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Rates of formation of anhydronucleosides and comparison with their corresponding oxy analogues.

Anhydronucleosides are reactive intermediates in the synthesis of modified nucleosides. Their rates of cyclization of 2,2'; 2,3' and 2,5'-anhydro 2-thiouridine were studied and compared with their oxy analogues. The order of displacement of the leaving group is as follows: 2' > 5' > 3'. It was also observed that the rates of cyclization onto the 5' or 3'-position was about 70 times faster in 2-thiouridine series as compared with the uridine series. Similarly the rate of formation of 2, 2'-anhydronucleoside is nearly 2 orders of the magnitude faster in the case of thionucleosides.