Synthesis of two isomeric pentasaccharides, the possible repeating unit of the β-glucan from the micro fungus Epicoccum nigrum Ehrenb. ex Schlecht

Abstract Two isomeric pentasaccharides, β- d -Glc p -(1→3)-[β- d -Glc p -(1→6)]-β- d -Glc p -(1→3)-[β- d -Glc p -(1→6)]-β- d -Glc p ( I ) and β- d -Glc p -(1→6)-β- d -Glc p -(1→3)-[β- d -Glc p -(1→3)-β- d -Glc p -(1→6)]-β- d -Glc p ( II ), the possible repeating unit of the β-glucan from the micro fungus Epicoccum nigrum Ehrenb. ex Schlecht, were synthesized as their 4-methoxyphenyl glycosides in a regio- and stereoselective manner. The pentasaccharide I was obtained from 3- O -selective glycosylation of 4-methoxyphenyl 4,6- O -benzylidene-β- d -glucopyranoside ( 12 ) with 2,3,4,6-tetra- O -benzoyl-β- d -glucopyranosyl-(1→3)-[2,3,4,6-tetra- O -benzoyl-β- d -glucopyranosyl-(1→6)]-2,4-di- O -acetyl-α- d -glucopyranosyl trichloroacetimidate ( 6 ) followed by acetylation, debenzylidenation, and 6- O -selective glucosylation with 2,3,4,6-tetra- O -benzoyl-β- d -glucopyranosyl trichloroacetimidate ( 1 ), and then by deprotection. The pentasaccharide II was obtained from 3- O -selective coupling of 12 with 2,3,4,6-tetra- O -benzoyl-β- d -glucopyranosyl-(1→6)-2,4-di- O -acetyl-3- O -allyl-α- d -glucopyranosyl trichloroacetimidate ( 10 ) followed by acetylation, debenzylidenation, and 6- O -selective glycosylation with 2,3,4,6-tetra- O -benzoyl-β- d -glucopyranosyl-(1→3)-2,4,6-tri- O -acetyl-α- d -glucopyranosyl trichloroacetimidate ( 11 ), and finally by deprotection.