Scalable Synthesis of a Nucleoside Phosphoramidate Prodrug Inhibitor of HCV NS5B RdRp: Challenges in the Production of a Diastereomeric Mixture

A scalable process is described for the synthesis of 2′-C-methyl­guanosine-5′-[2-[(3-hydroxy-2,2-dimethyl-1-oxopropyl)­thio]­ethyl-N-benzyl­phosphor­amidate], a nucleotide prodrug inhibitor of hepatitis C virus NS5B polymerase. The route features the use of phenyl­boronic acid to transiently protect the 2′,3′-hydroxyls of 2′-C-methyl­guanosine under mild conditions. The requirement to produce a 1:1 phosphorus diastereo­meric mixture precluded the use of traditional crystallization techniques. High sensitivity of the drug substance to acidic and basic conditions and its preferred solubility in mixed aqueous–organic solvents presented additional processing challenges. The use of reverse phase chromatography for the final purification was eliminated by the development of a dual liquid–liquid extraction protocol, which removed both non-polar and polar impurities whilst maintaining the 1:1 diastereo­meric ratio. Ethylene sulfide, a potential geno­toxic impurity that was observed at significant levels in the original procedure, was controlled to 99% purity in the requisite 1:1 diastereomeric ratio.