Industrial synthesis of the key precursor in the synthesis of the anti-influenza drug oseltamivir phosphate (Ro 64-0796/002, GS-4104-02):: Ethyl (3R,4S,5S)-4,5-epoxy-3-(1-ethyl-propoxy)-cyclohex-1-ene-1-carboxylate

Starting from (-)-quinic acid, the title compound was synthesized in seven chemical steps and an overall yield of 35-38%, The route of the improved Gilead synthesis was not changed, However, significant improvements in each step led to a doubled overall yield, a 30% reduction in the number of unit operations, and an excellent quality (greater than or equal to 99%) of the resulting epoxide. A highly regioselective method for the dehydration of a quinic acid to a shikimic acid derivative and for the reduction of a cyclic ketal was found, Alternatively, the title compound was synthesized in six chemical steps and 63-65% yield from commercially available (-)-shikimic acid, Compared to the optimized quinic acid route, the production time was reduced by about 50%, The quality of epoxide produced from either natural product was equivalent, Therefore (-)-shikimic acid is the preferred raw material, The absolute configuration of the epoxide was determined by X-ray single crystal structure analysis and it was demonstrated that the epoxide was stereoisomerically pure.