CHAPTER 25 CARBOHYDRATES SOLUTIONS TO TEXT PROBLEMS 25.1 (b) Redraw the Fischer projection so as to show the orientation of the groups in three dimensions. HOCH-CHo is equivalent to HOCH, -C--CHO OH Reorient the three-dimensional representation, putting the aldehyde group at the top and the primary alcohol at the bottom. CHO turn90° HOCH,C--CHO CHOH What results is not equivalent to a proper Fischer projection, because the horizontal bonds are n they should be"forward. The opposite is true for the vertical bonds. To make the drawing correspond to a proper Fischer projection, we need to rotate it 180 around a vertical axis CHO CHO HO-C-H is equivalent to HO H,OH CHOH rotate180° Now, having the molecule arranged properly, we see that it is L-glyceraldehyde 701 Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website
701 CHAPTER 25 CARBOHYDRATES SOLUTIONS TO TEXT PROBLEMS 25.1 (b) Redraw the Fischer projection so as to show the orientation of the groups in three dimensions. Reorient the three-dimensional representation, putting the aldehyde group at the top and the primary alcohol at the bottom. What results is not equivalent to a proper Fischer projection, because the horizontal bonds are directed “back” when they should be “forward.” The opposite is true for the vertical bonds. To make the drawing correspond to a proper Fischer projection, we need to rotate it 180° around a vertical axis. Now, having the molecule arranged properly, we see that it is L-glyceraldehyde. is equivalent to CHO CH2OH HO H CHO CH2OH HO H C CHO CH2OH H OH C rotate 180 H OH HOCH2 C CHO CHO CH2OH H OH C turn 90 is equivalent to H OH HOCH2 CHO OH HOCH2 C H CHO Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
702 CARBOHYDRATES (c) Again proceed by converting the Fischer projection into a three-dimensional representation CHO CHO HOCH,H HOCH, C-H Look at the drawing from a perspective that permits you to see the carbon chain oriented ver tically with the aldehyde at the top and the Ch,oh at the bottom. Both groups should point away from you. When examined from this perspective, the hydrogen is to the left and the hydroxyl to the right with both pointing toward you. CHO CHO HOCH, -C-H is equivalent to H-C-OH CHOH The molecule is D-glyceraldehyde 25. 2 Begin by drawing a perspective view of the molecular model shown in the problem. To view the compound as a Fischer projection, redraw it in an eclipsed conformation H OH H OH CH HOCH HOCH CHEO HO H taggered conformation Same molecule in eclipsed The eclipsed conformation shown, when oriented so that the aldehyde carbon is at the top, vertical bonds back, and horizontal bonds pointing outward from their stereogenic centers, is readily trans- formed into the Fischer projection of L-erythrose CHO HOH H is equivalent to HOC H, CHEO HO--H CHOH CHOH 25.3 L-Arabinose is the mirror image of D-arabinose, the structure of which is given in text Fig ure 25.2. The configuration at each stereogenic center of D-arabinose must be reversed to trans form it into L-arabinose CHO H H-OH HO-H oh Ho-h CHOH CHOH D-(-)-Arabinose L-(+)-Arabinose Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website
702 CARBOHYDRATES (c) Again proceed by converting the Fischer projection into a three-dimensional representation. Look at the drawing from a perspective that permits you to see the carbon chain oriented vertically with the aldehyde at the top and the CH2OH at the bottom. Both groups should point away from you. When examined from this perspective, the hydrogen is to the left and the hydroxyl to the right with both pointing toward you. The molecule is D-glyceraldehyde. 25.2 Begin by drawing a perspective view of the molecular model shown in the problem. To view the compound as a Fischer projection, redraw it in an eclipsed conformation. The eclipsed conformation shown, when oriented so that the aldehyde carbon is at the top, vertical bonds back, and horizontal bonds pointing outward from their stereogenic centers, is readily transformed into the Fischer projection of L-erythrose. 