Add time:09/07/2019         Source:sciencedirect.com

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-Galp-(1→4)-d-GlcNAc in fully substituted form (19).

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