Carbohydrates Monosaccharides

Chapter 34

Carbohydrates, Monosaccharides

Introduction

• In the leaf of a plant, the simple compounds carbon dioxide and water are combined to form the sugar ( +)-glucose. This process, known as photosynthesis, requires catalysis by the green coloring matter chlorophyll, and requires energy in the form of light. Thousands of (+)-glucose molecules can then be combined to form the much larger molecules of cellulose, which constitutes the supporting framework of the plant. (+ )-Glucose molecules can also be combined, in a somewhat different way, to form the large molecules of starch, which is then stored in the seeds to serve as food for a new, growing plant.

• When eaten by an animal, the starch and in the case of certain animals also the cellulose is broken down into the original -) -glucose units. These can be carried by the bloodstream to the liver to be recombined into glycogen, or animal starch; when the need arises, the glycogen can be broken down once more into ( + Vglucose. (H-)-Glucose is carried by the bloodstream to the tissues, where it is oxidized, ultimately to carbon dioxide and water, with the release of the energy originally supplied as sunlight. Some of the (-f )-glucose is converted into fats; some reacts with nitrogen-containing compounds to form amino acids, which in turn are combined to form the proteins that make up a large part of the animal body.

• -Glucose, cellulose, starch, and glycogen all belong to the class of organic compounds known as carbohydrates. Carbohydrates are the ultimate source of most of our food: we eat starch-containing grain or feed it to animals to be converted into meat and fat which we then eat. We clothe ourselves with cellulose in the form of cotton and linen, rayon and cellulose acetate. We build houses and furniture from cellulose in the form of wood. Thus, carbohydrates quite literally provide us with the necessities of life: food, clothing, and shelter.
• The study of carbohydrates is one of the most exciting fields of organic chemistry. It extends from the tremendously complicated problem of understanding the process of photosynthesis to the equally difficult problem of unraveling the tangled steps in the enzyme-catalyzed reconversion of ( + )-glucose into carbon dioxide and water. Between these two biochemical problems there lie the more traditional problems of the organic chemist: determination of the structure and properties of the carbohydrates, and the study of their conversion into other organic compounds.

Definition and classification

• Carbohydrates are polyhydroxy aldehydes, polyhydroxy ketones, or compounds that can be hydrolyzed to them. A carbohydrate that cannot be hydrolyzed to simpler compounds is called a monosaccharide. A carbohydrate that can be hydrolyzed to two monosaccharide molecules is called a disaccharide. A carbohydrate that can be hydrolyzed to many monosaccharide molecules is called a polysaccharide.

• A monosaccharide may be further classified. If it contains an aldehyde group, it is known as an aldose; if it contains a keto group, it is known as a ketose. Depending upon the number of carbon atoms it contains, a monosaccharide is known as a triose, tetrodes, pentose, hexose, and so on. An aldohexose, for example, is a six-carbon monosaccharide containing an aldehyde group; a Keto pentose is a five-carbon monosaccharide containing a keto group. Most naturally occurring monosaccharides are pentoses or hexoses.

• Carbohydrates that reduce Fehling's (or Benedict's) or Tollens* reagent (p. 1075) are known as reducing sugars. All monosaccharides, whether aldose or ketose, are reducing sugars. Most disaccharides are reducing sugars; sucrose (common table sugar) is a notable exception, for it is a non-reducing sugar.

(+)-GIucose: an aldohexoses

• Because it is the unit of which starch, cellulose, and glycogen are made up, and because of its special role in biological processes, (-f-)-glucose is by far the most abundant monosaccharide there are probably more (+ )-glucose units in nature than any other organic group and by far the most important monosaccharide.

• Most of what we need to know about monosaccharides we can learn from the study of just this one compound, and indeed from the study of just one aspect: its structure, and how that structure was arrived at. In learning about the structure of (4- )-glucose, we shall at the same time learn about its properties, since it is from these properties that the structure has been deduced. (+ )-Glucose is typical monosaccharide, so that in learning about its structure and properties, we shall be learning about the structure and properties of the other members of this family.

