CHEM 2222 Chapter 23 Notes
CHEM 2222 Chapter 23 Notes Chem 212 - Organic Chemistry II
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This 34 page Class Notes was uploaded by annafen on Sunday May 29, 2016. The Class Notes belongs to Chem 212 - Organic Chemistry II at Vanderbilt University taught by Dr. Alissa Hare in Spring 2016. Since its upload, it has received 5 views. For similar materials see Organic Chemistry II in Chemistry at Vanderbilt University.
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Date Created: 05/29/16
23.1 Classification of Carbohydrates Carbohydrates • Carbohydrates were considered to be “hydrates of carbon” because their molecular formulas correspond to C n(H2O) m • A carbohydrate is a polyhydroxy aldehyde or polyhydroxy ketone • Carbohydrates have several things to consider - 1. Number of Monomers • • • • 1 Carbohydrates 2. Position of carbonyl group • • 3. Number of carbons • • • 4. Cyclic form 2 23.2 Fischer Projections and D, L Notation Fischer Projections and D, L Notation • Fischer projections are the two crossed lines that represent a tetrahedral carbon • To assign R or S, with the H pointing toward you, solve the configuration and assign the opposite 3 Allowed Manipulations • Fischer projections can be rotate by 180º in the plane of the page • If one group is held steady, the other 3 groups can be rotated clockwise or counterclockwise 4 Fischer Projections 1. Assign priorities to the four substitutents according to the Cahn-Ingold- Prelog rules 2. Perform the two allowed manipulations of the Fischer projection to place the lowest priority group at thetop or bottom 3. If the priority of the other groups 1→2→3 is clockwise then assign the carbon as R, if priority of the other groups 1→2→3 is counterclockwise then assign the center as S. • However, if the lowest priority group is pointing forward, you can assign the wrong stereochemistry and thenswitch it 5 Fischer Projections and D, L Notation • Remember for most molecules, we can’t correlate absolute configuration with D or L = (+) or (-) • Fischer arbitrarily assigned (+)-glyceralderhyde to be D and (-)- glyceraldehyde to be L • D-carbohydrates 6 Fischer Projections and D, L Notation • Carbohydrates are designated as D- or L- according to the stereochemistry of the highest numbered chiral carbon of the Fischer projection. • If the hydroxyl group of the highest numbered chiral carbon is pointing to the right, the sugar is designated asD (Dextro : Latin for on the right side). • If the hydroxyl group is pointing to the left, the sugar is designated asL (Levo : Latin for on the left side). 7 23.3 The Aldotetroses The Aldotetroses • Glyceraldehyde is the smallest carbohydrate – it is an aldotriose and has 3 carbons • The four stereoisomers of 2,3,4-trihydroxybutanal are the aldotetroses 8 23.4 Aldopentoses and Aldohexoses The Aldopentoses 3 • Aldopentoses have three chirality centers and therefore 2 = eight stereoisomers • The 4 L-aldopentoses have the same name as their enantiomer except the prefix is L- rather than D- 9 The Aldohexoses • In aldohexoses there are four chirality centers and therefore 2 = sixteen stereoisomers • Eight D-aldohexoses and eight L-aldohexoses 10 23.5 A Mnemonic for Carbohydrate Configurations Remembering the Aldohexoses • Provided the 8 D-aldohexoses are drawn in the correct order then the mnemonic is: “All altruists gladly make gum in gallon tanks.” • The words of the sentence stand for: allose, altrose, glucose, mannose, gulose, idose, galactose, talose. 