An Unusual Side Reaction in the Rearrangement of 3,4,6-Tri-O-Acetyl-1,2-O-(Allyloxyethylidene)-β-D-Mannopyranose with TMS-Triflate: Formation of an α(1→2)-Linked Disaccharide

Synthesis of a Glycosylphosphatidylinositol (GPI) Fragment as a Potential Substrate for Mannoprotein Transglycosidases

A glycophosphatidylinositol tetrasaccharide fragment was synthesized to mimic the core features of primary model, that of Saccharomyces cerevisiae. The salient feature of this approach is centered on the quick access to various α-1,2- and α-1,6-mannosyl and α-1,4-glycosyl linkages by using simple glycosylation and protective-group techniques. 1D and 2D-J-resolved NMR spectroscopy was used to verify the α-configuration of the new linkages. The tetrasaccharides obtained in this work are useful for examining fungal cell-wall glycoprotein cross-linking by transglycosidase enzymes for antifungal drug development.

A convenient synthesis of short-chain α-(1 → 2) mannopyranosyl oligosaccharides

Sugar 1,2-orthoesters are by-products of chemical glycosylation reactions that can be subsequently rearranged in situ to give trans glycosides. They have been used as donors in the synthesis of the latter glycosides with good regio- and stereo-selectivity. Alkyl α-(1 → 2) linked mannopyranosyl disaccharides have been reported as the major products from the rearrangement of mannopyranosyl orthoesters. Recent studies in this laboratory have shown that α-(1 → 2) linked mannopyranosyl di-, tri- and tetrasaccharides can be obtained in one step from mannopyranosyl allyl orthoester under optimized reaction conditions. In addition to the expected mono- and disaccharides (56%), allyl 2,3,4,6-tetra-O-acetyl-α-d-mannopyranosyl-(1 → 2)-3,4,6-tri-O-acetyl-α-d-mannopyranosyl-(1 → 2)-tri-O-acetyl-α-d-mannopyranoside and allyl 2,3,4,6-tetra-O-acetyl-α-d-mannopyranosyl-(1 → 2)-3,4,6-tri-O-acetyl-α-d-mannopyranosyl-(1 → 2)-3,4,6-tri-O-acetyl-α-d-mannopyranosyl-(1 → 2)-3,4,6-tri-O-acetyl-α-d-mannopyranoside were obtained in 23% and 6% isolated yields, respectively, from the oligomerization of a β-d-mannopyranosyl allyl 1,2-orthoester, along with small amounts of higher DP oligomers. Possible mechanisms for the oligomerization and side reactions are proposed based on NMR and mass spectrometric data.

Recent studies on reaction pathways and applications of sugar orthoesters in synthesis of oligosaccharides

Formation of sugar-sugar orthoesters consisting of a fully acylated mono- or disaccharide donor and a partially protected mono- or disaccharide acceptor is regioselective, and rearrangement of the orthoesters via RO-(orthoester)C bond cleavage gives a dioxolenium ion intermediate leading to 1,2-trans glycosidic linkage. The activity order of hydroxyl groups in the partially protected mannose and glucose acceptors is 6-OH>3-OH>2- or 4-OH. The coupling reactions with acylated glycosyl trichloroacetimidates as the donors usually give orthoesters as the intermediates specially when the coupling is carried out at slowed rates, and this is successfully used in regio- and stereoselective syntheses of oligosaccharides. Mannose and rhamnose orthoesters readily undergo O-2-(orthoester)C bond breaking, and this is used for synthesis of alpha-(1-->2)-linked oligosaccharides. (1-->3)-Glucosylation is special since the rearrangement of its sugar orthoester intermediates can occur with either RO-(orthoester)C bond cleavage with formation of the dioxolenium ion leading to 1,2-trans linkage, or C-1-O-1 bond cleavage leading to 1,2-cis linkage, and this is dependent upon the structures of donor and acceptor that compose the orthoester.