Photocycloaddition of 5-bromouracil to uracil in a dinucleotide model compound

The photochemical reactivity of 5-bromouracil towards uracil in a dinucleotide model compound 1 has been examined. The formation of a cyclobutane adduct 2 as a major product (>90%), without debromination of the 5-bromouracil moiety was observed upon irradiation of 1 with near UV light (λ >300 nm) under both anaerobic and aerobic conditions. Upon heating, 2 undergoes cyclobutane ring cleavage with concomitant hydrolysis to form 5-hydroxyuracil analogue 3 as a major product.