《有机化学》课程教学资源（教材文献，英文版）CHAPTER 25 CARBOHYDRATES.pdf_大学文库

25.2 Fischer Projections and D-L Notation (C6H12O6), for example, is a monosaccharide. a disaccharide on hydrolysis is cleaved to two monosaccharides, which may be the same or different. Sucrose--common table ugar-is a disaccharide that yields one molecule of glucose and one of fructose on hydrolysis Sucrose(C12H22O11)+ H20- glucose(C6H1206)+ fructose(C6H12O6) An oligosaccharide(oligos is a greek word that in its plural form means"few") yields 3-10 monosaccharide units on hydrolysis. Polysaccharides are hydrolyzed to more than 10 monosaccharide units. Cellulose is a polysaccharide molecule that gives thousands of glucose molecules when completely hydrolyzed Over 200 different monosaccharides are known. They can be grouped according to the number of carbon atoms they contain and whether they are polyhydroxy alde hydes or polyhydroxy ketones. Monosaccharides that are polyhydroxy aldehydes are called aldoses; those that are polyhydroxy ketones are ketoses. Aldoses and ketoses are further classified according to the number of carbon atoms in the main chain Table 25.1 lists the terms applied to monosaccharides having four to eight carbon atoms 25.2 FISCHER PROJECTIONS AND D-L NOTATION Stereochemistry is the key to understanding carbohydrate structure, a fact that was clearly appreciated by the German chemist Emil Fischer. The projection formulas used by Fischer determined the struc Fischer to represent stereochemistry in chiral molecules are particularly well-suited to ture of gl studying carbohydrates. Figure 25 1 illustrates their application to the enantiomers of won the Nobse in 1900 and glyceraldehyde(2, 3-dihydroxypropanal), a fundamental molecule in carbohydrate stereo- chemistry. When the Fischer projection is oriented as shown in the figure, with the car bon chain vertical and the aldehyde carbon at the top, the C-2 hydroxyl group points to the right in(+)-glyceraldehyde and to the left in(-)-glyceraldehyde Techniques for determining the absolute configuration of chiral molecules were not developed until the 1950s, and so it was not possible for Fischer and his contemporaries to relate the sign of rotation of any substance to its absolute configuration. A system evolved based on the arbitrary assumption, later shown to be correct, that the enantiomers ure 25.1. Two stereochemical descriptors were defined: D and L. The absolute configu- glyceraldehyde as stereo rs of of glyceraldehyde have the signs of rotation and absolute configurations shown in Fig-Ade ration of(+)glyceraldehyde, as depicted in the figure, was said to be d and that of its pounds originated with enantiomer,(-)glyceraldehyde, L Compounds that had a spatial arrangement of sub- proposals made in 1906 by stituents analogous to D-(+)-and L-(-)-glyceraldehyde were said to have the D and L configurations, respectively. New York University. TABLE 25.1 Some Classes of Monosaccharides Number of carbon atoms Aldose Ketose Aldotetrose Ketotetrose Aldopentose Ketopentose Aldohexose Ketohexose Seven Aldoheptose Ketoheptose Eight Aldooctose Ketooctos Back Forward Main MenuToc Study Guide ToC Student o MHHE Website

(C6H12O6), for example, is a monosaccharide. A disaccharide on hydrolysis is cleaved to two monosaccharides, which may be the same or different. Sucrose—common table sugar—is a disaccharide that yields one molecule of glucose and one of fructose on hydrolysis. An oligosaccharide (oligos is a Greek word that in its plural form means “few”) yields 3–10 monosaccharide units on hydrolysis. Polysaccharides are hydrolyzed to more than 10 monosaccharide units. Cellulose is a polysaccharide molecule that gives thousands of glucose molecules when completely hydrolyzed. Over 200 different monosaccharides are known. They can be grouped according to the number of carbon atoms they contain and whether they are polyhydroxy aldehydes or polyhydroxy ketones. Monosaccharides that are polyhydroxy aldehydes are called aldoses; those that are polyhydroxy ketones are ketoses. Aldoses and ketoses are further classified according to the number of carbon atoms in the main chain. Table 25.1 lists the terms applied to monosaccharides having four to eight carbon atoms. 