![]() ![]() (2001) that lithium acts as the nucleophile that attacks the carbon at the 5-position and through a transition state the alcohol is substituted for the bromine group. On addition of the alcohol ROH and lithium carbonate, the OR replaces the bromine and on deprotecting the acetylated hydroxyls the product is synthesized in relatively high purity. D-glucose is first protected by forming the peracetate by addition of acetic anhydride in acetic acid, and then addition of hydrogen bromide which brominates at the 5-position. (2006) propose the Koenigs-Knorr method in the stereoselective synthesis of alkyl D-glucopyranosides via glycosylation, with the exception of using lithium carbonate which is less expensive and toxic than the conventional method of using silver or mercury salts. This method can be performed on a multi-kilogram scale. (2001) were able to achieve 100% yield of α- and β-D-glucosides. Employing a microwave oven equipped with refluxing apparatus in a rotor reactor with pressure bombs, Nüchter et al. (2001) have shown a new approach to Fischer glycosidation. Many other glycosides have important physiological functions. Pharmacologists often join substances to glucuronic acid via glycosidic bonds in order to increase their water solubility this is known as glucuronidation. ![]() When an anomeric center is involved in a glycosidic bond (as is common in nature) then one can distinguish between α- and β-glycosidic bonds by the relative stereochemistry of the anomeric position and the stereocenter furthest from C1 in the saccharide. Numbering, and α/β distinction of glycosidic bonds All of these modified glycosidic bonds have different susceptibility to hydrolysis, and in the case of C-glycosyl structures, they are typically more resistant to hydrolysis. C-glycosyl bonds have the glycosidic oxygen replaced by a carbon the term "C-glycoside" is considered a misnomer by IUPAC and is discouraged. Substances containing N-glycosidic bonds are also known as glycosylamines. In the same way, N-glycosidic bonds, have the glycosidic bond oxygen replaced with nitrogen. In analogy, one also considers S-glycosidic bonds (which form thioglycosides), where the oxygen of the glycosidic bond is replaced with a sulfur atom. Glycosidic bonds of the form discussed above are known as O-glycosidic bonds, in reference to the glycosidic oxygen that links the glycoside to the aglycone or reducing end sugar. The main storage polysaccharide of animal cellsĪ fibrous, tough, water-insoluble substance found in the cell walls of plants (esp.Adenosine, a component of RNA, results from the sugar ribose and adenine via the formation of an N-glycosidic bond (shown as the vertical line between the N and the sugar cycle) Heteropolysaccharides = contain two or more different kinds Homopolysaccharides = contain only a single monomeric species What is the difference between homopolysaccharides & heteropolysaccharides? identity of their recurring monosaccharide units The end of a chain in a disaccharide or polysaccharide with a free anomeric carbonĤ ways glycans (polysaccharides) differ from each otherġ. Joins the anomeric carbon of a sugar to a nitrogen atom in glycoproteins & nucleotides When a hydroxyl group of one sugar molecule (cyclic) reacts with the anomeric carbon of the other covalently joins two monosaccharides to form a disaccharide Sugars capable of reducing cupric ion ex) glucose When one ring form opens briefly into the linear form, then closes again to produce the other anomer ex) the alpha & beta anomers of D-glucose interconvert in aqueous solution Haworth perspective formulas (NOT Fischer projections used for linear sugar structures) What type of representation is best for cyclic sugar structures? ![]() Six-membered ring compounds named so because they resemble the six-membered ring compound "pyran" What is the relationship between anomers & anomeric carbon?Īnomers = isomeric forms of monosaccharides that differ only in their configuration about the hemiacetal or hemiketal carbon atomĪnomeric carbon = the carbonyl carbon atom Hemiketal = the derivative of a reaction between alcohols & ketones Hemiacetal = the derivative of a reaction between alcohols & aldehydes What is the difference between a hemiacetal & a hemiketal? Two sugars that differ only in the configuration around one carbon atom ex) D-glucose & G-mannose differ only in stereochemistry at C-2, D-glucose & D-galactose Three-dimensional sugar structures on paper ![]() What do Fischer projection formulas represent? Ketose = a monosaccharide where the carbonyl group is at any other position What is the difference between a ketose & an aldose?Īldose = a monosaccharide where the carbonyl group is at an end of the carbon chain ![]()
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