Abstract :
The peracetylated hexasaccharide 1,2,4-tri-O-acetyl-3-O-(2,3,4,6-tetra-O-acetyl-β-d-glucopyranosyl)-6-O-(2,3,4-tri-O-acetyl-6-O-(2,4-di-O-acetyl-3,6-di-O-(2,3,4,6-tetra-O-acetyl-β-d-glucopyranosyl)-β-d-glucopyranosyl)-β-d-glucopyranosyl)-α,β-d-glucopyranose 21 was synthesized in a blockwise manner, employing trisaccharide trichloroacetimidate 2,4-di-O-acetyl-3,6-di-O-(2,3,4,6-tetra-O-acetyl-β-d-glucopyranosyl)-α-d-glucopyranosyl trichloroacetimidate 17 as the glycosyl donor, and trisaccharide 4-O-acetyl-3-O-(2,3,4,6-tetra-O-acetyl-β-d-glucopyranosyl)-6-O-(2,3,4-tri-O-acetyl-β-d-glucopyranosyl)-1,2-O-(R,S)ethylidene-α-d-glucopyranose 18 as the acceptor. The donor 17 and acceptor 18 were readily prepared from trisaccharides 3-O-(2,3,4,6-tetra-O-acetyl-β-d-glucopyranosyl)-6-O-(2,3,4-tri-O-acetyl-6-O-chloroacetyl-β-d-glucopyranosyl)-1,2-O-(R,S)ethylidene-α-d-glucopyranose 10 and 3,6-di-O-(2,3,4,6-tetra-O-acetyl-β-d-glucopyranosyl)-1,2-O-(R,S)ethylidene-α-d-glucopyranose 11, respectively, which were obtained from rearrangement of orthoesters 3,4-di-O-acetyl-6-O-chloroacetyl-α-d-glucopyranose 1,2-(3-O-(2,3,4,6-tetra-O-acetyl-β-d-glucopyranosyl)-1,2-O-(R,S)ethylidene-α-d-glucopyranosid-6-yl orthoacetate) 8 and 3,4,6-tri-O-acetyl-α-d-glucopyranose 1,2-(3-O-(2,3,4,6-tetra-O-acetyl-β-d-glucopyranosyl)-1,2-O-(R,S)ethylidene-α-d-glucopyranosid-6-yl orthoacetate) 9, respectively. The orthoesters were prepared from selective coupling of the disaccharide 3-O-(2,3,4,6-tetra-O-acetyl-β-d-glucopyranosyl)-1,2-O-(R,S)ethylidene-α-d-glucopyranose 4 with ‘acetobromoglucose’ (tetra-O-acetyl-α-d-glucopyranosyl bromide) and 6-O-chloroacetylated ‘acetobromoglucose’, respectively. To confirm the selectivity of the orthoester formation and rearrangement, the disaccharide 4-O-acetyl-3-O-(2,3,4,6-tetra-O-acetyl-β-d-glucopyranosyl)-1,2-O-(R,S)ethylidene-α-d-glucopyranose 7 was prepared from 4 by selective tritylation, acetylation and detritylation. The title compound, an elicitor-active d-glucohexaose 3-O-(β-d-glucopyranosyl)-6-O-(6-O-(3,6-di-O-(β-d-glucopyranosyl)-β-d-glucopyranosyl)-β-d-glucopyranosyl)-α,β-d-glucopyranose 1, was finally obtained by Zemplén deacetylation of 21 in quantitative yield.