• (d->Glucose has the molecular formula C6H12O<^, as shown by elemental analysis and molecular weight determination. In is summarized other evidence about its structure: evidence consistent with the idea that (-j-)-glucose is a six-carbon, straight-chain, pentahydroxy aldehyde, that is, that (-f)-glucose is an aldohexose. But this is only the beginning. There are, as we shall see, 16 possible aldohexoses, all stereoisomers of each other, and we want to know which one (+)-glucose is. Beyond this, there is the fact that (+)-glucose exists in alpha and beta forms, indicating still further stereochemical possibilities that are not accommodated by the simple picture of a pentahydroxy aldehyde. Finally, we must pinpoint the predominant conformation in which the compound exists. All this is the structure of (-h)-glucose and, when we have arrived at it, we shall see the features that make it the very special molecule that it is.

(-)-Fructose: a 2-ketohexose

The most important ketose is (-)-fructose, which occurs widely in fruits and combined with glucose, in the disaccharide sucrose (common table sugar). The following sequence shows that (-)-fructose is a ketone rather than an aldehyde, and gives the position of the keto group in the chain:

Stereoisomers of (+)-glucose. Nomenclature of aldose derivatives

• If we examine the structural formula we have drawn for glucose, we see that it contains four chiral centers (marked by asterisks):

• Each of the possible stereoisomers is commonly represented by a "cross" formula, as, for example, in I. As always in formulas of this kind, it is understood that horizontal lines represent bonds coming toward us out of the plane of the paper, and vertical lines represent bonds going awayfrom us behind the plane of the paper.

• The products obtained from these other aldohexoses are generally given names that correspond to the names of the products obtained from glucose. This principle is illustrated in Table 34.1 for the aldohexose (H-)-mannose, which occurs naturally in many plants (the name is derived from the Biblical word manna).

• The structural formula we have drawn to represent (H-)-glucose so far could actually represent any of the 16 aldohexoses. Only when we have specified the configuration about each of the chiral centers will we have the structural formula that applies only to (+ )-glucose itself. Before we can discuss the brilliant way in which the configuration of (+)-glucose was worked out, we must first learn a little more about the chemistry of monosaccharides.

Oxidation. Effect of alkali

• Aldoses can be oxidized in four important ways: (a) by Fehling's or Tollens' reagent; (b) by bromine water; (c) by nitric acid; and (d) by periodic acid, HIO4 . Aldoses reduce Tollens 9 reagent, as we would expect aldehydes to do. They also reduce Fehling's solution, an alkaline solution of cupric ion complexed with tartrate ion (or Benedict's solution, in which complexing is with citrate ion); the deep-blue color of the solution is discharged, and red cuprous oxide precipitates. These reactions are less useful, however, than we might at first have expected. In the first place, they cannot be used to differentiate aldoses from ketoses Ketoses, too, reduce Fehling's and Tollens' reagents; this behavior is characteristic of a-hydroxy ketones.

• In the second place, oxidation by Fehling's or Tollens' reagent cannot be used for the preparation of adonic acids (monocarboxylic acids) from aldoses. Both Fehling's and Tollens' reagents are alkaline reagents, and the treatment of sugars with alkali can cause extensive isomerization and even decomposition of the chain. Alkali exerts this effect, in part at least, by establishing an equilibrium between the monosaccharide and an enediol structure.

• Bromine water oxidizes aldoses, but not ketoses as an acidic reagent it does not cause isomerization of the molecule. It can therefore be used to differentiate an aldose from a ketose, and is the reagent chosen to synthesize the adonic acid (monocarboxylic acid) from an aldose.

Osazone formation. Epimers

• As aldehydes, aldoses react with phenylhydrazine to form phenylhydrazones. If an excess of phenylhydrazine is used, the reaction proceeds further to yield products known as osazones, which contain two phenylhydrazine residues per molecule; a third molecule of the reagent is turned into aniline and ammonia. (Just how the OH group is oxidized is not quite clear.

