S-Linked Thiomimetics of Phytoalexin-Elicitor-Active, Branched Oligosaccharides, Their Synthesis, Protein-Binding Ability and Phytoalexin-Inducing Activity

Abstract

The sulfur‐linked pentathiohexasaccharide 3I,3IV‐di‐β‐D‐glucopyranosylthiogentiotetraose (12) has been prepared by a convergent approach involving the reaction of 1,2,4‐tri‐O‐acetyl‐6‐deoxy‐6‐iodo‐3‐S‐(2,3,4,6‐tetra‐O‐acetyl‐β‐D‐glucopyranosyl)‐3‐thio‐β‐D‐glucopyranose (10) with the sodium salt of 2,3,4‐tri‐O‐acetyl‐6‐S‐[2,4‐di‐O‐acetyl‐3,6‐di‐S‐(2,3,4,6‐tetra‐O‐acetyl‐β‐D‐glucopyranosyl)‐3,6‐dithio‐β‐D‐glucopyranosyl]‐1,6‐dithio‐β‐D‐glucopyranose (4). A further reaction, involving the sodium salt of the peracetylated β‐1‐thio derivative of 12 with 1,2,3,4‐tetra‐O‐acetyl‐6‐deoxy‐6‐iodo‐β‐D‐glucopyranose (26), afforded the homologous sulfur‐linked hexathioheptasaccharide 3II,3V‐di‐β‐D‐glucopyranosylthiogentiopentaose (28). Related sulfur‐linked positional isomers 3II,3IV‐di‐D‐β‐glucopyranosylthiogentiotetraose (34) and 3III,3V‐di‐β‐D‐glucopyranosylthiogentiopentaose (39) have been prepared using analogous synthetic strategies. Thus, SN2 displacement of the iodine atom in 10 by the sodium salt of 2,4‐di‐O‐acetyl‐3,6‐di‐S‐(2,3,4,6‐tetra‐O‐acetyl‐β‐D‐glucopyranosyl)‐1,3,6‐trithio‐β‐D‐glucopyranose afforded a tetrathiopentasaccharide, which resulted in the pentathiohexasaccharide 34 by a sequence 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‐6I, 3II, 6II, 3IV, 6IV‐pentathio derivative 37, followed by deacylation. The four isomeric pentathiohexa‐ and hexathioheptasaccharides 12,34 and 28,39, respectively, were all found to be active in eliciting phytoalexin accumulation in soybean cotyledon tissue and in binding to a glucan‐binding protein of soybean, although to a lesser extent than the corresponding O‐oligosaccharides, the alternate thiohexa‐ and thioheptasaccharides 12,28 being more active as compared to the geminally branched isomers 34,39.

Non-conventional Glycosidic Linkages: Syntheses and Structures of Thiooligosaccharides and Carbohydrates with Three-bond Glycosidic Connections

“…Defaye et al [26] employed this approach for the synthesis of the sulfur-linked pentathiohexasaccharide 44, and its O-deprotected product 45. Thus, SN2 displacement of the iodide in 43 by the sodium thiolate formed by treatment of 42 with sodium hydride, afforded 44 (Scheme 9).…”