Next, we used thermodynamic calculations to predict the maximum conversions in transglycosylation reactions employing different sugar donor excesses (Figure , Table ). These predictions revealed that the high equilibrium constants of phosphorolysis of the target nucleosides limited the maximum conversion severely. For example, to obtain 50% conversion to the Se-nucleoside 1a, an approximately 20-fold excess of the sugar donor a would be necessary. However, such a high sugar donor excess is not suitable for preparative experiments due to high substrate costs and waste. In addition, we expected that this would also prevent an efficient workup and purification. Thus, we applied a sugar donor excess of 10 as a compromise to the aforementioned issues and maximum conversion. Although other sugar donors such as 5ethynyluridine or 7-methylguanosine or direct glycosylation approaches with the sugar phosphates a' or b' might provide higher conversions than the application of natural uridine (a) and thymidine (b), these starting materials are considerably more expensive and would render this synthetic approach unfeasible (Table ). Therefore, we decided to employ cheaply available a and b as sugar donors, which conveniently also offer some of the more favorable equilibrium constants of phosphorolysis among the natural nucleosides. Finally, we confirmed our predictions for 10 equivalents of a or b and evaluated the reaction times until equilibrium in analytical-scale experiments under optimized conditions.