25.2 FISCHER PROJECTIONS AND D–L NOTATION Stereochemistry is the key to understanding carbohydrate structure, a fact that was clearly appreciated by the German chemist Emil Fischer. The projection formulas used by Fischer to represent stereochemistry in chiral molecules are particularly well-suited to studying carbohydrates. Figure 25.1 illustrates their application to the enantiomers of glyceraldehyde (2,3-dihydroxypropanal), a fundamental molecule in carbohydrate stereochemistry. When the Fischer projection is oriented as shown in the figure, with the carbon chain vertical and the aldehyde carbon at the top, the C-2 hydroxyl group points to the right in ()-glyceraldehyde and to the left in ()-glyceraldehyde. Techniques for determining the absolute configuration of chiral molecules were not developed until the 1950s, and so it was not possible for Fischer and his contemporaries to relate the sign of rotation of any substance to its absolute configuration. A system evolved based on the arbitrary assumption, later shown to be correct, that the enantiomers of glyceraldehyde have the signs of rotation and absolute configurations shown in Figure 25.1. Two stereochemical descriptors were defined: D and L. The absolute configuration of ()-glyceraldehyde, as depicted in the figure, was said to be D and that of its enantiomer, ()-glyceraldehyde, L. Compounds that had a spatial arrangement of substituents analogous to D-()- and L-()-glyceraldehyde were said to have the D and L configurations, respectively. Sucrose (C12H22O11) H 2O glucose (C6H12O6) fructose ( C6H12O6) 25.2 Fischer Projections and D–L Notation 973 TABLE 25.1 Some Classes of Monosaccharides Aldose Aldotetrose Aldopentose Aldohexose Aldoheptose Aldooctose Ketose Ketotetrose Ketopentose Ketohexose Ketoheptose Ketooctose Number of carbon atoms Four Five Six Seven Eight Adopting the enantiomers of glyceraldehyde as stereochemical reference compounds originated with proposals made in 1906 by M. A. Rosanoff, a chemist at New York University. Fischer determined the structure of glucose in 1900 and won the Nobel Prize in chemistry in 1902. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website

PROBLEM 25.1 Identify each of the following as either D- or L-glyceraldehyde: (a) (b) (c) SAMPLE SOLUTION (a) Redraw the Fischer projection so as to more clearly show the true spatial orientation of the groups. Next, reorient the molecule so that its relationship to the glyceraldehyde enantiomers in Figure 25.1 is apparent. The structure is the same as that of ()-glyceraldehyde in the figure. It is Dglyceraldehyde. Fischer projections and D–L notation have proved to be so helpful in representing carbohydrate stereochemistry that the chemical and biochemical literature is replete with their use. To read that literature you need to be acquainted with these devices, as well as the more modern Cahn–Ingold–Prelog system. 25.3 THE ALDOTETROSES Glyceraldehyde can be considered to be the simplest chiral carbohydrate. It is an aldotriose and, since it contains one stereogenic center, exists in two stereoisomeric forms: the D and L enantiomers. Moving up the scale in complexity, next come the aldotetroses. Examination of their structures illustrates the application of the Fischer system to compounds that contain more than one stereogenic center. The aldotetroses are the four stereoisomers of 2,3,4-trihydroxybutanal. Fischer projections are constructed by orienting the molecule in an eclipsed conformation with the aldehyde group at what will be the top. The four carbon atoms define the main chain of the Fischer projection and are arranged vertically. Horizontal bonds are directed outward, vertical bonds back. HO H CH2OH CHO is equivalent to turn 180° HO C CH2OH H CHO H C CHO OH CH2OH HOCH2 OH H CHO HOCH2 CHO H OH HO H CH2OH CHO 974 CHAPTER TWENTY-FIVE Carbohydrates CHœO CH2OH OH CHœO CHœO HO HO H H H CH2OH R-(+)-Glyceraldehyde S-(–)-Glyceraldehyde H C OH CHœO CH2OH C CH2OH FIGURE 25.1 Threedimensional representations and Fischer projections of the enantiomers of glyceraldehyde. Molecular models of the four stereoisomeric aldotetroses may be viewed on the CD that accompanies this text. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website