• Osazone formation is not limited to carbohydrates but is typical of a-hydroxy aldehydes and a-hydroxy ketones in general (e.g., benzoin, C6H5CHOHCOC6H5). Removal of the phenylhydrazine groups yields dicarbonyl compounds known as osones. For example:

• In 1858 Peter Griess (in time taken from his duties in an English brewery) discovered diazonium salts. In 1875 Emil Fischer (at the University of Munich) found that reduction of benzene diazonium chloride by sulfur dioxide yields phenylhydrazine. Nine years later, in 1884, Fischer reported that the phenylhydrazine he had discovered could be used as a powerful tool in the study of carbohydrates. '"One of the difficulties of working with carbohydrates is their tendency to form sirups; these are fine for pouring on pancakes at breakfast, but hard to work with in the laboratory. Treatment with phenylhydrazine converts carbohydrates into solid osazones, which are readily isolated and purified, and can be identified by their characteristic crystalline forms.

• Fischer found osazone formation to be useful not only in identifying carbohydrates, but also and this was much more important in determining their configurations. For example, the two diastereomeric aldohexoses (-f)-glucose and (H-)-mannose yield the same osazone. Osazone formation destroys the configuration about C-2 of an aldose but does not affect the configuration of the rest of the molecule.

• It therefore follows that (H-)-glucose and (+)-mannose differ only in configuration about C-2, and have the same configuration about C-3, C-4, and C-5. We can see that whenever the configuration of either of these compounds is established, the configuration of the other is immediately known through this osazone relationship. A pair of diastereomeric aldoses that differ only in configuration about C-2 are called epimers. One way in which a pair of aldoses can be identified as epimers is through the formation of the same osazone.

Lengthening the carbon chain of aldoses. The Kiliani-Fischer synthesis

• In the next few sections, we shall examine some of the ways in which an aldose can be converted into a different aldose. These conversions can be used not only to synthesize new carbohydrates, but also, as we shall see, to help determine their configurations.

• First, let us look at a method for converting an aldose into another aldose containing one more carbon atom, that is, at a method for lengthening the carbon chain. In 1886, Heinrich Kiliani (at the Tekniche Hochschule in Munich) showed that an aldose can be converted into two aldonic acids of the next higher carbon number by addition of HCN and hydrolysis of the resulting cyanohydrins. In 1890, Fischer reported that reduction of an aldonic acid (in the form of its lactone, can be controlled to yield the corresponding aldose. In the entire Kiliani-Fischer synthesis is illustrated for the conversion of an aldopentose into two aldohexoses.

• Addition of cyanide to the aldopentose generates a new chiral center, about which there are two possible configurations. As a result, two diastereomeric cyanohydrins are obtained, which yield diastereomeric carboxylic acids (aldonic acids) and finally diastereomeric aldoses.

• Since a six-carbon aldonic acid contains OH groups in the y- and 5-positions, we would expect it to form a lactone under acidic conditions. This occurs, the y-lactone generally being the more stable product. It is the lactone that is actually reduced to an aldose in the last step of a Kiliani-Fischer synthesis.

• The pair of aldoses obtained from the sequence differ only in configuration about C~2, and hence are epimers. A pair of aldoses can be recognized as epimers not only by their conversion into the same osazone  but also by their formation in the same Kiliani-Fischer synthesis.

• Like other diastereomers, these epimers differ in physical properties and therefore are separable. However, since carbohydrates are difficult to purify, it is usually more convenient to separate the diastereomeric products at the acid stage, where crystalline salts are easily formed, so that a single pure lactone can be reduced to a single pure aldose.

Shortening the carbon chain of aidoses. The Ruff degradation >

• There are a number of ways in which an aldose can be converted into another aldose of one less carbon atom. One of these methods for shortening the carbon chain is the Ruff degradation. An aldose is oxidized by bromine water to the aldonic acid; oxidation of the calcium salt of this acid by hydrogen peroxide in the presence of ferric salts yields carbonate ion and an aldose of one less carbon atom

Conversion of an aldose into its epimer

• In the presence of a tertiary amine, in particular pyridine, an equilibrium is established between an aldonic acid and its epimer. This reaction is the basis of the best method for converting an aldose into its epimer, since the only configuration affected is that at C-2. The aldose is oxidized by bromine water to the aldonic acid, which is then treated with pyridine. From the equilibrium mixture thus formed, the epimeric aldonic acid is separated, and reduced (in the form of its lactone) to the epimeric aldose. 

Configuration of (+)-glucose. The Fischer proofs

• Let us turn back to the year 1888. Only a few monosaccharides were known, among them (-f)-glucose, (-)-fructosc, (+)-arabinose. (-t-)-Mannose had just been synthesized. It was known that (-f)-glucose was an aldohexose and that (-f)-arabinose was an aldopentose. Emil Fischer had discovered (1884) that phenylhydrazones could convert carbohydrates into osazones. The Kiliany cyanohydrin method for lengthening the chain was just two years old.

• It was known that aldoses could be reduced to alditols, and could be oxidized to the monocarboxylic aldonic acids and to the dicarboxylic aldaric acids. A theory of stereoisomerism and optical activity had been proposed (1874) by van't Hoff and Le Bel. Methods for separating stereoisomers were known and optical activity could be measured. The concepts of racemic modifications, meso compounds, and epimers were well established

• (-f)-Glucose was known to be an aldohexose; but as an aldohexose it could have any one of 16 possible configurations. The question was: which configuration did it have? In 1888, Emil Fischer (at the University of Wurzburg) set out to find the answer to that question, and in 1891 announced the completion of a most remarkable piece of chemical research, for which he received the Nobel Prize in 1902. Let us follow Fischer's steps to the configuration of (+)-glucose. Although somewhat modified, the following arguments are essentially those of Fischer.

• The 16 possible configurations consist of eight pairs of enantiomers. Since methods of determining absolute configuration were not then available, Fischer realized that he could at best limit the configuration of (H-)-glucose to a pair of enantiomeric configurations; he would not be able to tell which one of the pair was the correct absolute configuration.

To simplify the problem, Fischer therefore rejected eight of the possible configurations, arbitrarily retaining only those (I-VI11) in which C-5 carried the OH on the right (with the understanding that H and OH project toward the observer). He realized that any argument that led to the selection of one of these formulas applied with equal force to the mirror image of that formula

• Since his proof depended in part on the relationship between (-t-)-glucose and the aldopentose (-)-arabinose, Fischer also had to consider the configurations of the five-carbon aldoses. Of the eight possible configurations, he. retained only four, IX-XII, again those in which the bottom chiral center carried the OH on the right

Configurations of aldose

• Today all possible aldoses (and ketoses) of six carbons or less, and many of more than six carbons, are known; most of these do not occur naturally and have been synthesized. The configurations of all these have been determined by application of the same principles that Fischer used to establish the configuration of (-f)-glucose; indeed, twelve of the sixteen aldohexoses were worked out by Fischer and his students.

• So far in our discussion, we have seen how configurations III, IV, V, and X of the previous section were assigned to (-f )-glucose, (4-) -mannose, (- )-gulose and (-)-arabinose, respectively. Let us see how configurations have been assigned to some other monosaccharides.

• The aldopentose (-)-ribose forms the same oxazine as ( -)-arabinose. Since (-)-arabinose was shown to have configuration X, (-)-ribose must have configuration IX. This configuration is confirmed by the reduction of (-)-ribose to the optically inactive (meso) pentahydroxy compound Rabito.

