706 CARBOHYDRATES Of the two chair conformations of B-L-ribose, the one with the greater number of equatorial AOH HO HO H Less stable chai More stable chair conformation of 25.7 The equation describing the equilibrium is HOCH2 O HO HOCH, OH OH HOCH OH CH=O OH a-D-Mannopyranose Open-chain form of D-mannose B-D-Mannopyranose Let a= percent a isomer; 100-A= percent B isomer. Then A(+29.39)+(100-A)(-17.0°)=100+14.2° 46.3A=3120 Percent a isomer =67%o Percent B isomer =(100-A)=33% 25.8 Review carbohydrate terminology by referring to text Table 25.1. A ketotetrose is a four-carbon ke- tose. Writing a Fischer projection for a four-carbon ketose reveals that only one stereogenic center is present, and thus there are only two ketotetroses. They are enantiomers of each other and are known as D- and L-erythrulose CHOH CHOH H-OH CHOH CHOH 25.9(b) Because L-fucose is 6-deoxy-L-galactose, first write the Fischer projection formula of D-galactose, and then transform it to its mirror image, L-galactose. Transform the C-6 CH,OH roup to CH3 to produce 6-deoxy-L-galactose CHO CHO DH HO HO H o-H H H H H HO-H CH,OH CH D-Galactose - Galactose 6-Deoxy-L-galactose (from Figure 25.2) fucose Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website
Of the two chair conformations of -L-ribose, the one with the greater number of equatorial substituents is more stable. 25.7 The equation describing the equilibrium is Let A percent isomer; 100 A percent isomer. Then A(29.3°) (100 A)(17.0°) 100(14.2°) 46.3A 3120 Percent isomer 67% Percent isomer (100 A) 33% 25.8 Review carbohydrate terminology by referring to text Table 25.1. A ketotetrose is a four-carbon ketose. Writing a Fischer projection for a four-carbon ketose reveals that only one stereogenic center is present, and thus there are only two ketotetroses. They are enantiomers of each other and are known as D- and L-erythrulose. 25.9 (b) Because L-fucose is 6-deoxy-L-galactose, first write the Fischer projection formula of D-galactose, and then transform it to its mirror image, L-galactose. Transform the C-6 CH2OH group to CH3 to produce 6-deoxy-L-galactose. H HO HO H OH CHO H OH H CH2OH d-Galactose (from Figure 25.2) HO H H OH CHO HO H H OH CH2OH l-Galactose HO H H OH CHO HO H H OH CH3 6-Deoxy-l-galactose (l-fucose) H C O OH CH2OH CH2OH d-Erythrulose HO C O H CH2OH CH2OH l-Erythrulose OH HOCH2 HO HO OH O -D-Mannopyranose [] 20 29.3 D OH HOCH2 HO OH HO O -D-Mannopyranose [] 20 17.0 D Open-chain form of D-mannose HOCH2 HO HO CH O OH OH HO H HO H HO H H O OH Less stable chair conformation of -l-ribopyranose OH HO OH OH O More stable chair conformation of -l-ribopyranose OH HO OH O HO 706 CARBOHYDRATES Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
CARBOHYDRATES 707 25.10 Reaction of a carbohydrate with an alcohol in the presence of an acid catalyst gives mixed acetals at CHO OH CHOH CHOH H CHOH OCH3 H-OH D-Galactose Methanol 25.11 Acid-catalyzed addition of methanol to the glycal proceeds by regioselective protonation of the dou ble bond in the direction that leads to the more stable carbocation. Here again the more stable car- bocation is the one stabilized by the ring oxygen. HO HOCH, O HOCH HOCH2 HO-\ Capture on either face of the carbocation by methanol yields the a and B methyl glycosides. 25.12 The hemiacetal opens to give an intermediate containing a free aldehyde function Cyclization of his intermediate can produce either the a or the B configuration at this center. The axial and equa torial orientations of the anomeric hydroxyl can best be seen by drawing maltose with the pyranose rings in chair conformations HOCH HOCH, CH,OH CHOH O CH=O hemiacetal (equatorial) Key intermediate forme cleavage of hemiac HO- OH CHOH a-Configuration of hemiacetal(axial) Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website