CHAPTER TWENTY-FIVE Carbohydrat As shown for the aldotetroses, an aldose belongs to the d or the L series accord- ing to the configuration of the stereogenic center farthest removed from the aldehyde function. Individual names, such as erythrose and threose, specify the particular arrange- ment of stereogenic centers within the molecule relative to each other. Optical activities cannot be determined directly from the D and L prefixes. As it turns out, both D-erythrose and D-threose are levorotatory, but D-glyceraldehyde is dextrorotatory 25.4 ALDOPENTOSES AND ALDOHEXOSES Aldopentoses have three stereogenic centers. The eight stereoisomers are divided into a et of four D-aldopentoses and an enantiomeric set of four L-aldopentoses. The aldopen- toses are named ribose, arabinose, xylose, and lyxose. Fischer projections of the D stereoisomers of the aldopentoses are given in Figure 25. 2. Notice that all these diastereo- mers have the same configuration at C-4 and that this configuration is analogous to that of D-(+)-glyceraldehyde PROBLEM 25.3 L-(+)-Arabinose is a naturally occurring L sugar. It is obtained by acid hydrolysis of the polysaccharide present in mesquite gum. Write a Fischer pro- jection for L-(+)-arabinose Among the aldopentoses, D-ribose is a component of many biologically important substances, most notably the ribonucleic acids, and D-xylose is very abundant and is iso- lated by hydrolysis of the polysaccharides present in corncobs and the wood of trees The aldohexoses include some of the most familiar of the monosaccharides. as well than glucose, but each cellu- as one of the most abundant organic compounds on earth, D-(+)-glucose. With four lose molecule is a polysac- charide composed of stereogenic centers, 16 stereoisomeric aldohexoses are possible: 8 belong to the D series housands of glucose units and 8 to the L series. All are known, either as naturally occurring substances or as the Section 25. 15). metha products of synthesis. The eight D-aldohexoses are given in Figure 25.2; it is the spatia ay also be more abundant, arrangement at C-5, hydrogen to the left in a Fischer projection and hydroxyl to the right, comes from glucose. that identifies them as carbohydrates of the D series PROBLEM 25.4 Name the following sugar CHO CHOH Of all the monosaccharides, D-(+)-glucose is the best known, most important, and most abundant. Its formation from carbon dioxide, water, and sunlight is the central theme of photosynthesis. Carbohydrate formation by photosynthesis is estimated to be on the order of 10 tons per year, a source of stored energy utilized, directly or indi rectly, by all higher forms of life on the planet. Glucose was isolated from raisins in 1747 and by hydrolysis of starch in 1811. Its structure was determined, in work culmi- nating in 1900, by Emil Fischer. D-(+)-Galactose is a constituent of numerous polysaccharides. It is best obtained by acid hydrolysis of lactose(milk sugar), a disaccharide of D-glucose and D-galactose Back Forward Main MenuToc Study Guide ToC Student o MHHE Website

As shown for the aldotetroses, an aldose belongs to the D or the L series according to the configuration of the stereogenic center farthest removed from the aldehyde function. Individual names, such as erythrose and threose, specify the particular arrangement of stereogenic centers within the molecule relative to each other. Optical activities cannot be determined directly from the D and L prefixes. As it turns out, both D-erythrose and D-threose are levorotatory, but D-glyceraldehyde is dextrorotatory. 25.4 ALDOPENTOSES AND ALDOHEXOSES Aldopentoses have three stereogenic centers. The eight stereoisomers are divided into a set of four D-aldopentoses and an enantiomeric set of four L-aldopentoses. The aldopentoses are named ribose, arabinose, xylose, and lyxose. Fischer projections of the D stereoisomers of the aldopentoses are given in Figure 25.2. Notice that all these diastereomers have the same configuration at C-4 and that this configuration is analogous to that of D-()-glyceraldehyde. PROBLEM 25.3 L-()-Arabinose is a naturally occurring L sugar. It is obtained by acid hydrolysis of the polysaccharide present in mesquite gum. Write a Fischer projection for L-()-arabinose. Among the aldopentoses, D-ribose is a component of many biologically important substances, most notably the ribonucleic acids, and D-xylose is very abundant and is isolated by hydrolysis of the polysaccharides present in corncobs and the wood of trees. The aldohexoses include some of the most familiar of the monosaccharides, as well as one of the most abundant organic compounds on earth, D-()-glucose. With four stereogenic centers, 16 stereoisomeric aldohexoses are possible; 8 belong to the D series and 8 to the L series. All are known, either as naturally occurring substances or as the products of synthesis. The eight D-aldohexoses