Mod. Chapter 25

Mod. Chapter 25 - Modified Chapter 25 Carbohydrates Dr....

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Unformatted text preview: Modified Chapter 25 Carbohydrates Dr. Wolf's CHM 424 25- 1 25.1 Classification of Carbohydrates Dr. Wolf's CHM 424 25- 2 Classification of Carbohydrates monosaccharide disaccharide oligosaccharide polysaccharide Dr. Wolf's CHM 424 25- 3 Monosaccharide iis not cleaved to a simpler carbohydrate on s hydrolysis hydrolysis glucose, for example, is a monosaccharide Dr. Wolf's CHM 424 25- 4 Disaccharide iis cleaved to two monosaccharides on hydrolysis s these two monosaccharides may be the same or different same C12H22O11 + H2O 11 sucrose (a disaccharide) Dr. Wolf's CHM 424 C6H12O6 + C6H12O6 glucose (a monosaccharide) fructose (a monosaccharide) 25- 5 Higher Saccharides oligosaccharide: gives two or more monosaccharide units on hydrolysis hydrolysis is homogeneous—all molecules of a particular oligosaccharide are the same, including chain length polysaccharide: yields "many" monosaccharide units on hydrolysis yields mixtures of the same polysaccharide differing only Dr. Wolf's CHM 424 25- 6 Table 25.1 Some Classes of Carbohydrates No. of carbons Aldose Ketose 4 Aldotetrose Ketotetrose 5 Aldopentose Ketopentose 6 Aldohexose Ketopentose 7 Aldoheptose Ketoheptose 8 Aldooctose Ketooctose Dr. Wolf's CHM 424 25- 7 25.2 Fischer Projections and D-L Notation Fischer Dr. Wolf's CHM 424 25- 8 Fischer Projections Dr. Wolf's CHM 424 25- 9 Fischer Projections Dr. Wolf's CHM 424 25- 10 Fischer Projections of Enantiomers Dr. Wolf's CHM 424 25- 11 Enantiomers of Glyceraldehyde CH H D O OH CH2OH (+)-Glyceraldehyde Dr. Wolf's CHM 424 CH HO L O H CH2OH (–)-Glyceraldehyde 25- 12 25.3 The Aldotetroses Dr. Wolf's CHM 424 25- 13 An Aldotetrose 1 H H D CH 2 3 4 O OH OH CH2OH stereochemistry assigned on basis of whether configuration of highest-numbered stereogenic center iis analogous to D or L-glyceraldehyde s Dr. Wolf's CHM 424 25- 14 An Aldotetrose 1 H H CH 2 3 4 O OH OH CH2OH D-Erythrose Dr. Wolf's CHM 424 25- 15 The Four Aldotetroses CH O CH O H OH HO H D-Erythrose and -Erythrose L-erythrose are -erythrose H OH HO H enantiomers enantiomers CH2OH CH2OH D-Erythrose L-Erythrose Dr. Wolf's CHM 424 25- 16 The Four Aldotetroses CH H H O OH OH CH2OH D-Erythrose Dr. Wolf's CHM 424 CH HO H O H OH D-Erythrose and -Erythrose D-threose are -threose diastereomers diastereomers CH2OH D-Threose 25- 17 The Four Aldotetroses CH HO HO O H H CH2OH L-Erythrose Dr. Wolf's CHM 424 CH HO H O H OH L-Erythrose and -Erythrose D-threose are -threose diastereomers diastereomers CH2OH D-Threose 25- 18 The Four Aldotetroses CH D-Threose and -Threose L-threose are -threose enantiomers enantiomers HO H H OH CH2OH D-Threose Dr. Wolf's CHM 424 CH O H HO O OH H CH2OH L-Threose 25- 19 The Four Aldotetroses CH O CH O CH H OH HO H HO H OH HO H H CH2OH D-Erythrose Dr. Wolf's CHM 424 CH2OH L-Erythrose O H CH H OH H OHHO CH2OH D-Threose O CH2OH L-Threose 25- 20 25.4 Aldopentoses and Aldohexoses Dr. Wolf's CHM 424 25- 21 The Aldopentoses There are 8 aldopentoses. Four belong to the D-series; four belong to Four -series; the L-series. Their names are ribose, arabinose, xylose, Their and lyxose. and Dr. Wolf's CHM 424 25- 22 The Four D-Aldopentoses The CH O CH O CH O CH O H OH HO H H OH HO H H OH H H OH H OH HO OH H H HO OH H H OH CH2OH D-Ribose CH2OH D-Arabinose Dr. Wolf's CHM 424 CH2OH D-Xylose CH2OH D-Lyxose 25- 23 Aldohexoses There are 16 aldopentoses. 