Provided are phosphoramidate nucleoside compounds of Formula (I), or pharmaceutically acceptable salts or esters or solvates thereof, useful in the treatment of cancer, viral infections and other diseases: Also provided is a process for the synthesis of compounds of Formula (I) where a desired enantiomer, having regard to the asymmetric chiral center of the phosphorus atom P, is provided in an enriched amount. The process comprises admixing a nucleoside with a phosphorochloridate in the presence of a catalyst comprising a metal salt selected from the group consisting of salts of Cu, Fe, La and Yb.