(5 ' S)- and (5 ' R)-5 ',8-cyclo-2 '-deoxyguanosine: Mechanistic insights on the 2 '-deoxyguanosin-5 '-yl radical cyclization

The two diastereomeric forms (5'S) and (5'R) of 5',8-cyclo-2'-deoxyguanosine have been synthesized and fully characterized. They have been used as references for the investigation of gamma-irradiation of 2'-deoxyguanosine and photolysis of 8-bromo-2'-deoxyguanosine in aqueous solutions. The observed (5'R)/(5'S) ratio of 8:1 was obtained in both sets of experiments. The mechanism of the cyclization reaction is discussed in some detail, and the diastereomeric outcome is rationalized in terms of favorable hydrogen-bonded structures in the pro-(5'R) conformation.