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1 CHAPTER 24 Carbohydrates: Polyfunctional Compounds in Nature 24-1 Names and Structures of Carbohydrates Sugars are classified as aldoses and ketoses. Carbohydrate is the general name for the various forms of sugars (monosaccharides, disaccharides, trisaccharides, polysaccharides). A monosaccharide, or simple sugar, is an aldehyde or ketone containing at least two additional hydroxy groups. Aldoses are aldehydic sugars. Ketoses are ketotic sugars. Complex sugars are those formed by the linkage of simple sugars through ether bridges. Based on chain length, sugars are called trioses (C3), tetroses (C4), pentoses (C5) and hexoses (C6). Glucose, also known as dextrose, blood sugar or grape sugar is an aldohexose. Glucose is present in many fruits and plants and is present in blood at concentrations of 0.08-0.1%. Fructose is an isomeric ketohexose of glucose. Fructose is the sweetest natural sugar and is present in many fruits and in honey. Ribose is an aldopentose and is a building block of the ribonucleic acids. A disaccharide is derived from two monosaccharides by the formation of an ether (usually acetal) bridge. Hydrolysis regenerates the monosaccharides. Trisaccharides, tetrasaccharides and eventually polysaccharides are formed through additional ether bridges. Starch and cellulose are two important biological polysaccharides. Most sugars are chiral and optically active. The simplest chiral sugar is 2,3-dihydroxypropanal (glyceraldehyde). With the exception of the ketose, 1,3-dihydroxyacetone, most biological sugars contain at least one stereocenter. The older D-L convention for naming sugars is still in general use. In this convention, monosaccharides whose highest numbered stereocenter has the same absolute configuration as that of D- (+)-2,3-dihydroxypropanal (D-glyceraldehyde) are labeled D. Those having the opposite absolute configuration are labeled L. Two diastereomers that differ only at one stereocenter are called epimers
he D.L nomencature divides sugars into two arous a8aaow D-glyceraldehyde is detrorotatory D-er 毛目 格 目 追重递用 雪 非用 24-2 Conformations and Cycic Forms of Sugars iacetals nbermpSdgonsdeplta-edipsed S hree-and f to th 22 2
2 The D,L nomenclature divides sugars into two groups. For a pentahydroxyhexanal there are 16 stereoisomers divided into two groups: 8 stereoisomers labeled D, and their 8 enantiomers labeled L. Systematic nomenclature of sugar molecules leads to long complex names. As a result, the common names of most sugars are usually used, for example: erythrose and threose for the four aldotetroses. Note that the D (or R) label does not necessarily imply (+) and L (or S) does not necessarily imply (-). D-glyceraldehyde is detrorotatory. D-erythrose is levorotatory. Almost all naturally occurring sugars have the D absolute configuration. 24-2 Conformations and Cyclic Forms of Sugars Fischer projections depict all-eclipsed conformations. The all-eclipsed Fischer projection representation of a sugar molecule can be translated into an all-eclipsed dashed-wedged line structure: Rotation at C3 and C5 by 180 degrees leads to the all-staggered conformation. Sugars form intramolecular hemiacetals. Hexoses and pentoses exist in solution as an equilibrium mixture with their cyclic hemiacetal isomers, in which the hemiacetals strongly predominate. Six-membered rings are the preferred products, however, five membered rings are known. Three- and four-membered rings are too strained to form. Six-membered rings are based upon the six-membered cyclic ether, pyran, and are called pyranoses. Five-membered rings are based upon the five-membered cyclic ether, furan, and are called furanoses
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3 Upon formation of the hemiacetal, a new stereocenter is created. When the new stereocenter is S in a D-series sugar or R in an Lseries sugar, the diastereoisomer is labeled α. When the new stereocenter is R in a D-series sugar or S in an Lseries sugar, the diastereoisomer is labeled β. This type of diastereoisomer formation is unique to sugars. These isomers have been given a separate name: anomers. The new stereocenter is called the anomeric carbon. Fischer, Haworth and chair cyclohexane projections help depict cyclic sugars. Fisher projection formulas represent cyclic sugars by drawing an elongated line indicating the bond formed upon cyclization. In the α form of a D sugar, the newly formed anomeric hydroxyl points to the right. In the β form, it points to the left. Haworth projection formulas of sugars are written in line notation as a pentagon or hexagon, with the anomeric carbon on the right and the ether oxygen on the top. The bottom bond (between C2 and C3) is assumed to be in front of the plane of the paper and the ring bonds containing the oxygen are assumed to be in the back. For a D sugar, the α anomer has the OH group on the anomeric carbon pointing down; the β anomer has it pointing up. The equivalent conformation pictures of glucofuranose and glucopyranose are:
