The predicted extent of product formation was observed for all four 2-Se-nucleosides (Figure , Table ) with reaction completion occurring within 3 to 4 h. Taken together, our thermodynamic characterization of the reaction system revealed unfavorable equilibrium constants of phosphorolysis of all Se-containing target nucleosides, severely limiting the maximum conversion achievable in transglycosylation reactions. However, a 10-fold excess of the sugar donors allowed for 40-48% conversion for all Senucleosides.