3,4,6-TRI-O-BENZYL-ALPHA-D-ARABINO-HEXOPYRANOS-2-ULOSYL BROMIDE - A VERSATILE GLYCOSYL DONOR FOR THE EFFICIENT GENERATION OF BETA-D-MANNOPYRANOSIDIC LINKAGES

An expedient four-step sequence is described for the conversion of acetobromoglucose into the title 2-oxohexosyl (''ulosyl'') bromide 4. Due to its O-benzyl protection, 4 is considerably more reactive than its acylated analogs 1-3: Ag2CO3-promoted glycosidations with 2-propanol, diacetone-galactose, and methyl 2,3-O-isopropylidene-alpha-L-rhamnoside are complete within minutes and, in addition, are endowed with beta-specificity. This renders ulosyl bromide 4 a most propitious, indirect beta-D-mannosyl donor, inasmuch as the borohydride reduction of the beta-D-glycosiduloses formed (14-16 --> 19, 21, and 22) proceeds with manno selectivities of >20:1. Comparative evaluation of the manno/gluco ratios obtained in all 21 beta-D-arabino hexosidulose reductions (Table 1) reveals the 3-O-blocking group to have a pronounced effect on the outcome: >20:1 in cases with a 3-O-benzyl group versus only 2:1 to 3:1 in the presence of 3-O-acyl functions.