Solid‐Phase Synthesis of a Phytoalexin Elicitor Pentasaccharide Using a 4‐Azido‐3‐chlorobenzyl Group as the Key for Temporary Protection and Catch‐and‐Release Purification

A phytoalexin elicitor pentasaccharide of the rice blast disease fungus, Pyricularia oryzae , was synthesized by a new route involving a solid‐phase method, in which a 4‐azido‐3‐chlorobenzyl (ClAzb) group was used for temporary protection of the hydroxy functions and catch‐and‐release purification. Thioglycosides possessing the ClAzb group were used as glycosyl donors and a macroporous polystyrene as a solid support. The saccharide chain was elongated by repeating a set of reactions: removal of the ClAzb group, glycosylation and capping of the unglycosylated hydroxy groups. Cleavage from the solid support by treatment with sodium methoxide afforded a crude pentasaccharide possessing the ClAzb group as a tag. The pentasaccharide was then purified by a catch‐and‐release procedure based on the specific reaction between the azido group and a solid‐supported phosphane. The final deprotection of all benzyl‐type protective groups gave the desired phytoalexin elicitor pentasaccharide 1 in overall 15% yield (from a solid‐supported monosaccharide 4 , average yield of 81% for each of a total of nine steps). The characteristic peaks of the azido group in the infrared spectrum were also useful for monitoring reactions in the solid phase. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)