CHEMICAL SYNTHESIS OF 6'-ALPHA-MALTOSYL-MALTOTSIOSE, A BRANCHED OLIGOSACCHARIDE REPRESENTING THE BRANCH POINT OF STARCH

Chemical synthesis of the branched pentasaccharide 6'-alpha-maltosyl-m altotriose (15) is reported, based on the use of one synthon as a glyc osyl acceptor and another synthon as a glycosyl donor. The synthon use d as glycosyl acceptor was phenyl 2,3,6-tri-O-benzyl-1-thio-beta-D-glu copyranoside (7) and was synthesized from D-glucose with phenyl yl-4,6 -O-benzylidene-1-thio-beta-D-glucopyranoside and phenyl yl-4,6-O-benzy lidene-1-thio-beta-D-glucopyranoside as key intermediates, The synthon used as glycosyl donor was (2,3,4,6-tetra-O-benzyl-alpha-D-glucopyran osyl)-(1 --> )-O-(2,3,6-tri-O-benzyl-alpha-D-glucopyranosyl)-(1 --> (2 ,3,4,6-tetra-O-benzyl-alpha-D-glucopyranosyl)-(1 --> 4)]-2,3-di-O-benz yl-alpha,beta-D-glucopyranosyl trichloroacetimidate (12) and was synth esized from phenyl (2,3,4,6-tetra-O-benzyl-alpha-D-glucopyranosyl)-(1 --> )-O-(2,3,6-tri-O-benzyl-alpha-D-glucopyranosyl)-(1 --> (2,3,4,6-te tra-O-acetyl-alpha-D-glucopyranosyl)-(1 --> 4)]-2,3-di-O-acetyl-1-thio -beta-D-glucopyranoside with (2,3,4,6-tetra-O-benzyl-alpha-D-glucopyra nosyl)-(1 --> )-O-(2,3,6-tri-O-benzyl-alpha-D-glucopyranosyl)-(1 --> [ (2,3,4,6-tetra-O-benzy-alpha-D-glucopyranosyl)-(1 --> 4)]-2,3-di-O-ben zyl-D-glucopyranose as an intermediate. Condensation of compounds 7 an d 12 followed by removal of the phenylthio group and debenzylation pro vided the branched pentasaccharide 15. Alternatively, the branched pen tasaccharide was produced from amylopectin by consecutive alpha- and b eta-amylase treatments and purified by chromatography. The identity of the products obtained by chemical synthesis and enzymatic hydrolysis is documented by H-1 and C-13 NMR spectra.