25.10 Reaction of a carbohydrate with an alcohol in the presence of an acid catalyst gives mixed acetals at the anomeric position. 25.11 Acid-catalyzed addition of methanol to the glycal proceeds by regioselective protonation of the double bond in the direction that leads to the more stable carbocation. Here again, the more stable carbocation is the one stabilized by the ring oxygen. Capture on either face of the carbocation by methanol yields the and methyl glycosides. 25.12 The hemiacetal opens to give an intermediate containing a free aldehyde function. Cyclization of this intermediate can produce either the or the configuration at this center. The axial and equatorial orientations of the anomeric hydroxyl can best be seen by drawing maltose with the pyranose rings in chair conformations. HO CH2OH OH HO O -Configuration of hemiacetal (equatorial) OH HO HO O HOCH2 O Key intermediate formed by cleavage of hemiacetal CH O CH2OH HO HO OH OH HO HO O HOCH2 O -Configuration of hemiacetal (axial) CH2OH HO HO HO O OH HO HO O HOCH2 O HOCH2 HO H O HO HO HOCH2 H HO O H HO HOCH2 H O HO CHO CH2OH H H H H HO HO OH OH d-Galactose Methanol Methyl -d-galactopyranoside Methyl -d-galactopyranoside CH3OH HCl OH CH2OH H OH HO OCH3 O OH H CH2OH OCH3 OH HO O CARBOHYDRATES 707 Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
708 CARBOHYDRATES Only the configuration of the hemiacetal function is affected in this process. The a configuration of the glycosidic linkage remains unchanged 25.13 Write the chemical equation so that you can clearly relate the product to the starting material CHO CH-OH H NabH H CHOH CHOH D-Ribose Ribitol is a meso form; it is achiral and thus not optically active. a plane of symmetry passin through C-3 bisects the molecule 25. 14(b) Arabinose is a reducing sugar; it will give a positive test with Benedict's reagent, because its open-chain form has a free aldehyde group capable of being oxidized by copper(l ion (c) Benedicts reagent reacts with a-hydroxy ketones by way of an isomerization process involv- ing an enediol intermediate Benedict CHOH tive test: cu,O formed CHOH CHOH CHO 1.3-Dihydroxyacetone Enedio Glyceraldehyde 1, 3-Dihydroxyacetone gives a positive test with Benedicts reagent. (d)D-Fructose is an a-hydroxy ketone and will give a positive test with Benedict's reagent CHOH H Benedicts H OH positive test; Cu,O formed H CH OH CHOH (e) Lactose is a disaccharide and will give a positive test with Benedict's reagent by way of an open-chain isomer of one of the rings. Lactose is a reducing sugar. OCH HOCH CHo Benedict s reagent, positive test; Cu,O formed OH Open-chain form (structure presented in Section 25. 14) Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website
Only the configuration of the hemiacetal function is affected in this process. The configuration of the glycosidic linkage remains unchanged. 25.13 Write the chemical equation so that you can clearly relate the product to the starting material. Ribitol is a meso form; it is achiral and thus not optically active. A plane of symmetry passing through C-3 bisects the molecule. 25.14 (b) Arabinose is a reducing sugar; it will give a positive test with Benedict’s reagent, because its open-chain form has a free aldehyde group capable of being oxidized by copper(II) ion. (c) Benedict’s reagent reacts with -hydroxy ketones by way of an isomerization process involving an enediol intermediate. 1,3-Dihydroxyacetone gives a positive test with Benedict’s reagent. (d) D-Fructose is an -hydroxy ketone and will give a positive test with Benedict’s reagent. (e) Lactose is a disaccharide and will give a positive test with Benedict’s reagent by way of an open-chain isomer of one of the rings. Lactose is a reducing sugar. HOCH2 HO O O HO OH HOCH2 O OH OH HO HOCH2 HO O O HO OH HOCH2 OH OH CHO HO positive test; Cu2O formed Lactose (structure presented in Section 25.14) Open-chain form Benedict’s reagent positive test; Cu2O formed HO H H OH CH2OH CHOH H OH d-Fructose HO H H OH CH2OH O CH2OH C H OH Benedict’s reagent C H O C O CH2OH CH2OH 1,3-Dihydroxyacetone Enediol Glyceraldehyde C CH2OH H OH C OH CHOH CH2OH Benedict’s reagent positive test; Cu2O formed C H O CHO CH2OH H H H OH OH OH d-Ribose NaBH4 H2O Plane of symmetry Ribitol CH2OH CH2OH H H H OH OH OH 708 CARBOHYDRATES Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
