S-linked thiomimetics of phytoalexin-elicitor-active, branched oligosaccharides, their synthesis, protein-binding ability and phytoalexin-inducing activity

The sulfur-linked pentathiohexasaccharide 3(I),3(IV)-di-beta-D-glucopyranos ylthiogentiotetraose (12) has been prepared by a convergent approach involv ing the reaction of 1,2,4-tri-O-acetyl-6-deoxy-6-iodo-3-S-(2,3,4,6-tetra-O- beta-D-glucopyranosyl)-3-thio-beta-D-glucopyranose (10) with the sodium sal t of 2,3,4-tri-O-acetyl-6-S-[2,4-di-O-acetyl-3,6-di-S-(2,3,4,6-tetra-O-acet yl-beta-D-glucopyranosyl)-3,6-di-S-(2,3,4,6-tetra-O-acetyl-beta-D-glucopyra nosyl]-1,6-dithio-beta-D-glucopyranose (4). A further reaction, involving t he sodium salt of the peracetylated beta-1-thio derivative of 12 with 1,2,3 ,4-tetra-O-acetyl-6-deoxy-6-iodo-beta-D-glucopyranose (26), afforded the ho mologous sulfur-linked hexathioheptasaccharide 3(II),3(V)-di-beta-D-glucopy ranosylthiogentiopentaose (28). Related sulfur-linked positional isomers 3( II),3(IV)-di-D-beta-glucopyranosylthiogentiotetraose (34) and 3(III),3(V)-d i-beta-D-glucopyranosylthiogentiopentaose (39) have been prepared using ana logous synthetic strategies. Thus, S(N)2 displacement of the iodine atom in 10 by the sodium salt of 2,4-di-O-acetyl-3,6-di-S-(2,3,1,6-tetraO-acetyl-b eta-D-glucopyranosyl)-1,3,6-trithio-beta-D-glucopyranose afforded a tetrath iopentasaccharide, which resulted in the pentathiohexasaccharide 34 by a se quence of reactions involving the 1-thioglycose 32 in reaction with 26. The hexathioheptasaccharide 39 was obtained conveniently by the reaction of 26 with the acetylated 1-thio-6(I), 3(II), 6(II), 3(IV), 6(IV)-pentathio deri vative 3?, followed by deacylation. The four isomeric pentathiohexa- and he xathioheptasaccharides 12,34 and 28,39, respectively, were all found to be active in eliciting phytoalexin accumulation in soybean cotyledon tissue an d in binding to a glucan-binding protein of soybean, although to a lesser e xtent than the corresponding O-oligosaccharides, the alternate thiohexa- an d thioheptasaccharides 12,28 being more active as compared to the geminally branched isomers 34,39.