The allyl ether as a protecting group in carbohydrate chemistry. Part VI. The allyl ether as a ‘temporary’ protecting group in oligosaccharide synthesis

Allyl 2,3,4-tri-O-benzyl-6-O-but-2-enyl-α-D-galactopyranoside was treated with chloro(tristriphenylphosphine)rhodium(I) to give predominantly the corresponding prop-1-enyl glycoside. The prop-1-enyl group was removed by hydrolysis and the free sugar was converted into 2,3,4-tri-O-benzyl-6-O-but-2-enyl-α-D-galactopyranosyl chloride, which was condensed with benzyl 2,3,4-tri-O-benzyl-α-D-galactopyranoside to give an α-linked disaccharide in high yield. The but-2-enyl group was removed to give benzyl 6-O-(2,3,4-tri-O-benzyl-α-D-galactopyranosyl)-2,3,4-tri-O-benzyl-α-D-galactopyranoside, a suitable intermediate for the synthesis of trisaccharides. The fully benzylated derivatives of benzyl 6-O-(α- and β-D-galactopyranosyl)-α-D-galactopyranose were also prepared. 6-O-Allyl-2,3,4-tri-O-benzyl-D-glucopyranose was prepared and the allyl group was removed by isomerisation with chloro(tristriphenylphosphine)rhodium(I) and subsequent acidic hydrolysis.