Optical families. D and L

• Most applications of stereochemistry, as we have already seen, are based upon the relative configurations of different compounds, not upon their absolute configurations. We are chiefly interested in whether the configurations of a reactant and its product are the same or different, not in what either configuration actually is. In the days before any absolute configurations had been determined there was the problem not only of determining the relative configurations ot various optically active compounds, but also of indicating these relationships once they had been established. This was a particularly pressing problem with the carbohydrates.

• The compound glyceraldehyde, CH2OHCHOHCHO, was selected as a standard of reference, because it is the simplest carbohydrate an Aldo triose capable of optical isomerism. Its configuration could be related to those of the carbohydrates, and because of its highly reactive functional groups, it could be converted into, and thus related to, many other kinds of organic compounds. ( + )-Glyceraldehyde's was arbitrarily assigned configuration I and was designated D-glyceraldehyde; (-)-glyceraldehyde was assigned configuration II and was designated L-glyceraldehyde. Configurations were assigned to the glyceraldehyde purely for convenience; the particular assignment had a 50:50 chance of being correct, and, as it has turned out, the configuration chosen actually is the correct. absolute configuration.

• To indicate the relationship thus established, compounds related to D-glyceraldehyde are given the designation D, and compounds related to L-glyceraldehyde are given the designation L. The symbols D and L (pronounced "dee" and "ell") thus refer to configuration, not to sign of rotation, so that we have, for example, D-(-)~glyceric acid and L-(-f )-lactic acid. (One frequently encounters the prefixes */and /, pronounced "dextro" and "Levo," but their meaning is not always clear. Today they usually refer to direction of rotation; in some of the older literature they refer to optical family. It was because of this confusion that D and L were introduced.)

• Unfortunately, the use of the designations D and L is not unambiguous. In relating glyceraldehyde to lactic acid, for example, we might envision carrying out a sequence of steps in which the CH2OH rather than the CHO group is converted into the COOH group:

Tartaric acid

• Tartaric acid, HOOCCHOHCHOHCOOH, has played a key role in the development of stereochemistry, and particularly the stereochemistry of the carbohydrates. In 1848 Louis Pasteur, using a hand lens and a pair of tweezers, laboriously separated a quantity of the sodium ammonium salt of racemic tartaric acid into two piles of mirror-image crystals and, in thus carrying out the first resolution of a racemic modification, was led to the discovery of enantiomerisrn. Almost exactly 100 years later, in 1949, Bijvoet, using x-ray diffraction--and also laboriously determined the actual arrangement in space of the atoms oY the sodium rubidium salt of (+ )-tartaric acid, and thus made the first determination of the absolute configuration of an optically active substance.

• As we shall see in the next section, tartaric acid is the stereochemical link between the carbohydrates and our standard of reference, glyceraldehyde. In 1917, the configurational relationship between glyceraldehyde and tartaric acid was worked out. When the reaction sequence outlined in Fig. 34.6 was carried out starting with D-glyceraldehyde, two products were obtained, one inactive anone which rotated the plane of polarized light to the left. The inactive product was, of course, mesotartaric acid, III. The active (-)-tartaric acid thus obtained was assigned configuration IV; since it is related to D-glyceraldehyde, we designate it D-(-)-tartaric acid.

• The designation of even the tartaric acids is subject to ambiguity. In this book, we have treated the tartaric acids as one does carbohydrates: by considering CHO of glyceraldehyde as the position from which the chain is lengthened, via the cyanohydrin reaction. Some chemists, on the other hand, view the tartaric acids as one does the amino acids (Sec. 36.5) and, considering COOH to be derived from CHO of glyceraldehyde, designate (-)-tartaric acid as i, and ( + )-tartaric acid as D. Regardless of which convention one follows, this fact remains: (-)- and (+)- tartaric acid and (+)- and (-)-glyccraldehyde have the absolute configurations shown on p. 108$ and 

Families of aldoses. Absolute configuration

The evidence on which Fischer assigned a configuration to (-f-)-glucose leads to either of the enantiomeric structures I and II. Fischer, we have seen, arbitrarily selected I, in which the lowest chiral center carries OH on the right.

