Total Synthesis of Entecavir: A Robust Route for Pilot Production

A practical synthetic route for pilot production of entecavir is described. It is safe, robust, and scalable to kilogram scale. Starting from (S)-(+)-carvone, this synthetic route consists of a series of highly efficient reactions including a Favorskii rearrangement-elimination-epimerization sequence to establish the cyclopentene skeleton, the Baeyer–Villiger oxidation/rearrangement to afford the correct configuration of the secondary alcohol, and a directed homoallylic epoxidation followed by epoxide ring-opening to introduce the hydroxyl group suitable for the Mitsunobu reaction. In addition, the synthesis contains only four brief chromatographic purifications.