Synthetic Studies on Glycopeptides Concerned with Defense Response of Plants. I. Syntheses of Supprescins A and B.

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Two glycopeptides, supprescins A and B, that suppress the production of pisatin, a phytoalexin of pea, were synthesized. In the synthesis of supprescin A, condensation of 3, 4, 6-tri-O-acetyl-2-azido-2-deoxy-α-D-galactopyranosyl trichloroacetimidate or its glycosidic β isomer with N-(carbobenzoxy)-L-seryl-O-benzyl-L-seryl-glycine methyl ester was carried out in the presence of trimethylsilyl trifluoromethanesulfonate (TMSOTf) to give the monoglycosyl tripeptide derivatives. For the synthesis of supprescin B, glycosylation of 2, 3, 4, 6-tetra-O-acetyl-α-D-galactopyranosyl bromide and 1, 2, 3, 6-tetra-O-benzoyl-α-D-galactopyranose was promoted by silver trifluoromethanesulfonate (AgOTf) to provide a disaccharide derivative. The coupling of diglycosyl imidate, 2, 3, 4, 6-tetra-O-acetyl-β-D-galactopyranosyl-(1→4)-3, 6-di-O-benzoyl-2-azido-2-deoxy-D-galactopyranosyl trichloroacetimidate, and N-(carbobenzoxy)-L-seryl-O-benzyl-L-seryl-glycyl-4-benzyl-L-aspartyl-5-benzyl-L-glutamyl-O-benzyl-L-threonine methyl ester in the presence of TMSOTf afforded the diglycosyl hexapeptide derivatives. Reduction, followed by N-acetylation, and then removal of the remaining protecting groups afforded the desired supprescin B.

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