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215-216 HH W13 Notes-Carbohydr Chem 215-216 HH W13 Notes – Dr. Masato Koreeda - Page 1 of 18. Date: April 8, 2013 Chapters 14.8; 23-1, 2, 5, and 7: Carbohydrates – Part I Carbohydrate nomenclature: http://www.chem.qmul.ac.uk/iupac/2carb/ Carbohydrates: ! e.g., • Polyhydroxylated aldehydes and ketones and their equivalents H O • Important constituents of both plants and animals (CHOH)n • D-glucose: The primary source of energy in the human body CH2OH • Hexoses: Sugars possessing six carbon atoms. • Pentoses: Sugars possessing five carbon atoms. • Aldoses: Sugars containing an aldehyde group. • Ketoses: Sugars containing a ketone group. • Monosaccharides: Carbohydrates that do not undergo cleavage on hydrolysis (treatment with water) to smaller molecules. I. Stereochemistry OH OH H H H OH 6 H 6 H 6 H 4 anomeric carbon 4 4 anomeric carbon HO O HO OH HO O 5 2 5 2 5 2 HO OH HO O HO H 3 1 3 1 H H 3 H 1 axial HO HO H H equatorial H H H HO OH one hemiacetal form of D-glucose open-chain form of D-glucose another hemiacetal form of D-glucose These are anomers and (C-1) epimers. Glucose reacts like an aldehyde since small amounts of the open-chain form are present at equilibrium. Glucose has 4 stereocenters → 24 = 16 stereoisomers possible → 8 pairs of enantiomers (1) Fischer projection formulas (a) (+)-Glyceraldehyde: configurational reference compound for all monosaccharides This stereochemistry is defined as "D" if the OH is projected to the right. O H O H O H vertical bonds go in R H OH H C OH C and horizontal bonds H HO CH2OH come out CH2OH CH2OH D-(+)-glyceraldehyde R-(+)-glyceraldehyde sign of optical rotation at the sodium D-line (589 nm) small upper-case D configurational designation • S-(-)-Glyceraldehyde has an L-configuration. • D-Stereochemistry and the sign of optical rotation have no direct correlation, although many D sugars are dextrorotatory (d or +). Chem 215-216 HH W13 Notes – Dr. Masato Koreeda - Page 2 of 18. Date: April 8, 2013 I. Stereochemistry (1) Fischer projection (cont’d) The configurational relatioship was established between D-glyceraldehyde and the naturally occurring positive-rotating grape sugar, (+)-glucose. OH OH OH O OH O 5 5 H 5 OH 4 2 HO 6 4 2 HO 6 4 2 HO 3 1 H 3 1 3 1 HO D HO HO O D HO HO D D-glucose D-fructose (found in many fruits) D-2-deoxyribose (found in DNA) [an aldohexose] [a ketohexose] [a deoxyaldopentose] (in its open-chain (in its open-chain (in its open-chain aldehyde form) aldehyde form) keto form) ------------------------------------------------------------------------------------------------------------------------ The Fischer projection of (+)-glucose • The more oxidized end of the chain (i.e., the aldehyde in this case) on top of the Fischer chain. • The first chiral center from the bottom of the chain determines the configuration (i.e., D or L). "view" through the "view" O O H surface of the paper 1 1 OH H H OH H OH H H OH 2 2 5 3 H HO 6 4 2 H H D-configuration HO 1 3 HO 3 H H OH HO H OH O H OH O H 6 H 4 4 "view" "view" HOH2C OH H OH (+)-glucose 5 5 H OH zig-zag conformation: 6 CH2OH CH2OH most favorable, all eclipsed! D-(+)-glucose natural conformation not a natural, D-(+)-glyceraldehyde in solution. stable conformation! For the conversion from the curved, eclipsed chain structure to the Fischer projection: 90° rotation O 1 HO HO OH H OH H OH 6 H H CH OH H H H 2 H 2 6 6 5 4 3 2 HO H H 1 5 H HO 1 5 H HO HO 4 1 H 3 O O HO H OH O H 2 3 OH HO 4 3 H 6 H 4 OH 2 "rotate" along HOH C HO H "rotate" along H the C -C -bond 2 5 OH HO H OH 1 2 the C3-C4-bond Look from this direction through the surface of the paper for the Fischer projection. Chem 215-216 HH W13 Notes – Dr. Masato Koreeda - Page 3 of 18. Date: April 8, 2013 Carbohydrate families: All aldoses are called “reducing sugars” because of the aldehyde group; they reduce certain metal ions and can be easily oxidized. Two well known reagents for reducing sugars are: 1) Ag(I) → Ag (0) in NaOH/H2O (Tollens test) 2) Cu(II)SO4 (blue) in NaOH/H2O → red Cu2O (Benedict’s reagent) Note: In addition to aldoses, ketoses are also reducing sugars. α-Hydroxyketones in general react with these reagents and can readily be oxidized. aldotriose: aldotetraoses: pentoses: O H O H O H O H O H H OH H OH HO H H OH H OH H OH H OH H OH HO H L! CH OH 2 H OH HO H CH2OH CH2OH CH OH CH OH D-glyceraldehyde D-erythrose D-threose 2 2 D-ribose: found in RNA L-arabinose Hexoses: There are 16 stereoisomers and 8 of these are D-sugars. Mnemonics for 8 D-aldohexoses: 1 1CHO 2 HO 2 H 3 HO 3 H H 4 HO 4 5 H 5 OH allose altrose glucose mannose gulose idose galactose talose 6 CH2OH All altruists gladly make gum in gallon tanks. You don’t need to memorize the structures of any sugars! 