CARBOHYDRATES 709 minal glucose residues at the ends of the chain and its branches are hemiacetals in equilibrium ctures. A positive test is expected. 25.15 Because the groups at both ends of the carbohydrate chain are oxidized to carboxylic acid functions two combinations of one CH,OH with one CHO group are possible CHO COH CHOH CHO H H-OH HO→H HNO, HO-H HO--H HO-H quivalent to H H H H-OH HO→H D-Glucose D-Glucaric acid L-Gulose yields the same aldaric acid on oxidation as does D-glucose 25.16 In analogy with the D-fructose D-glucose interconversion, dihydroxyacetone phosphate and D-glyceraldehyde 3-phosphate can equilibrate by way of an enediol intermediate CH OH triose phosphate CHOH triose phosphate CH,OP(OH) CH,OP(OH) CH,OP(OH) Dihydroxyacetone Enediol 25.17(b) The points of cleavage of D-ribose on treatment with periodic acid are as indicated. H HCOH H HCO,, H 4HIO H-OH HCOH H HCO.H CHOH HCH Four moles of periodic acid per mole of D-ribose are required. Four moles of formic acid and one mole of formaldehyde are produced (c) Write the structure of methyl B-D-glucopyranoside so as to identify the adjacent alcohol HOCH HOCH HCOH HO各 OCH OCH Methyl B-D-glucopyranoside Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website
( f ) Amylose is a polysaccharide. Its glycoside linkages are inert to Benedict’s reagent, but the terminal glucose residues at the ends of the chain and its branches are hemiacetals in equilibrium with open-chain structures. A positive test is expected. 25.15 Because the groups at both ends of the carbohydrate chain are oxidized to carboxylic acid functions, two combinations of one CH2OH with one CHO group are possible. L-Gulose yields the same aldaric acid on oxidation as does D-glucose. 25.16 In analogy with the D-fructose - D-glucose interconversion, dihydroxyacetone phosphate and D-glyceraldehyde 3-phosphate can equilibrate by way of an enediol intermediate. 25.17 (b) The points of cleavage of D-ribose on treatment with periodic acid are as indicated. Four moles of periodic acid per mole of D-ribose are required. Four moles of formic acid and one mole of formaldehyde are produced. (c) Write the structure of methyl -D-glucopyranoside so as to identify the adjacent alcohol functions. HOCH2 OCH3 HO HO HO O HOCH2 OCH3 HCO2H HC HC O O O 2HIO4 Methyl -D-glucopyranoside HCO2H HCO2H HCO2H HCO2H HCH 4HIO4 O H H H OH OH OH CH2OH d-Ribose C H O C O CH2OH CH2OP(OH)2 Dihydroxyacetone phosphate Enediol d-Glyceraldehyde 3-phosphate C O H C O C CHOH OH CH2OP(OH)2 O CH2OP(OH)2 O triose phosphate isomerase triose phosphate isomerase H OH equivalent to CHO H HO H H OH H OH OH CH2OH HNO3 heat HNO3 heat H HO H H OH CHO H OH OH CH2OH d-Glucose HO H HO H CHO HO H H OH CH2OH l-Gulose H HO H H OH H OH OH CO2H CO2H d-Glucaric acid CARBOHYDRATES 709 Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
710 CARBOHYDRATES Two moles of periodic acid per mole of glycoside are required. One mole of formic acid is produced (d) There are two independent vicinal diol functions in this glycoside. Two moles of periodic acid are required per mole of substrate CHOH HO-HO OCH CHO 2HIOA HCH CH HC H OH 25.18 (a) The structure shown in Figure 25. 2 is D-(+)-xylose: therefore(-)-xylose must be its mirror image and has the L-configuration at C-4 CHO CHO D-(+)-Xylose L-(-)-Xylose (b) Alditols are the reduction products of carbohydrates; D-xylitol is derived from D-xylose by conversion of the terminal -CHO to-CH,OH CH,OH CHOH (c) Redraw the Fischer projection of D-xylose in its eclipsed conformation H CH,O H OH CHO HOH Eclipsed conformation B-D-xylopyranose Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website
Two moles of periodic acid per mole of glycoside are required. One mole of formic acid is produced. (d) There are two independent vicinal diol functions in this glycoside. Two moles of periodic acid are required per mole of substrate. 25.18 (a) The structure shown in Figure 25.2 is D-()-xylose; therefore ()-xylose must be its mirror image and has the L-configuration at C-4. (b) Alditols are the reduction products of carbohydrates; D-xylitol is derived from D-xylose by conversion of the terminal GCHO to GCH2OH. (c) Redraw the Fischer projection of D-xylose in its eclipsed conformation. HO H H OH CHO H OH CH2OH d-Xylose Eclipsed conformation of d-xylose Haworth formula of -d-xylopyranose redrawn as H H HO OH H H OH H HO OH H H H OH OH CH2O CHO O d-Xylitol H OH HO H CH2OH CH2OH H OH d-()-Xylose H OH HO H CHO CH2OH H OH l-()-Xylose HO H H OH CHO CH2OH HO H 2HIO4 H CH CH HC O O OCH3 HCH H O O O H CH2OH OCH3 H OH HO H H H OH O 710 CARBOHYDRATES Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website