Intermediate compounds and process for the preparation of efinaconazole

A process for the preparation of Efinaconazole of formula (1), comprising the steps of:

• a) reacting a compound of formula (2) with an organometallic alkylating agent capable of introducing an ethyl group, to form an alcohol of formula (3):
• wherein X is a leaving group;
• b) converting compound (3) to an alkene of formula (4):
• c) reacting alkene (4) with 1,2,4-triazole, to form 1-(2-(2,4-difluorophenyl)but-2-en-1-yl)-1H-1,2,4-triazole (5):
• d) epoxidation of triazole-containing alkene (5), to form 1-((2-(2,4-difluorophenyl)-3-methyloxiran-2-yl)methyl)-1H-1,2,4-triazole (6);
• e) reacting epoxide (6) with 4-methylenepiperidine to form Efinaconazole of formula (1)