1) Which of the eight D-hexoses shown above represent epimeric pairs? 2) Draw the Fischer projection structures of L-glucose, D-galactose (C-4 epimer of D- glucose; a milk sugar), and D-mannose (C-2 epimer of D-glucose). + - 3) Draw the Fischer projection structure of L-alanine, (H3C)-CH(NH3 )C(=O)O . Chem 215-216 HH W13 Notes – Dr. Masato Koreeda - Page 4 of 18. Date: April 8, 2013 I. Stereochemistry: (1) Mutarotation OH OH H H H OH 6 H β-OH 6 H 6 H 4 H O 4 H O 4 HO 2 HO H 2 HO 5 2 5 2 5 2 HO OH HO O HO H 3 1 3 1 1 axial H H 3 H HO HO H H equatorial H H H HO OH α-OH open-chain form of D-glucose β−anomer α−anomer [α] +18.7° [α] +112° D virtually no concentration of this species D β−D-(+)-glucopyranose in the equilibrium mixture; only a transient α−D-(+)-glucopyranose or β−D-glucose intermediate. or α−D-glucose • After the equilibrium is reached, the optical rotation of the mixture shows: [α]D +53°. Therefore, the mixture consists of 63% of β-D- and 37% of α-D-glucopyranose. Based on: X • 18.7 + (1 - X) • 112 = 53 Note: (1) Pyranose vs furanose 6-membered ring 5-membered ring O O (HO)n (HO)n OH OH O O pyranose furanose pyran furan (2) β− vs α−stereochemistry: anomeric stereoisomers (see pages 5-6 for definitions) Cyclic sugars such as furanoses and pyranoses: the stereochemistry at the anomeric carbon relative to that at the stereo-defining center whether the sugar is D or L. If a D-sugar and the non-ether part of the ring For D-sugars: drawn in front and the ether portion drawn β behind a group (usually OH, OR, or X) O O ponting up at the anomeric center OH H 1 (i.e., at C-1) is defined as β 1 H α and the one pointing down is OH defined as α. non-ether part of the ring For L-sugars: • The C-6 group such as CH2OH, C(=O)OH usually α adopts an equatorial orientation. O O • The or has nothing to do with the axial or OH H α β 1 equatorial orientation of the group attached at C-1. 1 H β • For L-sugars definition is reversed OH axial Examples equatorial equatorial OH HO 6 OH 6 β-OH equatorial 4 H 4 O HO α-OH O 5 OH HO 5 2 HO 2 6 5 O 1 H HO OH HO OH 1 OH 3 1 3 3 H equatorial β-OH OH H 4 2 HO H H HO β−D-glucose enantiomers!! β−L-glucose α−L-glucose Chem 215-216 HH W13 Notes – Dr. Masato Koreeda - Page 5 of 18. Date: April 8, 2013 α- or β- at the anomeric carbon – Taken from: http://www.chem.qmul.ac.uk/iupac/2carb/06n07.html The anomeric center: The new center of chirality generated by hemiacetal or hemiketal ring closure is called the anomeric center. The two stereoisomers are referred to as anomers, designated α or β according to the configurational relationship between the anomeric center and a specified anomeric reference atom. The anomeric reference atom and the anomeric configurational symbol (α or β): The anomeric reference atom is the configurational atom of the parent, unless multiple configurational prefixes are used. If multiple configurational prefixes are used, the anomeric reference atom is the highest-numbered atom of the group of chiral centers next to the anomeric center that is involved in the heterocyclic ring and specified by a single configurational prefix. In the α anomer, the exocyclic oxygen atom at the anomeric center is formally cis, in the Fischer projection (i.e., the same side with respect to the carbon main chain), to the oxygen attached to the anomeric reference atom; in the β anomer these oxygen atoms are formally trans. The anomeric symbol α or β, followed by a hyphen, is placed immediately before the configurational symbol D or L of the trivial name or of the configurational prefix.
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