| Abstract
| - The synthesis of the C5‘ tert-butyl ketone of thymidine 1a and 2‘-deoxyguanosine 2 is achieved byreaction of 5‘-C-cyano derivatives with tert-butyl lithium followed by acid hydrolysis. The 5‘R configurationis assigned by X-ray crystal structure determination of an opportunely protected derivative of 1a. The(5‘S)-isomers of both nucleosides are not stable, and a complete decomposition occurs in the reactionmedium. The photochemistry of 1a and 2 effectively produced the thymidin-5‘-yl radical and the 2‘-deoxyguanosin-5‘-yl radical, respectively. In the thymidine system, the C5‘ radical is fully quenched inthe presence of a physiological concentration of thiols. In the 2‘-deoxyguanosine system, the C5‘ radicalundergoes intramolecular attack onto the C8−N7 double bond of guanine leading ultimately to the 5‘,8-cyclo-2‘-deoxyguanosine derivative. The cyclization of the 2‘-deoxyguanosin-5‘-yl radical occurs with arate constant of ca. 1 × 106 s-1 and is highly stereoselective affording only the (5‘S)-diastereomer.
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