THE CHEMISTRY OF GLUCAL HALOHYDRINS - THE EFFECT OF THE HALIDE ON EPOXIDE FORMATION

Chlorohydrins, bromohydrins, and iodohydrins, formed by hydroxyhalogen ation of tri-O-methyl glucal, undergo base induced cyclization to give glucal epoxides. The mechanism of the cyclization reaction was probed using H-1 NMR and deuterium incorporation studies. Cyclization and in situ trapping with Cs2CO3 in MeOD gave deuterated methyl 3,4,6-tri-O- methyl-alpha-D-mannoside and methyl 3,4,6-tri-O-methyl-beta-D-glucosid e, and an unsaturated aldehyde. These studies led to optimized stereos electivity for the epoxide formation. Reaction of gluco bromohydrins w ith NaH or LiHMDS in THF at 5 degrees C gave beta-epoxide, and reactio n of manno iodohydrins with KH and 18-crown-6 in toluene at -70 degree s C gave alpha-epoxide. The epoxides were opened by reaction with sodi um phenylthiolate to give phenyl 3,4,6-tri-O-methyl-1-thio-alpha-D-man nopyranoside (single isomer), and phenyl 3,4,6-tri-O-methy-I-thio-beta -D-glucopyranoside (>18:1), respectively.