(Z)-1,2:5,6-Di-O-isopropyl­idene-α-d-ribo-hexofuranos-3-ulose O-benzyl­oxime

The title compound, C19H25NO6, is a Z diastereomer in which the phenyl ring of the 3-benzyl­oxime substituent and the 5,6-O-isopropyl­idene acetal are both located on the Si-face of the C=N double bond. Inter­molecular C—H⋯O inter­actions result in helical chains along the b axis of the monoclinic unit cell

The stereospecific formation of 1,2-cis glycosides via allyl-mediated intramolecular aglycon delivery

This thesis describes investigations into the possibility and scope of allylderived intramolecular aglycon delivery as a tool for the stereospecific formation of 1,2-cis glycosides. The three-step procedure involves isomerisation of the 2-O-allyl protection of a glycosyl donor, followed by iodonium-mediated tethering to an aglycon alcohol. Activation of the donor induces intramolecular delivery of the aglycon, to afford the 1,2-cis glycoside.The synthesis of ten glycosyl donors, including gluco-, manno- and rhamnopyranosyl and glucofuranosyl donors, as selenoglycosides, thioglycosides and glycosyl fluorides, is described.Isomerisation was carried out in excellent yield in all cases using a catalyst derived from Wilkinson's catalyst.The scope of N-Iodosuccinimide-mediated tethering is described. Attempted tethering to a selenoglycoside resulted in activation of the donor. Tethering to a thiomethyl glycoside was successful for unhindered alcohols, but sluggish for secondary carbohydrates, and activation of the donor became competitive in these cases. The use of glycosyl fluorides gave good tethering yields for many alcohols, including secondary carbohydrates. For hindered alcohols, tethering of succinimide became competitive, resulting in reduced yields.Examples of high-yielding tethering reactions using a novel source of iodonium ion are reported. A large excess of aglycon was unnecessary for good yields to be obtained. A 'reverse' tethering approach, whereby a donor with a free 2-hydroxyl is tethered to the isomerised allyl group of an aglycon, was found to be effective using this reagent.In all cases, activation gave stereospecific glycosylation, furnishing the 1,2-cis glycosides. Other glycosylated products still bearing functionality derived from the tether, including mixed acetals and enol ethers, were cleaved by acid treatment, affording further 1,2-cis glycoside. Slightly lower glycosylation yields were obtained in the rhamno and glucofurano systems. Investigations into the glycosylation mechanism were also undertaken.The stereospecific synthesis of the Glc 3 Man N-glycan tetrasaccharide via an iterative tethering and allyl-mediated intramolecular glycosylation strategy is reported.</p

Review Article Chemical Modification of Polysaccharides

Copyright © 2013 Ian Cumpstey.This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. This review covers methods for modifying the structures of polysaccharides. The introduction of hydrophobic, acidic, basic, or other functionality into polysaccharide structures can alter the properties of materials based on these substances.The development of chemical methods to achieve this aim is an ongoing area of research that is expected to become more important as the emphasis on using renewable starting materials and sustainable processes increases in the future.Themethods covered in this review include ester and ether formation using saccharide oxygen nucleophiles, including enzymatic reactions and aspects of regioselectivity; the introduction of heteroatomic nucleophiles into polysaccharide chains; the oxidation of polysaccharides, including oxidative glycol cleavage, chemical oxidation of primary alcohols to carboxylic acids, and enzymatic oxidation of primary alcohols to aldehydes; reactions of uronic-acid-based polysaccharides; nucleophilic reactions of the amines of chitosan; and the formation of unsaturated polysaccharide derivatives. 1

Carbasugar analogues of galactofuranosides: α-O-linked derivatives

Using an indirect method, we have synthesised α-linked carbasugar analogues of galactofuranosides for the first time. Ring opening of a β-talo configured carbasugar 1,2-epoxide by alcohol nucleophiles under Lewis acidic conditions proceeded with very good regioselectivity to give α-talo configured C1-substituted ethers with a free OH-group at the C2 position. Inversion of configuration at C2 by an oxidation–reduction sequence gave the α-galacto configured carbahexofuranose C1 ethers. A carbadisaccharide corresponding to the Galf(α1→3)Manp substructure from Apodus deciduus galactomannan was synthesised to exemplify the method

An easy access to 2,3,4,6-tetra-O-benzyl-D-galactopyranose and 2,3,6-tri-O-benzyl-D-glucopyranose

The use of cheap starting material in the preparation of building blocks of higher value is an open field of research in the carbohydrate area. An easy and high yielding access to two partially protected monosacchrides from lactose is described

The stereospecific formation of 1,2-cis glycosides via allyl-mediated intramolecular aglycon delivery

The stereospecific synthesis of the Glc 3 Man N-glycan tetrasaccharide via an iterative tethering and allyl-mediated intramolecular glycosylation strategy is reported