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20H-Carbohydrates.Pdf Carbohydrates Carbohydrates are compounds that have the general formula CnH2nOn Because CnH2nOn can also be written Cn(H2O)n, they appear to be “hydrates of carbon” Carbohydrates are also called “sugars” or “saccharides” Carbohydrates can be either aldoses (ald is for aldehyde and ose means a carbohydrate) or ketoses (ket is for ketone) OH OH O OH CH2OH CH2OH OHC HOH2C OH OH OH OH An Aldose A Ketose (D-Glucose) (D-Fructose) Carbohydrates Due to the multiple chiral centers along a linear carbon chain for carbohydrates, Emil Fischer developed the “Fischer Projection” in order to represent these compounds Remember how to draw a Fischer projection: 1) View the linear carbon chain along the vertical axis (always place the more oxidized carbon [aldehyde in an aldose] towards the top) 2) The horizontal lines are coming out of the page toward the viewer 3) Will need to change the viewpoint for each carbon so the horizontal substituents are always pointing towards the viewer CHO OH OH H OH HO H CH2OH = OHC H OH OH OH H OH CH2OH Emil Fischer (1852-1919) Carbohydrates The aldoses are thus all related by having an aldehyde group at one end, a primary alcohol group at the other end, and the two ends connected by a series of H-C-OH groups CHO CHO CHO CHO CHO H OH H OH H OH H OH HO H CH2OH H OH H OH H OH HO H CH2OH H OH H OH HO H CH2OH H OH HO H CH2OH CH2OH Aldotriose Aldotetrose Aldopentose Aldohexose Aldohexose D-glyceraldehyde D-erythose D-ribose D-allose L-allose The D-aldoses are named according to glyceraldehyde, the D refers to the configurational carbon (H-C-OH group next to primary alcohol), if OH is to the right in Fischer it is called D (after dextrorotatory – “to the right” in Latin), if OH is to the left in Fischer it is called L (after levorotatory – “to the left” in Latin) Naturally occurring sugar molecules have the D configuration Reactions of Carbohydrates Carbohydrates react similar to other aldehydes and carbonyl groups observed earlier Due to the presence of the other alcohol groups in a carbohydrate, aldoses readily form acetal and hemiacetal linkages when the aldehyde reacts H H H OH HOHO O HO O O O HO H HO HO H HC H OH H H OH OH H OH H H tetrahydrofuran HO H α-D-glucofuranose β-D-glucofuranose H OH H OH HOH HOH CH OH H O H O O 2 HO HO HO H HO OH H OH H OH OH H H H tetrahydropyran α-D-glucopyranose β-D-glucopyranose The hemiacetal formation thus forms ring structures, either 5-membered (furanoses) or 6-membered (pyranoses) rings are favored When the aldehyde reacts, a new chiral center is formed, these isomers are called “anomers” and designated as the α- or β-anomer Reactions of Carbohydrates The majority of the sugar molecules in solution are in the cyclic hemiacetal form, although in equilibrium with the aldehyde open form CHO H HOH OH H OH O H HOHO HO H O HO H HO H OH HO OH H H H OH H OH H OH H H CH2OH Aldohexose Pyranose form Furanose form Allose 92 8 altrose 70 30 The ratio of the pyranose and furanose glucose ~100 <1 forms depends upon the aldohexose mannose ~100 <1 being considered gulose 97 3 idose 75 25 galactose 93 7 talose 69 31 Reactions of Carbohydrates The 1H NMR of glucose also indicates the presence of the two anomers of the predominant pyranose form β HOH HOH H O H O HO HO HO H HO OH H OH H OH α H OH H H α-D-glucopyranose β-D-glucopyranose Aldohexose α-Pyranose β-Pyranose α-Furanose β-Furanose Allose 16 76 3 5 Altrose 27 43 17 13 Glucose 36 64 <1 <1 Mannose 66 34 <1 <1 Gulose 16 81 <1 3 Idose 39 36 11 14 Galactose 29 64 3 4 Talose 37 32 17 14 Haworth Form Another representation of carbohydrates in the hemiacetal form is to draw a “Haworth form” In the Haworth form, the ring is drawn in a planar perspective and the substituents are drawn either above or below the plane of the ring The Haworth form does not indicate the axial and equatorial relationship as the chair conformation does, but it is a convenient representation for the pyranose and furanose rings CHO OH H OH H CH2OH CH2OH H O OOH O HO H HO OH OH H OH HO OH H OH OH OH OH H OH H H OH OH CH2OH Fischer projection Chair conformation Haworth form Haworth form D-glucose β-D-glucopyranose β-D-glucopyranose α-D-glucopyranose Reactions of Carbohydrates Carbohydrates can undergo a variety of reactions similar to any other carbonyl compound The Kiliani-Fischer synthesis allows the conversion of a carbohydrate into another carbohydrate with one additional carbon, a so-called chain lengthening procedure N N HN H HN H C C C C O H C H C OH HO C H H C OH HO C H H OH H OH H OH H OH H OH NaCN H2/Pd HO H HO H HO H HO H HO H H OH H OH H OH "poisoned" H OH H OH CH2OH CH2OH CH2OH CH2OH CH2OH D-Xylose epimers H+, H2O O H O H Reaction of aldehyde with cyanide creates a cyanohydrin C C But two stereoisomers are created with new chiral center H C OH HO C H H OH H OH Reduction of nitrile with poisoned catalyst creates imine HO H HO H