Chemical synthesis of 6′-α-maltosyl-maltotriose, a branched oligosaccharide representing the branch point of starch

Chemical synthesis of the branched pentasaccharide 6′-α-maltosyl-maltotriose (15) is reported, based on the use of one synthon as a glycosyl acceptor and another synthon as a glycosyl donor. The synthon used as glycosyl acceptor was phenyl 2,3,6-tri-O-benzyl-1-thio-β-d-glucopyranoside (7) and was synthesized from d-glucose with phenyl 2,3-di-O-acetyl-4,6-O-benzylidene-1-thio-β-d-glucopyranoside and phenyl 2,3-di-O-benzyl-4,6-O-benzylidene-1-thio-β-d-glucopyranoside as key intermediates. The synthon used as glycosyl donor was O-(2,3,4,6-tetra-O-benzyl-α-d-glucopyranosyl)-(1 → 4)-O-(2,3,6-tri-O-benzyl-α-d-glucopyranosyl)-(1 → 6)-O-[(2,3,4,6-tetra-O-benzyl-α-d-glucopyranosyl)-(1 → 4)]-2,3-di-O-benzyl-α,β-d-glucopyranosyl trichloroacetimidate (12) and was synthesized from phenyl O-(2,3,4,6-tetra-O-benzyl-α-d-glucopyranosyl)-(1 → 4)-O-(2,3,6-tri-O-benzyl-α-d-glucopyranosyl)-(1 → 6)-O-[(2,3,4,6-tetra-O-acetyl-α-d-glucopyranosyl)-(1 → 4)]-2,3-di-O-acetyl-1-thio-β-d-glucopyranoside with O-(2,3,4,6-tetra-O-benzyl-α-d-glucopyranosyl)-(1 → 4)-O-(2,3,6-tri-O-benzyl-α-d-glucopyranosyl)-(1 → 6)-O-[(2,3,4,6-tetra-O-benzyl-α-d-glucopyranosyl)-(1 → 4)]-2,3-di-O-benzyl-d-glucopyranose as an intermediate. Condensation of compounds 7 and 12 followed by removal of the phenylthio group and debenzylation provided the branched pentasaccharide 15. Alternatively, the branched pentasaccharide was produced from amylopectin by consecutive alpha- and beta-amylase treatments and purified by chromatography. The identity of the products obtained by chemical synthesis and enzymatic hydrolysis is documented by 1H and 13C NMR spectra.