8 belong to the D-series; 8 belong to the Lbelong -series; series. Their names and configurations are best Their remembered with the aid of the mnemonic described in Section 25.5. described Dr. Wolf's CHM 424 25- 24 25.5 A Mnemonic for Carbohydrate Mnemonic Configurations Configurations Dr. Wolf's CHM 424 25- 25 The Eight D-Aldohexoses The CH CH H O OH CH2OH Dr. Wolf's CHM 424 25- 26 The Eight D-Aldohexoses The All CH CH Altruists O Gladly Make Gum In Gallon H OH CH2OH Tanks Dr. Wolf's CHM 424 25- 27 The Eight D-Aldohexoses The All Allose Altruists Altrose Gladly Glucose Make Mannose Gum Gulose In Idose Gallon Galactose Tanks Talose Dr. Wolf's CHM 424 CH CH H O OH CH2OH 25- 28 The Eight D-Aldohexoses The Allose CH CH Altrose O Glucose Mannose Gulose Idose Galactose Talose Dr. Wolf's CHM 424 H OH CH2OH 25- 29 The Eight D-Aldohexoses The Allose CH CH Altrose O Glucose Mannose Gulose H OH Idose H OH Galactose Talose Dr. Wolf's CHM 424 CH2OH 25- 30 The Eight D-Aldohexoses The Allose CH CH Altrose O Glucose Mannose Gulose Idose Galactose Talose Dr. Wolf's CHM 424 HO H H OH CH2OH 25- 31 The Eight D-Aldohexoses The Allose CH CH Altrose O Glucose Mannose Gulose H OH Idose H OH Galactose Talose Dr. Wolf's CHM 424 CH2OH 25- 32 The Eight D-Aldohexoses The Allose CH CH Altrose O Glucose Mannose H OH Gulose H OH Idose H OH Galactose Talose Dr. Wolf's CHM 424 CH2OH 25- 33 The Eight D-Aldohexoses The Allose CH CH Altrose O Glucose HO Gulose H H OH Idose H OH Mannose Galactose Talose Dr. Wolf's CHM 424 CH2OH 25- 34 The Eight D-Aldohexoses The Allose CH CH Altrose O Glucose Mannose Gulose Idose Galactose Talose Dr. Wolf's CHM 424 HO H H OH CH2OH 25- 35 The Eight D-Aldohexoses The Allose CH CH Altrose O Glucose Mannose Gulose Idose Galactose Talose Dr. Wolf's CHM 424 H OH HO H OH H CH2OH 25- 36 The Eight D-Aldohexoses The Allose CH CH Altrose O Glucose Mannose Gulose Idose Galactose Talose Dr. Wolf's CHM 424 HO HO H H H OH CH2OH 25- 37 The Eight D-Aldohexoses The Allose CH CH Altrose O Glucose Mannose H OH Gulose H OH Idose H OH Galactose Talose Dr. Wolf's CHM 424 CH2OH 25- 38 The Eight D-Aldohexoses The Allose CH CH Altrose O Glucose H OH Mannose H OH Gulose H OH Idose H OH Galactose Talose Dr. Wolf's CHM 424 CH2OH 25- 39 The Eight D-Aldohexoses The Allose CH CH Altrose O Glucose HO Mannose H OH Gulose H OH Idose H OH Galactose Talose Dr. Wolf's CHM 424 H CH2OH 25- 40 The Eight D-Aldohexoses The Allose CH CH Altrose O Glucose Gulose H H OH Idose H OH Mannose Galactose Talose Dr. Wolf's CHM 424 HO CH2OH 25- 41 The Eight D-Aldohexoses The Allose CH CH Altrose Glucose H HO O OH Gulose H H OH Idose H OH Mannose Galactose Talose Dr. Wolf's CHM 424 CH2OH 25- 42 The Eight D-Aldohexoses The Allose CH CH Altrose O Glucose HO H Mannose HO Gulose H H OH Idose H OH Galactose Talose Dr. Wolf's CHM 424 CH2OH 25- 43 The Eight D-Aldohexoses The Allose CH CH Altrose O Glucose Mannose Gulose Idose Galactose Talose Dr. Wolf's CHM 424 H OH HO H OH H CH2OH 25- 44 The Eight D-Aldohexoses The Allose CH CH Altrose Glucose Mannose Gulose Idose Galactose Talose Dr. Wolf's CHM 424 O H H OH HO H OH H OH CH2OH 25- 45 The Eight D-Aldohexoses The Allose CH CH Altrose O Mannose HO H H OH Gulose HO H OH Glucose Idose Galactose Talose Dr. Wolf's CHM 424 H CH2OH 25- 46 The Eight D-Aldohexoses The Allose CH CH Altrose O Glucose Mannose Gulose Idose Galactose Talose Dr. Wolf's CHM 424 HO HO H H H OH CH2OH 25- 47 The Eight D-Aldohexoses The Allose CH CH Altrose Glucose H Mannose HO HO Gulose Idose Galactose Talose Dr. Wolf's CHM 424 H O OH H H OH CH2OH 25- 48 The Eight D-Aldohexoses The Allose CH CH Altrose O Glucose HO H Mannose HO HO H Gulose Idose Galactose Talose Dr. Wolf's CHM 424 H H OH CH2OH 25- 49 L-Aldohexoses There are 8 There aldohexoses of the L-series. H They have the They HO same name as H their mirror image except the prefix is H L- rather than D-. rather CH CH O OH HO H OH H OH CH2OH D-(+)-Glucose Dr. Wolf's CHM 424 CH CH HO HO O H OH H H CH2OH L-(–)-Glucose 25- 50 25.6 Cyclic Forms of Carbohydrates: Furanose Forms Dr. Wolf's CHM 424 25- 51 Recall from Section 17.8 R R C O • + R"OH • •• R' •• R"O •• C •• O •• R' Product is a hemiacetal. Dr. Wolf's CHM 424 25- 52 H Cyclic Hemiacetals R R C O OH C OH O Aldehydes and ketones that contain an OH Aldehydes group elsewhere in the molecule can undergo intramolecular hemiacetal formation. intramolecular The equilibrium favors the cyclic hemiacetal if The the ring is 5- or 6-membered. the Dr. Wolf's CHM 424 25- 53 Carbohydrates Form Cyclic Hemiacetals 1 CH O 2 4 O 4 3 OH 3 1 2 H CH2OH equilibrium lies far to the right cyclic hemiacetals that have 5-membered rings are called furanose forms are furanose Dr. Wolf's CHM 424 25- 54 D-Erythrose 1 H H CH 2 3 4 O OH HH O 4 OH CH2OH H H 1 3 OH OH 2 H OH stereochemistry is maintained during cyclic hemiacetal formation Dr. Wolf's CHM 424 25- 55 D-Erythrose 1 2 3 1 4 turn 90° 3 2 4 Dr. Wolf's CHM 424 25- 56 D-Erythrose move O into position by rotating about bond between carbon-3 and carbon-4 Dr. Wolf's CHM 424 1 4 3 2 25- 57 D-Erythrose 1 4 3 2 Dr. Wolf's CHM 424 1 4 3 2 25- 58 D-Erythrose 1 4 3 2 Dr. Wolf's CHM 424 close ring by hemiacetal formation between OH at C-4 and carbonyl group 25- 59 D-Erythrose 1 4 3 2 Dr. Wolf's CHM 424 1 4 3 2 25- 60 D-Erythrose 1 H H CH 2 3 4 anomeric carbon O OH HH O 4 OH CH2OH H 3 OH H OH 1 2 H OH stereochemistry is variable at anomeric carbon; two diastereomers are formed Dr. Wolf's CHM 424 25- 61 D-Erythrose HH O 4 H 3 OH H H 1 2 OH OH OH α-D-Erythrofuranose Dr. Wolf's CHM 424 HH O 4 H 3 OH H OH 1 2 H OH β-D-Erythrofuranose 25- 62 D-Ribose 1 CH O 2 OH H 3 OH H 4 OH H 5 CH OH 2 2 furanose ring formation involves OH group at C-4 furanose Dr. Wolf's CHM 424 25- 63 D-Ribose 1 H CH 2 O OH 5 CH2OH H H H H 3 OH 4 H 4 OH HO 5 CH OH 2 3 1 CH CH 2 OH OH 2 need C(3)-C(4) bond rotation to put OH iin proper n need orientation to close 5-membered ring orientation Dr. Wolf's CHM 424 25- 64 O D-Ribose 5 HOCH2 OH H 4 H 3 OH H 2 OH Dr. Wolf's CHM 424 CH2OH H H H 1 CH CH 5 O 4 HO 3 1 CH CH 2 OH OH 25- 65 O D-Ribose 5 5 HOCH2 OH H 4 H 3 OH H 2 OH 1 CH CH O HOCH2 HOCH HOH 4 H 3 OH 2 OH 1 H OH β-D-Ribofuranose CH2OH group becomes a substituent on ring Dr. Wolf's CHM 424 25- 66 25.7 Cyclic Forms of Carbohydrates: Pyranose Forms Dr. Wolf's CHM 424 25- 67 Carbohydrates Form Cyclic Hemiacetals 1 CH O 5 2 3 4 5 OH O 1 4 3 2 H CH2OH cyclic hemiacetals that have 6-membered rings cyclic are called pyranose forms are pyranose Dr. Wolf's CHM 424 25- 68 D-Ribose 1 CH O H 2 H 3 OH H 4 OH 5 OH CH2OH 5 CH2OH H H H 4 HO 3 1 CH CH O 2 OH OH pyranose ring formation involves OH group at C-5 pyranose Dr. Wolf's CHM 424 25- 69 D-Ribose H H 4 HO 5 H H 3 OH O OH H 1 2 H OH 5 CH2OH H H H 4 HO 3 1 CH CH 2 OH OH β-D-Ribopyranose Dr. Wolf's CHM 424 25- 70 O D-Ribose H H 4 HO 5 H H 3 OH H O OH H 1 2 H OH β-D-Ribopyranose Dr. Wolf's CHM 424 H 4 HO 5 H H 3 OH O H 2 H 1 OH OH OH α-D-Ribopyranose 25- 71 D-Glucose 1 CH O H 2 OH HO 3 H H 4 H 5 6 H H 4 OH OH HO 6 CH2OH 5 OH OH 3 CH CH H 1 2 H OH CH2OH pyranose ring formation involves OH group at C-5 pyranose Dr. Wolf's CHM 424 25- 72 O D-Glucose 6 H H HOCH2 OH H5 H 4 OH H HO 3 CH CH 1 2 H OH O 4 HO 6 CH2OH 5 OH OH 3 CH CH H 1 2 H OH need C(4)-C(5) bond rotation to put OH iin proper n need orientation to close 6-membered ring orientation Dr. Wolf's CHM 424 25- 73 O D-Glucose 6 6 HOCH2 HOCH2 OH H5 H 4 OH H HO 3 H CH CH 1 2 H OH O 4 HO 5 H OH 3 H O OH H 1 2 H OH β-D-Glucopyranose Dr. Wolf's CHM 424 25- 74 D-Glucose 6 6 HOCH2 HOCH2 H 4 HO 5 H OH 3 H O H 2 H 1 4 OH OH HO OH α-D-Glucopyranose Dr. Wolf's CHM 424 H 5 H OH 3 H O OH H 1 2 H OH β-D-Glucopyranose 25- 75 D-Glucose 6 HOCH2 H 4 HO 5 H OH 3 H O OH H 1 2 H OH β-D-Glucopyranose pyranose forms of carbohydrates adopt chair pyranose conformations conformations Dr. Wolf's CHM 424 25- 76 D-Glucose 6 H6 HOCH2 H 4 HO HO 5 3 H HOCH2 2 H H O OH 1 H OH OH 4 HO 5 H OH 3 H O OH H 1 2 H OH β-D-Glucopyranose all substituents are equatorial in β-D-glucopyranose all Dr. Wolf's CHM 424 25- 77 D-Glucose H HOCH2 H HO HO H H H HOCH2 H O OH 1 OH OH H β-D-Glucopyranose HO HO H H O OH OH α-D-Glucopyranose OH group at anomeric carbon is axial iin α-D-glucopyranose n Dr. Wolf's CHM 424 1 25- 78 H Figure 25.5 CH O H OH H OH H OH CH2OH Less than 1% of the open-chain form of D-ribose Less -ribose is present at equilibrium in aqueous solution. is Dr. Wolf's CHM 424 25- 79 Figure 25.5 76% of the D-ribose is a mixture of the α and β76% -ribose pyranose forms, with the β-form predominating H H H H O HO OH OH OH H β-D-Ribopyranose (56%) Dr. Wolf's CHM 424 H O HO H H OH H H H OH OH 1 H OH α-D-Ribopyranose (20%) 25- 80 Figure 25.5 The α and β-furanose forms comprise 24% of The -furanose the mixture. the HOCH2 HOCH HOH H OH H OH OH β-D-Ribofuranose (18%) Dr. Wolf's CHM 424 HOCH2 HOCH HOH H H OH OH OH α-D-Ribofuranose (6%) 25- 81 25.8 Mutarotation Dr. Wolf's CHM 424 25- 82 Mutarotation Mutarotation is a term given to the change in Mutarotation the observed optical rotation of a substance with time. Glucose, for example, can be obtained in either its α or β-pyranose form. The two forms -pyranose have different physical properties such as melting point and optical rotation. melting When either form is dissolved in water, its When initial rotation changes with time. Eventually both solutions have the same rotation. both Dr. Wolf's CHM 424 25- 83 Mutarotation of D-Glucose Mutarotation H HOCH2 H HO HO H H H