Development of a Diastereoselective Phosphorylation of a Complex Nucleoside via Dynamic Kinetic Resolution

The development of a diastereoselective nucleoside phosphorylation is described, which produces a single isomer of a complex nucleoside monophosphate pro-drug. A stable phosphoramidic acid derivative is coupled to the nucleoside, in a process mediated by HATU and quinine, to deliver the coupled product in high chemical yield and good diastereoselectivity. This unusual process was shown to proceed through a dynamic kinetic resolution of a 1:1 mixture of activated phosphonate ester diastereoisomers. The optimized conditions afforded the product with a combined [S,S(P)] and [S,R(P)] in-process yield of 89% and a similar to 7:1 [S,S(P):S,R(P)] diastereomeric ratio. Isolation of the major isomer was facilitated by single crystallization from anisole, where the product was obtained in 57% isolated yield, excellent purity (>95%), and a high diastereomeric ratio (>50:1).