Block co-oligomers of tri-O-methylated and unmodified cello-oligosaccharides as model compounds for methylcellulose and its dissolution/gelation behavior

A novel synthetic method for co-oligomers of tri-O-methylated and unmodified cello-oligosaccharides was designed. These oligomers are of importance as model compounds for investigations on the dissolution behavior of commercial methylcelluloses. In this connection, insights into the chemical structure of 'cross linking loci' in the thermo reversible gelation of aqueous solution of methylcellulose are of particular significance. The synthetic procedure consists of glycosylation using glycosyl fluoride and oligomerization of sugar orthoester. Thus, phenyl 2,3,6-tri-O-methyl-beta-D-glucopyranosyl-(1 -> 4)-2,3,6-tri-O-methyl-1-thio-beta-D-glueopyranoside (1) as a glycosyl acceptor was glycosylated with 4-O-acetyl-2,3,6-tri-O-methyl-beta-D-glucopyranosyl-(1 -> 4)-2,3,6-tri-O-methyl-D-glucopyranosyl fluoride (2) as a glycosyl donor converted to give a cellotetraose derivative (3). Both reactants have been prepared from commercially available cellobiose. After deacetylation of 3, 3-O-benzyl-6-O-pivaloyl-alpha-D-glucopyranose 1,2,4-orthopivalate (5) was reacted with 4-hydroxyl group at non-reducing-end of cellotetraose derivative (4) to give the block cooligomer (6). After the deprotection of compound 6, tri-O-methylated-block-unmodified cello-oligosaccharides (18 and 18') (DP = 4-8, DS = 2.79-1.38) were obtained, monitored by MALDI-TOF MS spectra. Chloroform-soluble methylated cellotetraose derivatives (18 and 18' (DP = 4, n = 0), DS = 2.57, and 2.79, respectively) were also soluble in the water solution of tri-O-methylated-block-unmodified cello-oligosaccharides (DP = 4-8, DS = 2.79-1.50). This fact indicated that hydrophobic methylated cello-tetraose derivatives were encapsulated within a micelle of amphiphlic tri-O-methylated-block-unmodified cello-oligosaccharides. It was found that solubilities of 18 and 18' (DP = 4-8, DS = 2.79-1.38) in water and chloroform were obviously different in the mixtures, depending on their DP and DS values. The substituent distribution of the tri-O-methylated-block-unmodified cello-oligosaccharides along one molecule and between molecules plays an important role in its solubility in water and chloroform.