25.3 L-Arabinose is the mirror image of D-arabinose, the structure of which is given in text Figure 25.2. The configuration at each stereogenic center of D-arabinose must be reversed to transform it into L-arabinose. HO H H OH CHO H OH CH2OH d-()-Arabinose H HO HO H OH CHO H CH2OH l-()-Arabinose is equivalent to or CHO CH2OH HO C H HO C H L-Erythrose HO H HO H CHO CH2OH OH OH HOCH2 H H CH O OH H HOCH2 H HO CH O Staggered conformation Same molecule in eclipsed conformation OH OH HOCH2 H H CH O is equivalent to CHO CH2OH H OH C CHO OH HOCH2 C H is equivalent to CHO OH HOCH2 H CHO OH HOCH2 C H Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
CARBOHYDRATES 703 25.4 The configuration at C-5 is opposite to that of D(+)-glyceraldehyde. This particular carbohydrate therefore belongs to the L series. Comparing it with the Fischer projection formulas of the eight D-aldohexoses reveals it to be in the mirror image of D-(+)-talose; it is L-( 25.5 (b) The Fischer projection formula of D-arabinose may be found in text Figure 25.2. The Fischer projection and the eclipsed conformation corresponding to it are CHO HO→H HOCH, OH about H H HO H HO H CHOH Conformation suitable for of D-arabinose Cyclic hemiacetal formation between the carbonyl group and the C-4 hydroxyl yields the a-and B-furanose forms of D-arabinose. HOCH O H HO HO H HO H a-D- Arabinofuranose (c) The mirror image of D-arabinose [from part(b)] is L-arabinose CHO Ho-H OH HO CHOH H H OH HO-+H HOH∠C H OH Ho-+H CH,OH CHOH H OH L-Arabinose The C-4 atom of the eclipsed conformation of L-arabinose must be rotated 120 in a clock wise sense so as to bring its hydroxyl group into the proper orientation for furanose ring formation H H oH H o rotate about OH C-3-C-4 bond OCH Original eclipsed conformation Conformation suitable for uranose ring formation Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website
25.4 The configuration at C-5 is opposite to that of D-()-glyceraldehyde. This particular carbohydrate therefore belongs to the L series. Comparing it with the Fischer projection formulas of the eight D-aldohexoses reveals it to be in the mirror image of D-()-talose; it is L-()-talose 25.5 (b) The Fischer projection formula of D-arabinose may be found in text Figure 25.2. The Fischer projection and the eclipsed conformation corresponding to it are Cyclic hemiacetal formation between the carbonyl group and the C-4 hydroxyl yields the - and -furanose forms of D-arabinose. (c) The mirror image of D-arabinose [from part (b)] is L-arabinose. The C-4 atom of the eclipsed conformation of L-arabinose must be rotated 120° in a clockwise sense so as to bring its hydroxyl group into the proper orientation for furanose ring formation. Original eclipsed conformation of l-arabinose HO H 4 OH H H OH 3 2 1 5 CH2OH Conformation suitable for furanose ring formation H HOCH2 OH H H OH OH rotate about C-3 C-4 bond C O H C O H CHO CH2OH HO H OH H OH H d-Arabinose CHO CH2OH H HO H HO H OH l-Arabinose Eclipsed conformation of l-arabinose HO H 4 HO H H OH 3 2 1 5 CH2OH C O H -d-Arabinofuranose H HOCH2 OH H H HO HO H O -d-Arabinofuranose H HOCH2 H OH H HO HO H O d-Arabinose H OH H OH CHO CH2OH HO H rotate about C-3 C-4 bond Eclipsed conformation of d-arabinose H HO 4 H HO HO H 3 2 1 5 CH2OH Conformation suitable for furanose ring formation HOCH2 H 4 H HO HO H C O H 3 2 1 5 OH C O H CARBOHYDRATES 703 Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
704 CARBOHYDRATES Cyclization gives the a-and B-furanose forms of L-arabinose OH H OH H HOCH HOCH ar-L-Arabinofuranose B-L-Arabinofuranose In the L series the anomeric hydroxyl is up in the a isomer and down in the B isomer. (d) The Fischer projection formula for D-threose is given in the text Figure 25. 2. Reorientation of that projection into a form that illustrates its potential for cyclization is shown 10-tH is equivalent to 4 H CHOH Cyclization yields the two stereoisomeric furanose forms H H 0 H HO H HO B-D-Threofuranose a-D-Threofuranose 25.6 (b) The Fischer projection and Haworth formula for D-mannose are CHO HO-H HOCH HO-H OH OH 1o HO CS HOHO OH CHOH D-Mannose mannopyranose The Haworth formula is more realistically drawn as the following chair conformation HOCH HO- OH H Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website