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4 Most aldohexoses adopt the chair conformation placing the bulky hydroxymethyl group at C5 in the equatorial position. For glucose, this means that in the α form four of the five substituents are equatorial, while only one is axial. The other seven D aldohexoses contain one or more axial substituents. Anomers of Simple Sugars: Mutarotation of Glucose 24-3 The specific rotation of pure α-D-(+)-glucopyranose in water is +112o, while specific rotation of pure β-D-(+)-glucopyranose is +18.7o. If either anomer is placed in water, the initial value of the specific rotation slowly changes to a constant value of +52.7o. This change in specific rotation is caused by the slow formation of an equilibrium mixture of α and β anomers, a process called mutarotation. This is a property of all sugars. Polyfunctional Chemistry of Sugars: Oxidation to Carboxylic Acids 24-4 Fehling’s and Tollens’s tests detect reducing sugars. The formyl group in an aldose and the α-hydroxy group in a ketose can be oxidized by Cu2+ (Fehling’s test) or by Ag+ (Tollens’s test). Aldoses are oxidized to aldonic acids, while ketoses are oxidized to dicarbonyl compounds. Oxidation of aldoses can give mono- or dicarboxylic acids. Aldonic acids can be prepared by oxidizing aldoses using bromine in a buffered aqueous solution (pH = 5-6). Upon evaporation of water, the γ lactone spontaneously forms. More vigorous oxidation causes reaction at the primary hydroxyl group, as well as at the carbonyl group to form an aldric acid. 24-5 Oxidative Cleavage of Sugars Periodic acid, HIO4, causes C-C bond cleavage between vicinal diols to give carbonyl compounds
24-6 Reduction of Monosaccharides to Alditols teamgt8auaarhaiO nsumed indicates the CH.OH CH,O 24-7 Carbonyl Condensations with Amine Derivatives s are stable and no further molecules of phenylhydrazin an os N一NB -NC 24-8 Ester and Ether Formation:Glycosides Williamson ether synthesis allows complete methylation of sugar Sugars can be esterfied and methylated. ncluding the anomeric hydroxyl. OH- 。c。8 NO H 5
5 Exhaustive oxidation of a sugar with HIO4 results in a mixture of one carbon compounds. Analysis of this mixture is useful in the elucidation of the structure of the original sugar. The number of equivalents of HIO4 consumed indicates the number of carbon atoms in the sugar. Each one-carbon fragment produced carries the same number of hydrogen substituents as it did in the original sugar. CHO Î HCOOH C=O Î CO2 CHOH Î HCOOH CH2OH Î CH2O 24-6 Reduction of Monosaccharides to Alditols The same reagents used to reduce aldehydes and ketones to alcohols can be used to reduce aldoses and ketoses to polyhydroxy compounds called alditols. D-Glucitol is found in red seaweed in concentrations as high as 14%. It is also found in many berries, cherries, plums, pears and apples. 24-7 Carbonyl Condensations with Amine Derivatives The carbonyl function in aldoses and ketoses will undergo condensation reactions with amine derivatives. Treatment of a sugar with phenylhydrazine yields the corresponding hydrazone. A second molecule of phenylhydrazine then adds forming an osazone. Osazones are stable and no further molecules of phenylhydrazine react. Osazones, unlike their parent sugars, crystallize readily to form solids with well defined melting points, simplifying isolation and characterization of many sugars. 24-8 Ester and Ether Formation: Glycosides Sugars can be esterfied and methylated. Monosaccharides can be converted into esters by standard techniques. Excess reagent converts all hydroxyl groups, including the anomeric hydroxyl. Williamson ether synthesis allows complete methylation of sugars. The acetal function can be selectively hydrolized: