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

he major classes of organic compounds common to living systems are lipids, pro- teins, nucleic acids, and carbohydrates. Carbohydrates are very familiar to us- we call many of them"sugars." They make up a substantial portion of the food we eat and provide most of the energy that keeps the human engine running. Carbohy- drates are structural components of the walls of plant cells and the wood of trees. Genetic

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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

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