Synthesis of Hexasaccharide Fragments of Pectin

Abstract

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.

Superarmed and Superdisarmed Building Blocks in Expeditious Oligosaccharide Synthesis

“…Madsen et al [43] clearly demonstrated that a single electron-withdrawing moiety at the remote C-6 position will sufficiently disarm the leaving group of a glycosyl acceptor, in comparison to the per-alkylated glycosyl donor. This effect was especially pronounced with the use of a pentafluorobenzoyl (PFBz) ester, capable of a very powerful electron-withdrawal [44].…”

Synthesis of Several Cleistrioside and Cleistetroside Natural Products via a Divergent De Novo Asymmetric Approach

“…Removal of chloroacetyl group using thiourea (3 equiv) in the presence of NaHCO 3 (3.3 equiv.) and catalytic Bu 4 NI,12 followed by deprotection of the acetonide group using 80% AcOH/H 2 O2a furnished the target cleistrioside- 5 (75% yield, two steps).…”

An array of possibilities for pectin

“…The synthesis of target molecules is well established in the production of nucleotide and protein microarrays but the synthesis of complex glycans is notoriously difficult. Nevertheless, Clausen et al produced a series of short oligogalacturides (OGAs) by chemical synthesis that were substituted with methyl esters attached at known positions along the short backbone, 19,20 and Guillaumie et al demonstrated the feasibility of enzymatic synthesis of OGAs. 21 The sourcing of samples for extracted glycan microarrays presents a completely different set of challenges because the aim is not to obtain pure pectins but rather to obtain a mixture that represents as far as possible the entire pectin population of a particular biological sample.…”