Facile synthesis of the sugar cores from phenylpropanoid glycosides

Allyl 2-O-benzoyl-4,6-O-benzylidene-alpha-D-glucopyranoside (3), obtained by selective benzoylation of allyl 4,6-O-benzylidene-alpha-D-glucopyranoside (2) with benzoyl chloride-imidazole in anhydrous chloroform, reacted with 2, 3, 4-tri-O-benzoyl-alpha-L-rhamnopyranosyl bromide to give a disaccharide derivative (4), and an important intermediate (5) was obtained by cleavage of its acetal. Treatment of 5 with a series of glycopyranosyl bromides, protected by acetyl or benzoyl groups in benzene-nitromethane in the presence of Hg(CN)2 as a catalyst afforded four trisaccharides (6-9). The disaccharide (5) and trisaccharides (6-9) constitute the sugar cores of phenylpropanoid glycosides. A new glycosyl anomeric leaving group, trichloroacetoxy, was employed to prepare the disaccharide (4) and trisaccharides residue (8) efficiently and with high stereoselectivity.