Oligosaccharide Analogues of Polysaccharides

The α- and γ-CD analogues 6 and 18, which possess a hexa-2,5-diyne-1,6-dioxy unit, were synthesised by intramolecular coupling of the bis-O-propargylated maltohexaoside 4, or the analogous maltooctaoside 16, followed by deprotection. The dialkynylated linear oligosaccharides were obtained by glycosidation of propargyl alcohol with the thioglycosides 1 and 13, reductive cleavage of the benzylidene acetal, and propargylation of the terminal HO−C(4) group, respectively. The β-CD analogues 23 and 25, which possess a penta-1,3-diyn-1-yl-5-oxy unit, were similarly obtained by intramolecular oxidative coupling of 20 and 21, respectively. The linear dialkynylated oligosaccharides 20 and 21 were obtained by two consecutive glycosylations, first with the maltohexaosyl-S-glycoside 1 as donor, and then by glycosylation of the resulting propargyl maltohexoside with the C(4)-ethynylated donor 19. The proximity of the terminal units of maltooligosaccharides allowed a facile intramolecular cycloaddition of the azido alkyne 29 to the isomeric triazoles 30 and 31, which were deprotected to 32 and 33, respectively. Analysis of the intramolecular H-bonds in 6, 23, 25, 32, and 33 showed that insertion of a noncarbohydrate link interrupts a single flip-flop H-bond.