Syntheses of β-(1→6)-branched β-(1→3)-linked d-galactans that exist in the rhizomes of Atractylodes lancea DC

Abstract

Effective syntheses of galactose hepta-, octa-, nona-, and decasaccharides that exist in the rhizomes of Atractylodes lancea DC were achieved with 2,3,4,6-tetra- O-benzoyl-α- d-galactopyranosyl trichloroacetimidate ( 1), 4-methoxyphenyl 2,3,4-tri- O-benzoyl-β- d-galactopyranoside ( 2), 6- O-acetyl-2,3,4-tri- O-benzoyl-α- d-galactopyranosyl trichloroacetimidate ( 5), 4-methoxyphenyl 6- O-acetyl-2,4-di- O-benzoyl-β- d-galactopyranoside ( 22), and 4-methoxyphenyl 2,4,6-tri- O-benzoyl-β- d-galactopyranoside ( 26) as the key synthons. Coupling of 2 with 1, followed by oxidative cleavage of 1-OMP and subsequent trichloroacetimidate formation gave the β-(1→6)-linked disaccharide donor 4. Condensation of 2 with 5 and subsequent selective deacetylation by methanolysis produced the β-(1→6)-linked disaccharide acceptor 7. Reaction of 7 with 4, oxidative cleavage of 1-OMP, and trichloroacetimidate formation produced the tetrasaccharide donor 9. The penta- ( 15), the hexa- ( 17), and the heptasaccharide donor 19 were synthesized similarly. Meanwhile, treatment of 1 with 22 yielded β-(1→3)-linked disaccharide 23 and α-(1→3)-linked disaccharide 25. Oxidative cleavage of 1-OMp of 23 followed by trichloroacetimidate formation produced the disaccharide donor 24. Coupling of 26 with 24, again, gave β-linked 27 and α-linked 29. Selective 6-O-deacetylation of 27 afforded the trisaccharide acceptor 28. TMSOTf-promoted condensation 28 of with the tetra- ( 9), penta- ( 15), hexa-( 17), and heptasaccharide donor 19, followed by deprotection, gave the target compounds.