An efficient method for the diastereoselective synthesis of α-fluoro-β-hydroxy esters based on the radical reduction of α-bromo-α-fluoro-β-hydroxy esters

When α-bromo-α-fluoro-β-hydroxy esters were allowed to react successively with trimethylaluminum in toluene at −15°C for 0.5 h and with tributyltin hydride in the presence of a catalytic amount of triethylborane at −15°C for 4 h, the corresponding threo-α-fluoro-β-hydroxyalkanoates were obtained highly diastereoselectively in good yields. On the other hand, the α-bromo-β-hydroxy esters having an aromatic substituent were subjected to the reaction with tris(trimethylsilyl)silane in the presence of a catalytic amount of triethylborane in THF at −78°C for 10 h to give the corresponding erythro-α-fluoro-β-hydroxyalkanoates with high erythro-selectivity in good yields, but those carrying an aliphatic substituent being reduced in an almost nonstereoselective manner.