Efficient synthesis of methyl lycotetraoside, the tetrasaccharide constituent of the tomato defence glycoalkaloid α-tomatine

Branched oligosaccharide lycotetraose, β-D-glucopyranosyl-(1→2)- [β-D-xylopyranosyl-(1→3)]-β-D-glucopyranosyl(1→4) -β-D-galactopyranose, is a key constituent of many steroidal saponins, including glycoalkaloid α-tomatine, which is involved in protection of plants from invading pathogens. A new synthesis of the methyl β-lycotetraoside (2) is described. Key steps of the synthesis include two successive glycosylation reactions of disaccharide acceptor methyl (4,6-O-benzylidene-3-O-p-methoxybenzyl-β-D-glucopyranosyl)-(1→4)-2,3, 6-tri-O-benzyl-β-D-galactopyranoside with readily available benzoylated trichloroacetimidates of α-D-glucopyranose and α,β-D- xylopyranose. This scheme allows sequential glycosylation in one-pot on account of the convenient in situ removal of a p-methoxybenzyl protecting group under the acid conditions of the first glycosylation step. Following deprotection, tetrasaccharide 2 was obtained in 19% yield over eight steps.