Thymidine selective oxidation

Chromium trioxide in pyridine selectively oxidizes the hydroxymethyl groups in thymidine, 2 -deoxyadenosine, 2 -deoxyguano-sine, and 2 -deoxycytidine to carboxyl groups,525 but the partial liberation of the free, heterocyclic bases in the reactions suggested that oxidation at C-3 also occurs to some extent. 

Photolysis of pyruvic esters (see p. 129) has been used for the selective oxidation ol nucleosides. For example, this photochemical process was used to prepare 5 -0-trityl(or benzoyI)-3 -ketothymidine (610) from either 5 -0-trityl(or benzoyl)-thymidine (Scheme 93) or their 3 -epimers. The 5 -0-trityl-3 -kctonucleoside (610 ... 

The ability to incorporate deuterium or tritium selectively into appropriate positions on nucleotides has had a tremendous impact on mechanistic studies on sugar oxidation. In particular, Stubbe and coworkers have shown that when thymidine deuterated at the 4 position is incorporated into restriction fragments, sequence-selective isotope effects are observed for cleavage by Fe-BLM (12). These isotope effects are essentially equal for the radical and carbocation pathways (Scheme... 

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