3,4,6-Tri-O-benzyl-α-d-arabino-hexopyranos-2-ulosyl Bromide: A Versatile Glycosyl Donor for the Efficient Generation of β-d-Mannopyranosidic Linkages

Abstract

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, diacetonegalactose, and methyl 2,3-O-isopropylidene-α-l-rhamnoside are complete within minutes and, in addition, are endowed with β-specificity. This renders ulosyl bromide 4 a most propitious, indirect β-d-mannosyl donor, inasmuch as the borohydride reduction of the β-d-glycosiduloses formed (14–16 → 19, 21, and 22) proceeds with manno selectivities of >20:1. Comparative evaluation of the mannolgluco ratios obtained in all 21 β-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. © 1994, American Chemical Society. All rights reserved.