“Standardized intermediates” for oligosaccharide synthesis. Precursors of β-linked, interior d-galactopyranose units having chain extension at position 4, or positions 4 and 2

Abstract Allyl 6- O -benzyl-3,4- O -isopropylidene-α-d-galactopyranoside (1) was used to prepare a series of 2- O -benzoyl-3,6-di- O -benzyl-α-d-galactopyranosyl halides carrying either a second benzoyl group (11, 17a) or a selectively removable, temporary protecting group (17b-d) at position 4. In one synthetic scheme, the 2-butenyl (crotyl) group was used for the transient protection of position 2, and the 2- O -benzoyl group was incorporated by selective acylation of a 2,4-diol (6) . In a more direct scheme, the 2- O -benzoyl group was introduced at the first step (1→12) . Selective benzylation of O-3 was accomplished by the action of α-bromotoluene on 3,4- O -dibutylstannylene derivatives (4 and 14) . Benzoyl, allyl, tert -butyldimethylsilyl, and tetrahydro-2-pyranyl groups, respectively, were incorporated at position 4 by derivatization of the otherwise fully substituted 1-propenyl glycoside 7 , or its precursor 15 . The fully substituted propenyl glycosides (8a-d) were converted into the 1-hydroxy compounds (9a-d) by mercuric ion-catalyzed hydrolysis, and thence directly into the chlorides (17a-d) by rection with hexamethylphosphorous triamide-carbon tetrachloride. The 2,4-di- O -benzoylated bromide 11 was made from 8a by a conventional sequence. On coupling to the 2-amino-2-deoxyglucoside 18 , it gave the disaccharide β-d-Gal p -(1→4)-d-GlcNAc in fully substituted form (19) .