HOCH2 H O OH 1 OH OH HO HO H H β-D-Glucopyranose Initial: [α]D +18.7° H O OH 1 OH α-D-Glucopyranose Initial: [α]D +112.2° Final: [α]D +52.5° Dr. Wolf's CHM 424 25- 84 H Mutarotation of D-Glucose Mutarotation H HOCH2 H HO HO H H H HOCH2 H O OH 1 OH OH H β-D-Glucopyranose HO HO H H O OH 1 OH α-D-Glucopyranose Explanation: After being dissolved in water, the Explanation: α and β forms slowly interconvert via the openchain form. An equilibrium state is reached that chain contains 64% β and 36% α. Dr. Wolf's CHM 424 25- 85 H 25.9 Ketoses Dr. Wolf's CHM 424 25- 86 Ketoses Ketoses are carbohydrates that have a ketone Ketoses carbonyl group in their open-chain form. carbonyl C-2 is usually the carbonyl carbon. Dr. Wolf's CHM 424 25- 87 Examples CH2OH CH2OH CH2OH O O O H OH H H OH HO CH2OH OH H CH2OH HO H H OH H OH CH2OH D-Ribulose Dr. Wolf's CHM 424 L-Xyulose D-Fructose 25- 88 25.13 Glycosides Dr. Wolf's CHM 424 25- 89 Glycosides Glycosides have a substituent other than OH at Glycosides the anomeric carbon. the Usually the atom connected to the anomeric Usually carbon is oxygen. carbon Dr. Wolf's CHM 424 25- 90 Example HOCH2 HO HO O OH OH D-Glucose Linamarin is Linamarin an O-glycoside -glycoside derived from D-glucose. Dr. Wolf's CHM 424 HOCH2 HO HO O CH3 OCC OH CH3 25- 91 N Glycosides Glycosides have a substituent other than OH at Glycosides the anomeric carbon. the Usually the atom connected to the anomeric Usually carbon is oxygen. carbon Examples of glycosides in which the atom Examples connected to the anomeric carbon is something other than oxygen include S-glycosides and N-glycosides glycosides. Dr. Wolf's CHM 424 25- 92 Example Adenosine is an NAdenosine glycoside derived from glycoside D-ribose NH2 N OH HOCH2 HOCH HOH H OH H OH D-Ribose Dr. Wolf's CHM 424 N N HOCH2 N HOCH HOH H OH H OH Adenosine 25- 93 Example HOCH2 HO HO O OH OH D-Glucose Sinigrin is an Sinigrin HOCH2 S-glycoside HO -glycoside derived from HO D-glucose. Dr. Wolf's CHM 424 O NOSO3K SCCH2CH OH 25- 94 CH2 Glycosides O-Glycosides are mixed acetals. Dr. Wolf's CHM 424 25- 95 O-Glycosides are mixed acetals CH O O OH hemiacetal H CH2OH ROH O OR H Dr. Wolf's CHM 424 acetal 25- 96 Preparation of Glycosides Glycosides of simple alcohols (such as Glycosides methanol) are prepared by adding an acid catalyst (usually gaseous HCl) to a solution of a carbohydrate in the appropriate alcohol. carbohydrate Only the anomeric OH group is replaced. An equilibrium is established between the α and An β-glycosides (thermodynamic control). The -glycosides more stable stereoisomer predominates. more Dr. Wolf's CHM 424 25- 97 Preparation of Glycosides CH CH O H OH HO H H OH H OH CH2OH D-Glucose Dr. Wolf's CHM 424 HOCH2 HO HO CH3OH HCl O OCH3 OH + HOCH2 HO HO O OH OCH3 25- 98 Preparation of Glycosides Methyl β-D-glucopyranoside HOCH2 HO HO O OCH3 OH + Methyl α-D-glucopyranoside (major product) Dr. Wolf's CHM 424 HOCH2 HO HO O OH OCH3 25- 99 Mechanism of Glycoside Formation HOCH2 HO HO •• O• • OH OH HCl HOCH2 HO HO Dr. Wolf's CHM 424 •• O• carbocation is stabilized carbocation •+ by lone-pair donation from oxygen H oxygen of the ring OH 25- 100 Mechanism of Glycoside Formation HOCH2 •• CH3 O• HO HO HO O • + HO + • H • OH HOCH2 HO HO O OH + O H3C •• H CH3 •• •O• • •H O• •+ OH Dr. Wolf's CHM 424 HOCH2 H 25- 101 Mechanism of Glycoside Formation HOCH2 HO HO •• O• • OH HOCH2 HO HO HO O • + HO + • H + –H •• •• O• • HO HO •• OCH3 + •• O OH + O H3C •• H HOCH2 OH Dr. Wolf's CHM 424 CH3 HOCH2 •• O• • OH • OCH3 •• 25- 102 25.14 Disaccharides Dr. Wolf's CHM 424 25- 103 Disaccharides Disaccharides are glycosides. The glycosidic linkage connects two The monosaccharides. monosaccharides. Two structurally related disaccharides are Two cellobiose and maltose. Both are derived from glucose. glucose. Dr. Wolf's CHM 424 25- 104 Maltose and Cellobiose HOCH2 HO HOCH2 O α 1 O O 4 OH Maltose OH HO OH HO Maltose is composed of two glucose units linked together by a glycosidic bond between C-1 of one glucose and C-4 of the other. one The stereochemistry at the anomeric carbon of The the glycosidic linkage is α. The glycosidic linkage is described as α(1,4) The Dr. Wolf's CHM 424 25- 105 Maltose and Cellobiose HOCH2 HO HOCH2 O β 1 O O 4 OH Cellobiose OH HO OH HO Cellobiose is a stereoisomer of maltose. The only difference between the two is that The cellobiose has a β(1,4) glycosidic bond while (1,4) that of maltose is α(1,4). that Dr. Wolf's CHM 424 25- 106 Maltose and Cellobiose Maltose Dr. Wolf's CHM 424 Cellobiose 25- 107 Cellobiose and Lactose HOCH2 HO HOCH2 O β 1 O O 4 OH Cellobiose OH HO OH HO Cellobiose and lactose are stereoisomeric lactose disaccharides. disaccharides. Both have β(1,4) glycosidic bonds. Both The glycosidic bond unites two glucose units in The cellobiose. It unites galactose and glucose in lactose. lactose. Dr. Wolf's CHM 424 25- 108 Cellobiose and Lactose HOCH2 HO HOCH2 O β 1 O O 4 OH Lactose OH HO OH HO Cellobiose and lactose are stereoisomeric lactose disaccharides. disaccharides. Both have β(1,4) glycosidic bonds. Both The glycosidic bond unites two glucose units in The cellobiose. It unites galactose and glucose in lactose. lactose. Dr. Wolf's CHM 424 25- 109 25.18 Reduction of Carbohydrates Dr. Wolf's CHM 424 25- 110 Reduction of Carbohydrates Carbonyl group of open-chain form is reduced to Carbonyl an alcohol. an Product is called an alditol. Alditol lacks a carbonyl group so cannot cyclize Alditol to a hemiacetal. to Dr. Wolf's CHM 424 25- 111 Reduction of D-Galactose Reduction reducing agent: NaBH4, H2O (catalytic hydrogenation can also be used) CH α-D-galactofuranose β-D-galactofuranose α-D-galactopyranose β-D-galactopyranose H OH H OH HO H HO H HO H HO H H OH CH2OH Dr. Wolf's CHM 424 CH2OH O H OH CH2OH D-Galactitol (90%) 25- 112 25.19 Oxidation of Carbohydrates Dr. Wolf's CHM 424 25- 113 Benedict's Reagent O RCH + 2Cu2+ + 5HO– O RCO– + Cu2O + 3H2O Benedict's reagent is a solution of the citrate complex of Benedict's CuSO4 in water. It is used as a test for "reducing CuSO sugars." Cu2+ is a weak oxidizing agent. sugars." A reducing sugar is one which has an aldehyde function, reducing or is in equilibrium with one that does. or A positive test is the formation of a red precipitate of positive Cu2O. Cu Dr. Wolf's CHM 424 25- 114 Examples of Reducing Sugars Aldoses: because they have an aldehyde Aldoses: function in their open-chain form. function Ketoses: because enolization establishes an Ketoses: equilibrium with