Process Development of Voriconazole: A Novel Broad-Spectrum Triazole Antifungal Agent

In the synthesis of (2 R, 3 S )-2-(2,4-difluorophenyl)-3-(5-fluoro-4-pyrimidinyl)-1-(1 H -1,2,4-triazol-1-yl)-2-butanol (voriconazole), the relative stereochemistry is set in the addition of a 4-(1-metalloethyl)-5-fluoropyrimidine derivative to 1-(2,4-difluorophenyl)-2-(1 H -1,2,4-triazol-1-yl)-1-ethanone. The diastereocontrol of this reaction has been examined by variation of pyrimidine substitution pattern and by changes in the metalation and reaction conditions. Excellent diastereoselection (12:1) is obtained using an organozinc derivative of 6-(1-bromoethyl)-4-chloro-5-fluoropyrimidine. After removal of the chlorine from the pyrimidine ring, the absolute stereochemistry of voriconazole is established via a diastereomeric salt resolution process using (1 R )-10-camphorsulfonic acid. Synthetic routes to the pyrimidine partner have also been evaluated. The initial six-step development route from 5-fluorouracil has been superseded by a four-step synthesis involving fluorination of methyl 3-oxopentanoate and cyclisation with formamidine acetate.