PREARRANGED GLYCOSIDES .2. INTRAMOLECULAR GLYCOSYLATION OF PREARRANGED SACCHARIDES AS A NOVEL STRATEGY FOR THE CONSTRUCTION OF BETA-L-RHAMNOSIDIC LINKAGES

Disaccharides containing a beta-L-rhamnosyl residue were prepared by a novel intramolecular glycosylation strategy. Thus, suitably benzyl-pr otected L-rhamnosyl donors (ethyl and phenyl 1-thiorhamnoside 7, 18) w ere prearranged in positions 2 and 3 with benzyl -benzoyl-4,6-O-benzyl idene-alpha-D-glucopyranoside 8 in position 3 by a malonyl, succinyl a nd phthaloyl bridge, respectively, to give the corresponding connected saccharides 9, 12, 16 and 20. After regioselective opening of the ben zylidene ring of the glucoside residue to give compounds 10, 13, 17 an d 21, the latter afforded the 3,2'- and 3,3'-bridged disaccharides 22- 25 upon intramolecular glycosylation with a thiophilic reagent. The an omeric selectivity of this intramolecular glycosylation is strongly in fluenced by the nature of the alkanoyl and aroyl bridge, its position at the rhamnosyl residue and the solvent used for the coupling. Best r esults were obtained in acetonitrile with the succinyl ''spacer'' atta ched to position 2 of the phenyl 1-thiorhamnoside and position 3 of th e glucoside that afforded the corresponding beta-(1 --> 4)-linked disa cchride 22 beta in 76% yield.