Synthesis of hexasaccharide fragments of pectin

Short syntheses of partially methyl-esterified hexagalacturonates 1-5 are described as part of the development of strategies for the preparation of larger pectic oligosaccharides. The methodology is based on the repeated coupling of galactose mono- and disaccharide donors onto a galactose acceptor until a hexagalactan is obtained. All glycosylations are carried out with n-pentenyl glycosides to provide good yields of the desired alpha anomers. Pentenyl disaccharide donors are prepared by the coupling of two pentenyl galactosides controlled by either the armed-disarmed effect or by converting one pentenyl galactoside into the corresponding galactosyl bromide or fluoride. Two orthogonal protecting groups are employed at C6, which makes it possible to oxidize these positions to either the carboxylic acid or to the methyl ester. Each hexagalactan is therefore able to bifurcate into two different hexagalacturonates with a reverse methyl-esterification pattern. The methyl ester distribution in the hexagalacturonates is confirmed by tandem mass spectrometry.