. This demonstrates that, although the syn–anti photoisomerization concerns rotation of a large methoxyl group, steric interactions with the matrix cage do not preclude it. The final stage of the reaction was a photostationary state. Subsequent UV (λ>335 nm) irradiation led to photoisomerization in the opposite direction, resulting in partial recovery of the syn isomers of the investigated N4-methoxycytosines.