an aldose. equilibrium O CH2OH CHOH CH C C CHOH O R R OH R oxidized by Cu2+ oxidized Cu Dr. Wolf's CHM 424 25- 115 Examples of Reducing Sugars Disaccharides that have a free hemiacetal Disaccharides function. function. HOCH2 HOCH2 O O Maltose HO Dr. Wolf's CHM 424 OH O HO OH HO OH 25- 116 Examples of Reducing Sugars Disaccharides that have a free hemiacetal Disaccharides function. function. HOCH2 HOCH2 O Maltose OH CH O HO HO OH HO O OH oxidized by Cu2+ oxidized Cu Dr. Wolf's CHM 424 25- 117 Glycosides are not reducing sugars HOCH2 HO HO O OH OCH3 Methyl α-D-glucopyranoside lacks a free Methyl hemiacetal function; cannot be in equilibrium with a species having an aldehyde function Dr. Wolf's CHM 424 25- 118 Oxidation of Reducing Sugars The compounds formed on oxidation of The reducing sugars are called aldonic acids. reducing Aldonic acids exist as lactones when 5- or 6membered rings can form. A standard method for preparing aldonic acids standard uses Br2 as the oxidizing agent. uses Dr. Wolf's CHM 424 25- 119 Oxidation of D-Xylose Oxidation CH O H OH HO H OH H CH2OH D-Xylose Dr. Wolf's CHM 424 CO2H Br2 H2O H OH HO H OH H CH2OH D-Xylonic acid (90%) 25- 120 Oxidation of D-Xylose Oxidation O HO HO OH CO2H O H OH H HOCH2 OH O O OH Dr. Wolf's CHM 424 OH HO + H CH2OH D-Xylonic acid (90%) 25- 121 Ruff Degradation Ruff Part 1,Oxidation of D-Glucose CH CH O H OH HO H H OH H OH CO2H CH2OH D-Glucose Dr. Wolf's CHM 424 H OH Br2 HO H2O H H OH H OH CH2OH CH D-Gluconic acid 25- 122 Ruff Degradation Ruff Part 2,Oxidized Carbon Removed Part CO2H H HO H H OH H OH CH OH CH2OH CH D-Gluconic acid Dr. Wolf's CHM 424 1) CaCO3 2) H2O2, Fe+3 HO H H O H OH OH CH2OH D-Arabinose 25- 123 Nitric Acid Oxidation Nitric acid oxidizes both the aldehyde function Nitric and the terminal CH2OH of an aldose to CO2H. and The products of such oxidations are called The aldaric acids. aldaric Dr. Wolf's CHM 424 25- 124 Nitric Acid Oxidation CH CH O H OH HO H H OH H OH CH2OH D-Glucose Dr. Wolf's CHM 424 CO2H HNO3 H OH HO 60°C H H OH H OH CO2H CO D-Glucaric acid (41%) 25- 125 25.20 Cyanohydrin Formation and Cyanohydrin Carbohydrate Chain Extension Carbohydrate Kiliani-Fischer Synthesis Dr. Wolf's CHM 424 25- 126 Extending the Carbohydrate Chain Carbohydrate chains can be extended by using Carbohydrate cyanohydrin formation as the key step in C—C bond-making. bond-making. The classical version of this method is called the The Kiliani-Fischer synthesis. The following example is a more modern modification. example Dr. Wolf's CHM 424 25- 127 Extending the Carbohydrate Chain CN CH α-L-arabinofuranose O CHOH α-L-arabinopyranose HO OH HCN H H HO β-L-arabinopyranose HO H β-L-arabinofuranose H CH2OH HO OH H H CH2OH the cyanohydrin is a mixture of two stereoisomers that the differ in configuration at C-2; these two diastereomers are separated in the next step separated Dr. Wolf's CHM 424 25- 128 Extending the Carbohydrate Chain CN CN CN H OH H OH HO + H CHOH H OH separate OH H HO H HO H HO H HO H HO H HO H CH2OH L-Mannononitrile Dr. Wolf's CHM 424 CH2OH CH2OH L-Gluconononitrile 25- 129 Extending the Carbohydrate Chain CN CH H OH H OH HO H HO H CH2OH L-Mannononitrile Dr. Wolf's CHM 424 O H H2, H2O Pd, BaSO4 OH H OH HO H HO H CH2OH L-Mannose (56% from L-arabinose) (56% 25- 130 Likewise... CN HO H CH H OH HO H HO H CH2OH L-Gluconononitrile Dr. Wolf's CHM 424 HO H2, H2O Pd, BaSO4 H O H OH HO H HO H CH2OH L-Glucose (26% from L-arabinose) (26% 25- 131 25.21 Epimerization and Isomerization Epimerization of Carbohydrates of Dr. Wolf's CHM 424 25- 132 Enol Forms of Carbohydrates Enolization of an aldose scrambles the Enolization stereochemistry at C-2. stereochemistry This process is called epimerization. This epimerization Diastereomers that differ in stereochemistry at only one of their stereogenic centers are called epimers. epimers. D-Glucose and D-mannose, for example, are -Glucose -mannose, epimers. Dr. Wolf's CHM 424 25- 133 Epimerization CH CH CHOH O H OH HO HO H H OH H OH CH2OH D-Glucose C OH CH CH O HO H HO H H OH H H OH H OH H OH CH2OH Enediol CH2OH D-Mannose This equilibration can be catalyzed by hydroxide ion. Dr. Wolf's CHM 424 25- 134 Enol Forms of Carbohydrates The enediol intermediate on the preceding slide The can undergo a second reaction. It can lead to the conversion of D-glucose or D-mannose -glucose -mannose (aldoses) to D-fructose (ketose). Dr. Wolf's CHM 424 25- 135 Isomerization CH CH CHOH CHOH HO HO H H OH H OH CH2OH D-Glucose or D-Mannose Dr. Wolf's CHM 424 CH2OH CH C O C OH O HO H H OH H H OH H OH H OH CH2OH Enediol CH2OH D-Fructose 25- 136 25.22 Acylation and Alkylation of Acylation Hydroxyl Groups in Carbohydrates Carbohydrates Dr. Wolf's CHM 424 25- 137 Reactivity of Hydroxyl Groups in Reactivity Carbohydrates Carbohydrates Hydroxyl groups in carbohydrates undergo Hydroxyl reactions typical of alcohols. reactions acylation alkylation Dr. Wolf's CHM 424 25- 138 Example: Acylation of α-D-glucopyranose Example: Acylation HOCH2 HO HO OO O 5 CH3COCCH3 + OH OH pyridine O O CH3COCH2 O CH3CO CH3CO O CH3CO O Dr. Wolf's CHM 424 (88%) OCCH3 O 25- 139 Example: Alkylation of methyl α-D-glucopyranoside Example: Alkylation HOCH2 HO HO O OH 4CH3I + OCH3 Ag2O, CH3OH CH3OCH2 CH3O CH3O O (97%) CH3O Dr. Wolf's CHM 424 OCH3 25- 140 Classical Method for Ring Size Ring sizes (furanose or pyranose) have been Ring determined using alkylation as a key step. determined HOCH2 HO HO CH3OCH2 O OH Dr. Wolf's CHM 424 CH3O CH3O OCH3 O CH3O OCH3 25- 141 Classical Method for Ring Size Ring sizes (furanose or pyranose) have been Ring determined using alkylation as a key step. determined CH3OCH2 CH3O CH3O CH3OCH2 O CH3O H2O H+ OH (mixture of α + β) (mixture Dr. Wolf's CHM 424 CH3O CH3O O CH3O OCH3 25- 142 Classical Method for Ring Size Ring sizes (furanose or pyranose) have been Ring determined using alkylation as a key step. determined CH CH3OCH2 CH3O CH3O H CH3O O O OCH3 H H CH3O OH (mixture of α + β) (mixture Dr. Wolf's CHM 424 OCH3 H OH CH2OCH3 25- 143 Classical Method for Ring Size Ring sizes (furanose or pyranose) have been Ring determined using alkylation as a key step. determined CH H This carbon has OH This instead of OCH3. Therefore,its O was the oxygen in the ring. Dr. Wolf's CHM 424 CH3O O OCH3 H H OCH3 H OH CH2OCH3 25- 144 End of Chapter 25 Dr. Wolf's CHM 424 25- 145 ...
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This document was uploaded on 01/03/2012.

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