INTRAMOLECULAR REACTIONS OF CARBOHYDRATE TRIFLATES

Each of the four possible methyl 2,6-dideoxy-beta-D-hexopyranosides (3-6) was treated with an equivalent amount of triflic anhydride in the presence of 2,6-di-tert-butyl-4-methylpyridine (7). The ribo isomer 3 produced both the 3-O-triflyl (8, 15%) and the 4-O-triflyl (9, 73%) derivatives while the lyxo isomer 4 gave only the 3-O-triflyl compound (11, 84%). Upon heating, the triflate 8 lost the elements of triflic acid to form methyl 2,3,6-trideoxy-beta-D-glycero-hexopyranosid-4-ulose (23, 72%), while 9 experienced ring contraction involving the pyranoid ring oxygen to form methyl 3,5-anhydro-2,6-dideoxy-beta-D-xylo-hexofuranoside (20, 54%). Compound 11 produced methyl 3-C-aldehydo-2,5-dideoxy-beta-D-threo-pentofuranoside (13, 65%). Upon reaction with triflic anhydride, methyl 2,6-dideoxy-beta-D-arabino- and -xylo-hexopyranosides (5 and 6, respectively) generated hydroxy triflates which were not isolated due to their further reaction below room temperature. Compound 5 produced aldehyde 13 (54%) while reaction of 6 gave methyl 3,4-anhydro-2,6-dideoxy-beta-D-lyxo- and -ribo-hexopyranosides [15 (48%) and 16 (24%), respectively]. The mechanisms for these reactions are discussed and the reactivity of the hydroxy triflates is compared to that of related carbohydrate derivatives.