With hypoxic and alkaline reaction conditions in hand which served well to ensure solubility and stability of the nucleobases 1 and 2, we turned our attention to the improvement of the previously observed low conversions. Recent work from our group had demonstrated the application of analytical-scale experiments and thermodynamic calculations for the yield optimization in NP-catalyzed transglycosylations. Therefore, we sought to transfer the same principles which had succeed for the synthesis of dihalogenated nucleosides to the preparation of Se-pyrimidines to obtain improved yields. Since nucleoside phosphorolysis (and consequently also the reverse reaction, glycosylation) is a thermodynamically controlled reaction, one may calculate the equilibrium state of transglycosylations via the corresponding equilibrium constants. Thus, when both equilibrium constants in the system are known, conversions can be optimized in silico to suggest conditions that enable the desired extent of product formation (for further details, please see ).