11 23.6 Cyclic Forms of Carbohydrates Furanoses • Aldoses contain an aldehyde and alcohols which react to form cyclic hemiacetals • Five-membered cyclic hemiacetals are furanose forms • Six-membered cyclic hemiacetals are pyranose forms 12 Furanoses • Cyclization of carbohydrates to the hemiacetal creates a new chiral center • The hemiacetal or hemiketal carbon of the cyclic form of carbohydrates is the anomeric carbon • Carbohydrate isomers that differ only in the stereochemistry of the anomeric carbon are called anomers 13 From Fischer Projection to Haworth • Substituents that are right in a Fischer projection are “down” in the Haworth structure; those to the left are “up.” 14 Haworth Formulas and α and β 1. For carbohydrates of the D series, the configuration of the anomeric carbon is α if its OH is down, β if the OH at the anomeric carbon is up 3. For carbohydrates of the L series, the configuration of the anomeric carbon is α if its OH is up, β if the OH at the anomeric carbon is down 15 23.7 Cyclic Forms of Carbohydrates: Pyranose Forms Pyranose Formation From D-Ribose • Draw the eclipsed conformation and then the α- and β-pyranose forms 16 Pyranose-Furanose-Open Chain • Six-membered rings more stable so more abundant – exist in the chair conformation • Small amounts of open chain form and can also form the furanose 17 23.8 Mutarotation Mutarotation • The α- and β- anomers are in equilibrium and can interconvert through the open form • While anomers can be separated, if allowed to react with water, the anomer will undergo mutarotation and a mixture will form 18 23.9 Carbohydrate Conformation: The Anomeric Effect The Anomeric Effect • The anomeric effect stabilizes an electronegative substituent on the anomeric carbon by orbital mixing • The axial substituent is stabilized by interaction of a non-bonding electron pair on the ring O with the σ* orbital which lowers the energy • The electronegative group prefers to be axial 19 23.10 Ketoses Ketoses • Ketoses are less common than aldoses • Carbonyl is normally on C-2 20 Ketoses and Cylclic Hemiketals • Ketoses exist mainly as cyclic hemiketals • The anomeric carbon of a furanose or pyranose form of a ketose bears both a hydroxyl group and a CH 2OH group 21 23.11 Deoxy Sugars Deoxy Sugars • In deoxy sugars, a hydroxyl group is replaced by hydrogen 22 23.14 Glycosides: The Fischer Glycosidation Glycosides • Glycoside formation is related to acetal formation • The anomeric hydroxyl group is replaced by some other substituent • In Fischer glycosidation carbohydrates react with an alcohol in the presence of an acid catalyst • The reaction is thermodynamically controlled 23 Mechanism of the Fischer Glycosidation 24 23.15 Disaccharides Disaccharides • Disaccharides are comprised of two monomers connected at the anomeric position • The -OR group of the glycoside is a second sugar molecule 25 23.16 Polysaccharides Polysaccharides • Cellulose: glucose polymer made up of • Amylose: glucose polymer made up of 26 23.17 Application of Familiar Reactions to Monosaccharides Reactions 27 Reactions 28 Reactions 29 23.18 Oxidation of Monosaccharides Oxidation • C1 of aldoses can be selectively oxidized to the carboxylic acid (aldonic acids) with Br 2or Ag(I) (Tollen’s test) • Most commonly used method for preparing aldonic acids is by oxidation with bromine in aqueous solution or nitric acid 30 Oxidation of Monosaccharides • Oxidation of aldoses to aldaric acids with HNO 3 • Uronic Acids are carbohydrates in which only the terminal -CH OH is 2 oxidized to a carboxylic acid • Oxidative cleavage of vicinal diol functions in carbohydrates uses periodic acid (HIO 4) or sodium metaperiodate (NaIO ). 4 31 23.19 Glycosides: Synthesis of Oligosaccharides Synthesis of Oligosaccharides • Fischer glycosidation is thermodynamically controlled and favors the formation of pyranose over furanose • The anomeric effect causes the α stereoisomer to predominate over the β 32 Synthesis of Oligosaccharides General strategy: 1. Prepare a suitably protected glycosyl donor and a glycosyl acceptor. The glycosyl donor has a leaving group at the anomeric carbon. The glycosyl acceptor contains one nucleophilic OH. All the other hydroxyl groups are protected 2. Formation of the glycosidic C ⎯ O bond by a nucleophilic substitution 3. Removal of all protecting groups 33 Mechanism of Glycosidation 34
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