Selective Generation and Reactivity of 5′-Adenosinyl and 2′-Adenosinyl Radicals

The reaction of hydrated electrons (e${{- \hfill \atop {\rm aq}\hfill}}$) with 8‐bromoadenosine 7 has been investigated by radiolytic methods coupled with product studies. Pulse radiolysis revealed that one‐electron reductive cleavage of the CBr bond gives the C8 radical 8 followed by a fast radical translocation to the sugar moiety. The reaction is partitioned between C5′ and C2′ positions in a 60:40 ratio leading to 5′‐adenosinyl radical 9 and 2′‐adenosinyl radical 11. This radical translocation from C8 to different sites of the sugar moiety has also been addressed computationally by means of DFT B3LYP calculations. In addition, ketone 21 was prepared and photolyzed providing an independent generation of C2′ radical 11. Both C5′ and C2′ radicals undergo unimolecular reactions. Radical 9 attacks adenine with a rate constant of 1.0×104 s−1 and gives the aromatic aminyl radical 10, whereas C2′ radical 11 liberates adenine with a rate constant of 1.1×105 s−1. Migration of the radical center in adenosinyl radical 1 from the heterocycle to the 5′‐ and 2′‐positions of the ribose is observed. These ribosyl radicals exhibit extremely different chemical reactions. Radical 2 attacks adenine and leads to cyclic products, whereas radical 3 cleaves off adenine in an elimination reaction.