N6-substituent effect on the photooxidation of 2′,3′-O- isopropylideneadenosines with a pyrimido[5,4-g]pteridinetetraone N-oxide. Chemical evidence for the generation and reactivity of adenosyl cation radicals

Abstract

A comparative study on the photooxidation of 2′,3′-O-isopropylideneadenosine 1a and its N⁶-benzoyl, N⁶-monomethyl, and N⁶,N⁶-dimethyl derivatives, 1b–d, with a pyrimido[5,4-g]pteridinetetraone N-oxide (PPO) was carried out. The ease of photooxidative consumption of the adenosines by the PPO is in the order of 1d > 1c > 1a > 1b, which is parallel to their oxidation-peak potentials. Although substrates 1a and 1b underwent oxidative intramolecular cyclisation to the corresponding 5′-O,8-cycloadenosines, 2a and 2b, even in low yield, substrates 1c and 1d were exclusively oxidised at the N⁶-methyl group to give the corresponding N⁶-formyl derivatives, 3 and 4, together with minor amounts of demethylated products, 1a and 1c. The present observations provide chemical evidence for the generation and reactivity of adenosyl cation radicals.