are given in Figure 25.2; it is the spatial arrangement at C-5, hydrogen to the left in a Fischer projection and hydroxyl to the right, that identifies them as carbohydrates of the D series. PROBLEM 25.4 Name the following sugar: Of all the monosaccharides, D-()-glucose is the best known, most important, and most abundant. Its formation from carbon dioxide, water, and sunlight is the central theme of photosynthesis. Carbohydrate formation by photosynthesis is estimated to be on the order of 1011 tons per year, a source of stored energy utilized, directly or indirectly, by all higher forms of life on the planet. Glucose was isolated from raisins in 1747 and by hydrolysis of starch in 1811. Its structure was determined, in work culminating in 1900, by Emil Fischer. D-()-Galactose is a constituent of numerous polysaccharides. It is best obtained by acid hydrolysis of lactose (milk sugar), a disaccharide of D-glucose and D-galactose. H OH CHO H OH H OH HO H CH2OH 976 CHAPTER TWENTY-FIVE Carbohydrates Cellulose is more abundant than glucose, but each cellulose molecule is a polysaccharide composed of thousands of glucose units (Section 25.15). Methane may also be more abundant, but most of the methane comes from glucose. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website

CHAPTER TWENTY-FIVE Carbohydrat L()-Galactose also occurs naturally and can be prepared by hydrolysis of flaxseed gum and agar. The principal source of D-(+)-mannose is hydrolysis of the polysaccharide of the ivory nut, a large, nut-like seed obtained from a South American pal 25.5 A MNEMONIC FOR CARBOHY DRATE CONFIGURATIONS The task of relating carbohydrate configurations to names requires either a world-class emory or an easily recalled mnemonic. A mnemonic that serves us well here was pop- Chemical Educa. ularized by the husband-wife team of Louis F. Fieser and Mary Fieser of Harvard Uni- 34). An article gi of the lo the jury onics riety of versity in their 1956 textbook, Organic Chemistry. As with many mnemonics, it's not clear who actually invented it, and references to this particular one appeared in the chem- ical education literature before publication of the Fiesers'text. The mnemonic has two eatures a e a system for setting down all the stereoisomeric D-aldohexoses in a logical order; and(2)a way to assign the correct name to each one A systematic way to set down all the D-hexoses(as in Fig. 25.2) is to draw skele tons of the necessary eight Fischer projections, placing the hydroxyl group at C-5 to the right in each so as to guarantee that they all belong to the D series. Working up the car bon chain, place the hydroxyl group at C-4 to the right in the first four structures, and to the left in the next four. In each of these two sets of four, place the C-3 hydroxyl group to the right in the first two and to the left in the next two; in each of the result ng four sets of two, place the C-2 hydroxyl group to the right in the first one and to the left in the second Once the eight Fischer projections have been written, they are named in order with the aid of the sentence: All altruists gladly make gum in gallon tanks. The words of the sentence stand for allose, altrose, glucose, mannose, gulose, idose, galactose, talose An analogous pattern of configurations can be seen in the aldopentoses when they are arranged in the order ribose, arabinose, xylose, lyxose. (RAXL is an easily remer bered nonsense word that gives the correct sequence. )This pattern is discernible even 25.6 CYCLIC FORMS OF CARBOHY DRATES: FURANOSE FORMS Aldoses incorporate two functional groups, C=O and OH, which are capable of react- ing with each other. We saw in Section 17.8 that nucleophilic addition of an alcohol function to a carbonyl group gives a hemiacetal. When the hydroxyl and carbonyl groups are part of the same molecule, a cyclic hemiacetal results, as illustrated in Figure 25.3 Cyclic hemiacetal formation is most common when the ring that results is five-or six-membered Five-membered cyclic hemiacetals of carbohydrates are called furanose orms: six-membered ones are called pyranose forms. The ring carbon that is derived from the carbonyl group, the one that bears two oxygen substituents, is called the anomeric carbon Aldoses exist almost exclusively as their cyclic hemiacetals; very little of the open- chain form is present at equilibrium. To understand their structures and chemical reac tions, we need to be able to translate Fischer projections of carbohydrates into their cyclic hemiacetal forms. Consider first cyclic hemiacetal formation in D-erythrose. So as to visualize furanose ring formation more clearly, redraw the Fischer projection in a form more suited to cyclization, being careful to maintain the stereochemistry at each stereo- genic center. Back Forward Main MenuToc Study Guide ToC Student o MHHE Website

L()-Galactose also occurs naturally and can be prepared by hydrolysis of flaxseed gum and agar. The principal source of D-()-mannose is hydrolysis of the polysaccharide of the ivory nut, a large, nut-like seed obtained from a South American palm. 25.5 A MNEMONIC FOR CARBOHYDRATE CONFIGURATIONS The task of relating carbohydrate configurations to names requires either a world-class memory or an easily recalled mnemonic. A mnemonic that serves us well here was popularized by the husband–wife team of Louis F. Fieser and Mary Fieser of Harvard University in their 1956 textbook, Organic Chemistry. As with many mnemonics, it’s not clear who actually invented it, and references to this particular one appeared in the chemical education literature before publication of the Fiesers’ text. The mnemonic has two features: (1) a system for setting down all the stereoisomeric D-aldohexoses in a logical order; and (2) a way to assign the correct name to each one. A systematic way to set down all the D-hexoses (as in Fig. 25.2) is to draw skeletons of the necessary eight Fischer projections, placing the hydroxyl group at C-5 to the right in each so as to guarantee that they all belong to the D series. Working up the carbon chain, place the hydroxyl group at C-4 to the right in the first four structures, and to the left in the next four. In each of these two sets of four, place the C-3 hydroxyl group to the right in the first two and to the left in the next two; in each of the resulting four sets of two, place the C-2 hydroxyl group to the right in the first one and to the left in the second. Once the eight Fischer projections have been written, they are named in order with the aid of the sentence: All altruists gladly make gum in gallon tanks. The words of the sentence stand for allose, altrose, glucose, mannose, gulose, idose, galactose, talose. An analogous pattern of configurations can be seen in the aldopentoses when they are arranged in the order ribose, arabinose, xylose, lyxose. (RAXL is an easily remembered nonsense word that gives the correct sequence.) This pattern is discernible even in the aldotetroses erythrose and threose. 25.6 CYCLIC FORMS OF CARBOHYDRATES: FURANOSE FORMS Aldoses incorporate two functional groups, CœO and OH, which are capable of reacting with each other. We saw in Section 17.8 that nucleophilic addition of an alcohol function to a carbonyl group gives a hemiacetal. When the hydroxyl and carbonyl groups are part of the same molecule, a cyclic hemiacetal results, as illustrated in Figure 25.3. Cyclic hemiacetal formation is most common when the ring that results is five- or six-membered. Five-membered cyclic hemiacetals of carbohydrates are called furanose forms; six-membered ones are called pyranose forms. The ring carbon that is derived from the carbonyl group, the one that bears two oxygen substituents, is called the anomeric carbon. Aldoses exist almost exclusively as their cyclic hemiacetals; very little of the openchain form is present at equilibrium. To understand their structures and chemical reactions, we need to be able to translate Fischer projections of carbohydrates into their cyclic hemiacetal forms. Consider first cyclic hemiacetal formation in D-erythrose. So as to visualize furanose ring formation more clearly, redraw the Fischer projection in a form more suited to cyclization, being careful to maintain the stereochemistry at each stereogenic center. 978 CHAPTER TWENTY-FIVE Carbohydrates See, for example, the November 1955 issue of the Journal of Chemical Education (p. 584). An article giving references to a variety of chemistry mnemonics appears in the July 1960 issue of the Journal of Chemical Education (p. 366). Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website