Which upon hydrolysis creates two new sugar compounds H OH H OH with one additional carbon CH2OH CH2OH (aldopentose becomes an aldohexose) D-Gulose D-Idose Reactions of Carbohydrates Carbohydrates can also have a chain shortening procedure through a “Ruff degradation” Ca O O CHO C 1) Fe2(SO4)3, H OH 1) Br2, H2O H OH H2O CHO HO H 2) Ca(OH)2 HO H 2) H2O2 (30%) HO H H OH H OH H OH H OH H OH H OH CH2OH CH2OH CH2OH D-Glucose D-Arabinose First the carbohydrate is oxidized to a carboxylic acid (Br2 is a selective oxidant) and the calcium salt is obtained by reaction with calcium hydroxide The calcium salt is then decarboxylated with ferric ion (need to use weak hydrogen peroxide to stop at aldehyde stage) Thus overall a aldohexose is converted into an aldopentose, maintaining the chirality at all remaining chiral centers Reactions of Carbohydrates In solution, carbohydrates are in the cyclic hemiacetal form the majority of the time CHO H OH HO H H OH HO O HO H HO O HO HO HO H H OH HO OH H H H H H OH H OH H H CH2OH α-D-mannopyranose β-D-mannopyranose The cyclic form equilibrates, however, with the open chain aldehyde form When the open form recloses to the hemiacetal, it could create two anomers (α and β) In solution, therefore, a carbohydrate equilibrates between the α and β forms (called mutarotation) Each carbohydrate has its own ratio of these forms at equilibrium Aldohexose α-Pyranose β-Pyranose α-Furanose β-Furanose Mannose 66 34 <1 <1 Reactions of Carbohydrates While in neutral solution carbohydrates equilibrate between the two anomers, when treated with base a carbohydrate equilibrates into both an epimer (by inversion of the stereocenter adjacent to the aldehyde) and by conversion of the aldose to a ketose Squiggly line means Chirality has CH2OH HOH both anomers HOH changed O H OH H O Ca(OH)2 HO O Ca(OH)2 HO HO H OH H H H OH H H OH OH H OH OH OH CH2OH D-Allose D-Altrose D-Psicose Epimerization occurs through enolate formation at α-position O H When enolate is CHO CH2OH H OH OH O protonated at α position, two epimers are obtained H OH Ca(OH)2 H OH H OH H OH H OH H OH When enolate equilibrates H OH H OH H OH with enol, a ketose is CH2OH CH2OH CH2OH obtained Reactions of Carbohydrates Any carbohydrate that contains a hemiacetal can equilibrate to the aldose form CHO HOH H OH H O H OH HO H H OH H OH OH OH H OH CH2OH In the presence of sodium borohydride, the aldehyde can be reduced to a primary alcohol (this is why the aldohexoses are called “reducing sugars”, the aldehyde is reduced to alcohol) CHO CH2OH H OH H OH H OH NaBH4 H OH H OH H OH H OH H OH CH2OH CH2OH Notice that the carbohydrate after reduction has two terminal primary alcohol groups, depending upon the chirality of the initial carbohydrate a meso compound can be obtained Reactions of Carbohydrates Carbohydrate can also be oxidized, but due to the presence of an aldehyde in aldoses and a multitude of alcohol groups (primary and secondary), different oxidizing conditions can selectively oxidize different parts of the carbohydrate Bromine in water selectively oxidizes only the aldehyde group into a carboxylic acid (the other alcohols in the molecule are unaffected) CHO CO2H H OH H OH H OH Br2 H OH H OH H2O H OH H OH H OH CH2OH CH2OH D-Allose D-Allonic acid The two ends of the allonic acid are different, thus allonic acid is a chiral molecule Reactions of Carbohydrates If stronger oxidizing conditions are used, both the aldehyde and the primary alcohol can be oxidized to carboxylic acids (typically reagent is nitric acid) [called aldaric acids] CHO CO2H H OH H OH H OH HNO3 H OH H OH H OH H OH H OH CH2OH CO2H Similar to the reduction of carbohydrates with NaBH4, this reaction also creates two identical end groups (both carboxylic acids) which can result in meso compounds CHO CO2H CHO CO2H H OH H OH H OH H OH HO H HNO3 HO H HO H HNO3 HO H H OH H OH HO H HO H H OH H OH H OH H OH CH2OH CO2H CH2OH CO2H D-Glucose Glucaric acid D-Galactose Galactaric acid chiral achiral Reactions of Carbohydrates Another oxidation observed earlier is when periodate reacts with vicinal diols O O O I O I O O O O O O I O O HO OH O O CH2 CH2 Vicinal primary alcohols are thus oxidized to formaldehyde O O I O O OH H2O HO OH O O O HO CH H H HO H 2 Aldehydes hydrate to a geminal diol which can be oxidized to formic acid O O O I O O I O OH O O O O O O HO OH CH2 OH HO H CH2 Secondary alcohols of a carbohydrate will be also be oxidized twice to formic acid Reactions of Carbohydrates Due to the variety of carbonyl or alcohol groups on adjacent carbons of carbohydrates, periodate oxidation of sugars was historically convenient to determine structure O O CHO HCO2H CH2OH H2C O O O O O H OH I HCO2H O I O C O HO H O HCO2H HO H O HCO2H H OH HCO2H H OH HCO2H H OH HCO2H H OH HCO2H CH2OH H2C O CH2OH H2C O D-Glucose D-Fructose Oxidation of glucose, or any aldohexose, produces 5 CH2OH O H2C O O O equiv.
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