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Reactions of Monosaccharides

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Reactions of Monosaccharides Monosaccharides Oxidation-Reduction Products Oxidation - Reduction H OH OH H OH OH [O] S R OH OH O O [O] = Ag+, NH , H O R R OH OH 3 2 OH OH or Br2 / H2O Aldoses are "reducing" sugars, that is, they are oxidized CHO CO H under very mild conditions. (Only the aldehyde is oxidized.) 2 H OH + H OH Ag H OH OH HO H HO H [H] + Ag OH H OH H OH O H OH H OH OH OH [H] = ? CH OH CH OH 2 NaBH4 2 1) Identify the starting sugar as D- or L-. 2) Draw Fisher projections for the products. 3) Draw a Haworth projection of the beta pyrano anomer. Sorbitol Redox Reactions of Monosaccharides Oxidation In a basic solution, ketoses are converted into aldoses Ketones and alcohols cannot be oxidized by Br2 Osazone Formation A strong oxidizing agent such as HNO can oxidize the 3 Aldoses and ketoses react with three equivalents of aldehyde and the alcohol groups phenylhydrazine The C-2 epimers of aldoses form identical osazones Reaction of Ketoses with Phenylhydrazine The carbon chain of an aldose can be increased by one The Ruff degradation shortens an aldose chain by one carbon in a Kiliani–Fischer synthesis carbon Preparation of the Calcium D-Gluconate for the Monosaccharides Ruff Degradation D-glucose beta anomer O CHO OH H OH HO H O HO H OH H OH OH H OH OH CH OH all groups are equatorial 2 or 1,2 - trans to each other OH HOCH2 O OH HO HO Cyclic Structure of Monosaccharides Note … Hemiacetal Formation • If an aldose can form a five- or six-membered ring, it will exist predominantly as a cyclic hemiacetal • A sugar with an aldehyde, a ketone, a hemiacetal, or a hemiacetal group is a reducing sugar anomer anomer The specific rotation of pure α-D-glucose or β-D-glucose changes over time to reach an equilibrium (mutarotation) Monosaccharides ester formation Acylation of Monosaccharides CHO O O H OH CH3CO2 O2CCH3 HO H O H3CO O OCH3 H OH O2CCH3 alpha + anomer H OH O2CH3 O CCH CH2OH 2 3 O2CCH3 CH3CO2CH2 O O2CCH3 CH3CO2 CH3CO2 Reactions of Monosaccharides Alkylation of the OH Groups Glycoside (acetal) formation H OH OH O OCH3 HO OH CH3OH O + H HO OH OH OH CH3OH OH + + H H OH O HOCH2CH OCH3 HO OH Mechanism of Glycoside Formation Formation of Glycosides The acetal (or ketal) of a sugar is called a glycoside The formation of a glycoside favors the α-glucoside Reactions of Monosaccharides product: the anomeric effect Acetonide (acetal) formation CH3 O OH cis CH3 O O O OH O OH HCl O + H3C CH3 HO O OH OH CH trans H3C 3 Disaccharides Composed of two monosaccharide subunits hooked Formation of an N-Glycoside together by an acetal linkage In α-maltose, the OH group bonded to the anomeric carbon is axial Maltose is a reducing sugar In cellobiose, the two subunits are hooked together by a In lactose, the two different subunits are joined by a β-1,4’-glycosidic linkage β-1,4’-glycosidic linkage Cellobiose is a reducing sugar Lactose is a reducing sugar The most common disaccharide is sucrose Polysaccharides Amylose is a component of starch Sucrose is not a reducing sugar Amylopectin is another polysaccharide component of starch that has a branched structure An example of a naturally occurring product derived from carbohydrates Monosaccharides Identification: 1H NMR couplings Examples Dihedral angles 1,2-disubstituted cyclohexanes Coupling Constants Using Coupling Values • A mixture of anomers of D-glucose was separated and analyzed by nmr. • The coupling constants of the epimeric protons were 3.5 Hz for anomer X and 8.6 Hz for anomer Y. • Identify the alpha and beta anomer. Possible Exam Questions Dihedral Angles • Given a Haworth structure: a) draw the Fisher structure of the open straight chain, identify it as: eg. aldose, pentose, reducing, etc. b) draw a Haworth structure for the opposite anomer of the given structure, c) draw the most stable chair conformer, is it the α- or β-? • Identify an isomer: D- or L- from a structure. How many possible stereoisomers could the structure possibly have? • Give an example of mutarotation. • Identify/name structures: saccharides, glycosides, glycosamine, etc. The larger coupling constant corresponds to the larger dihedral angle. Therefore X is alpha. Olestra, Fats & Lipids http://www.cspinet.org/olestra/ OH HO OH OH O HO OH HO O O HO OOCH3 CH3COO OOCCH 3 OOCCH3 O CH3COO OOCCH3 CH COO O 3 O Olestra CH3COO R Can Be: O ( ) O ( ) O ( ) Simplesse O CH2OH O H H N N ( N ) H CH3 O CH2CH2CO2H.
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