Cyclization gives the - and -furanose forms of L-arabinose. In the L series the anomeric hydroxyl is up in the isomer and down in the isomer. (d) The Fischer projection formula for D-threose is given in the text Figure 25.2. Reorientation of that projection into a form that illustrates its potential for cyclization is shown. Cyclization yields the two stereoisomeric furanose forms. 25.6 (b) The Fischer projection and Haworth formula for D-mannose are The Haworth formula is more realistically drawn as the following chair conformation: HO HO H H OH H H H OH HOCH2 O -d-Mannopyranose CHO CH2OH HO H OH H OH H HO H d-Mannose H HO HO OH H H HO H CH2OH -d-Mannopyranose (Haworth formula) H HO OH H HO H HOCH2 H HO H O C O H H HO OH H 4 3 2 O H 1 -d-Threofuranose OH H H HO HO H O -d-Threofuranose H OH H HO HO H O C O H H HO OH H 4 3 2 O H 1 d-Threose CH2OH HO H OH H C H 1 O 2 3 4 is equivalent to C O H -l-Arabinofuranose H HOCH2 OH H OH H H OH O -l-Arabinofuranose H HOCH2 H OH OH H H OH O 704 CARBOHYDRATES Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
CARBOHYDRATES 705 Mannose differs from glucose in configuration at C-2. All hydroxyl groups are equatorial in B-D-glucopyranose; the hydroxyl at C-2 is axial in B-D-mannopyranose (c) The conformational depiction of B-L-mannopyranose begins in the same way as that of B-D-mannopyranose. L-Mannose is the mirror image of D-mannose. CHO CHO CH,OH H 10-H H HH HO-H CHOH D-Mannose L-Mannose Eclipsed conformation To rewrite the eclipsed conformation of L-mannose in a way that permits hemiacetal forma tion between the carbonyl group and the C-5 hydroxyl, C-5 is rotated 120 in the clockwise HO CH,OH H te about CHOH CHOH H C-S bond Translating the Haworth formula into a proper conformational depiction requires that a choice be made between the two chair conformations shown H HO CH,OHH H H H HH OH OH HO CHOH OH H OH Haworth formula of Less stable chair conformation More stable chair confor B-L-mannopyranose ChOH is axial CH,OH is equatoria (d) The Fischer projection formula for L-ribose is the mirror image of that for D-ribose CHO CHO CH2OH H OHHO→H HO HO HOHO H-OH L-Ribose Eclipsed conformation of L-ribose is Haworth formula of Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website
Mannose differs from glucose in configuration at C-2. All hydroxyl groups are equatorial in -D-glucopyranose; the hydroxyl at C-2 is axial in -D-mannopyranose. (c) The conformational depiction of -L-mannopyranose begins in the same way as that of -D-mannopyranose. L-Mannose is the mirror image of D-mannose. To rewrite the eclipsed conformation of L-mannose in a way that permits hemiacetal formation between the carbonyl group and the C-5 hydroxyl, C-5 is rotated 120° in the clockwise sense. Translating the Haworth formula into a proper conformational depiction requires that a choice be made between the two chair conformations shown. (d) The Fischer projection formula for L-ribose is the mirror image of that for D-ribose. CHO CH2OH H H H OH OH OH d-Ribose CHO CH2OH HO HO HO H H H l-Ribose Eclipsed conformation of l-ribose is oriented properly for ring closure. HO H HO H HO H CH2OH Haworth formula of -l-ribopyranose HO H HO H HO H H O OH C O H HO H H CH2OH HO H OH H H O OH Haworth formula of -l-mannopyranose Less stable chair conformation; CH2OH is axial OH H H CH2OH H H HO H OH OH O More stable chair conformation; CH2OH is equatorial OH OH OH H H H H HO HOCH2 O H HO H HO H HO H H OH CH2OH C H O O 1 3 2 4 5 5 6 HO H H CH2OH HO H H OH OH C H 1 3 2 4 -l-Mannopyranose (remember, the anomeric hydroxyl is down in the l series) HO H H CH2OH HO H H OH H OH O rotate about C-4 C-5 bond CHO CH2OH H HO H HO H OH H OH l-Mannose CHO CH2OH HO H OH H OH H HO H d-Mannose Eclipsed conformation of l-mannose HO H H H HO HO H OH CH2OH C O H CARBOHYDRATES 705 Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
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