Synthesis of (1----4)-linked galacturonic acid trisaccharides, a proposed plant wound-hormone and a stereoisomer.

Syntheses of alpha-GalA-(1----4)-alpha-GalA-(1----4)-GalA, a proposed plant wound-hormone, and its stereoisomer beta-GalA-(1----4)-alpha-GalA-(1----4)-GalA are described. The key intermediates, tert-butyldiphenylsilyl O-(6-O-acetyl-2,3,4-tri-O-benzyl-alpha-D-galactopyranosyl)-(1----4)-O-(6 -O- acetyl-2,3-di-O-benzyl-alpha-D-galactopyranosyl)-(1----4)-6-O-acetyl-2,3 -di-O- benzyl-beta-D-galactopyranoside and tert-butyldiphenylsilyl O-(6-O-acetyl-2,3,4-tri-O-benzyl-beta-D-galactopyranosyl)-(1----4)-O-(6- O- acetyl-2,3-di-O-benzyl-alpha-D-galactopyranosyl)-(1----4)-6-O-acetyl-2,3 -di-O- benzyl-beta-D-galactopyranoside, were prepared by stereoselective alpha-glycosylation of tert-butyldiphenylsilyl 6-O-acetyl-2,3-di-O-benzyl-beta-D-galactopyranoside with O-(6-O-acetyl-2,3,4-tri-O-benzyl-alpha-D-galactopyranosyl)-(1----4)-6-O- acetyl-2,3-di-O-benzyl-D-galactopyranosyl fluoride and O-(6-O-acetyl-2,3,4-tri-O-benzyl-beta-D-galactopyranosyl)-(1----4)-6-O-a cetyl- 2,3-di-O-benzyl-D-galactopyranosyl fluoride, respectively. The corresponding beta isomers were minor products. Characteristic features in the n.m.r. data of the key intermediates are discussed.