CHAPTER TWENTY-FIVE Carbohydrat Structural drawings of carbohydrates of this type are called Haworth formulas, after the British carbohydrate chemist Sir Walter Norman Haworth(St. Andrews Uni versity and the University of Birmingham. Early in his career Haworth contributed to the discovery that sugars exist as cyclic hemiacetals rather than in open-chain forms Later he collaborated on an efficient synthesis of vitamin C from carbohydrate precur- sors. This was the first chemical synthesis of a vitamin and provided an inexpensive route to its preparation on a commercial scale. Haworth was a corecipient of the Nobel Prize or chemistry in 1937 The two stereoisomeric furanose forms of D-erythrose are named a-D-erythrofura- nose and rofuranose The prefixes a and B describe relative configuration. The configuration of the anomeric carbon is a when its hydroxyl group is on the same side of a Fischer projection as the hydroxyl group at the highest numbered stereogenic cen- ter. When the hydroxyl groups at the anomeric carbon and the highest numbered stereo- genic center are on opposite sides of a Fischer projection, the configuration at the anomeric carbon is Substituents that are to the right in a Fischer projection are""in the corre- sponding Haworth formula. Generating Haworth formulas to show stereochemistry in furanose forms of higher aldoses is slightly more complicated and requires an additional operation. Furanose forms of D-ribose are frequently encountered building blocks in biologically important organic molecules. They result from hemiacetal formation between the aldehyde group and the hyd at C-4 CH CH,OH H-OH CH=O Furanose ring oH formation involves this hydroxyl group D-Ribose Eclipsed conformation of D-ribose Notice that the eclipsed conformation of D-ribose derived directly from the Fischer pro- jection does not have its C-4 hydroxyl group properly oriented for furanose ring forma- tion. We must redraw it in a conformation that permits the five-membered cyclic hemi- acetal to form. This is accomplished by rotation about the C(3)-C(4)bond, taking care that the configuration at C-4 is not changed ry using a model to help see t CH,OH CH=O HO OH HO OH suitable for furanose ring formation Back Forward Main MenuToc Study Guide ToC Student o MHHE Website

Structural drawings of carbohydrates of this type are called Haworth formulas, after the British carbohydrate chemist Sir Walter Norman Haworth (St. Andrew’s University and the University of Birmingham). Early in his career Haworth contributed to the discovery that sugars exist as cyclic hemiacetals rather than in open-chain forms. Later he collaborated on an efficient synthesis of vitamin C from carbohydrate precursors. This was the first chemical synthesis of a vitamin and provided an inexpensive route to its preparation on a commercial scale. Haworth was a corecipient of the Nobel Prize for chemistry in 1937. The two stereoisomeric furanose forms of D-erythrose are named -D-erythrofuranose and -D-erythrofuranose. The prefixes and describe relative configuration. The configuration of the anomeric carbon is when its hydroxyl group is on the same side of a Fischer projection as the hydroxyl group at the highest numbered stereogenic center. When the hydroxyl groups at the anomeric carbon and the highest numbered stereogenic center are on opposite sides of a Fischer projection, the configuration at the anomeric carbon is . Substituents that are to the right in a Fischer projection are “down” in the corresponding Haworth formula. Generating Haworth formulas to show stereochemistry in furanose forms of higher aldoses is slightly more complicated and requires an additional operation. Furanose forms of D-ribose are frequently encountered building blocks in biologically important organic molecules. They result from hemiacetal formation between the aldehyde group and the hydroxyl at C-4: Notice that the eclipsed conformation of D-ribose derived directly from the Fischer projection does not have its C-4 hydroxyl group properly oriented for furanose ring formation. We must redraw it in a conformation that permits the five-membered cyclic hemiacetal to form. This is accomplished by rotation about the C(3)±C(4) bond, taking care that the configuration at C-4 is not changed. CH2OH H H H HO HO OH 4 CH O 5 3 2 1 O H HOCH2 H H H HO OH 4 CH O 5 3 2 1 Conformation of D-ribose suitable for furanose ring formation rotate about C(3)±C(4) bond H CHO CH2OH OH H OH H OH 3 4 5 2 1 D-Ribose CH2OH H H H HO HO OH 4 CH O 5 3 2 1 Eclipsed conformation of D-ribose Furanose ring formation involves this hydroxyl group 980 CHAPTER TWENTY-FIVE Carbohydrates Try using a molecular model to help see this. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website