• In 1906 the American chemist Rosanoff (then an instructor at New York University) proposed glyceraldehyde as the standard to which the configurations of carbohydrates she Mid be related. Eleven years later experiment showed that it is the dextrorotatory (+)-glyceraldehyde that is related to (+ )-glucose. On that basis, (+)-glyceraldehyde was then given the designation n and was assigned a configuration to conform with the one arbitrarily assigned to (-f)-glucose by Fischer. Although rejected by Fischer, the Rosanoff convention became universally accepted.

• Regardless of the direction in which they rotate polarized light, all monosaccharides are designated as D or L on the basis of the configuration about the lowest chiral center, the carbonyl group being at the top: D if the OH is on the right, L if the OH is on the left. (As always, it is understood that H and OH project toward us from the plane of the paper.) (+)-Mannose and (-)-arabinose, for example, are both assigned to the o-family on the basis of their relationship to D-(+)-glucose, and, through it, to D-(+)-glyceraldehyde.

• Until 1949, these configurations were accepted on a purely empirical basis; they were a convenient way to show configurational relationships among the various carbohydrates, and between them and other organic compounds. But so far as anyone knew, the configurations of these compounds might actually have been the mirror images of those assigned ; the lowest chiral center in the D-series of monosaccharides might have carried OH on the left. As we have seen, however, when Bijvoet determined the absolute configuration of (-f-)-tartaric acid by x-ray analysis in 1949, he found that it actually has the configuration that had been up to then merely assumed. The arbitrary choice that Emil Fischer made in 1891 was the correct one; the configuration he assigned to (+)-glucose and, through it. to every carbohydrate is the correct absolute configuration.

Cyclic structure of D-(+)-glueose. Formation of ghicosides

• We have seen evidence indicating t^at D-(-f )-glucose is a pentahydroxy aldehyde. We have seen how its configuration has been established. It might seem, therefore, that D+)-glucose had been definitely proved to have structure I.

• But during the time that much of the work we have just described was going on, certain facts were accumulating that were inconsistent with this structure of D-(-f-)-glucose. By 1895 it had become clear that the picture of n-(-f)-glucose as a pentahydroxy aldehyde had to be modified.

• Furthermore, not just one but two of these monomethyl derivatives of D-(-f )- glucose are known, one with m.p. 165 and specific rotation 4- 158, and the other with m.p. 107 and specific rotation 33. The isomer of higher positive rotation is called methyl a-D-glucoside, and the other is called methyl-p-D-glucoside. These glucosides do not undergo mutarotation, and do not reduce Tollens' or Fehling's reagent.

• To fit facts like these, ideas about the structure of D-(H-)-glucose had to be changed. In 1895, as a result of work by many chemists, including Tollens, Fischer, and Tannert, there emerged a picture of D-( -I- )-glucose as a cyclic structure. In 1926 the ring size was corrected, and in recent years the preferred conformation has been elucidated.

• To fit facts like these, ideas about the structure of D-(H-)-glucose had to be changed. In 1895, as a result of work by many chemists, including Tollens, Fischer, and Tanret, there emerged a picture of D-(-I-)-glucose as a cyclic structure. In 1926 the ring size was corrected, and in recent years the preferred conformation has been elucidated.

• D-(-H)-Glucose is the hemiacetal corresponding to reaction between the aldehyde group and the C-5 hydroxyl group of the open-chain structure (I). It has a cyclic structure simply because aldehyde and alcohol are part of the same molecule. There are two isomeric forms of D+)-glucose because this cyclic structure has one more chiral center than Fischer's original open-chain structure (I). -D-( +)-Glucose and 0-D-(-f)-glucose are diastereomers, differing in configuration about C-l. Such a pair of diastereomers are called anomers. As hemiacetals, - and j9-D-(-f)-glucose are readily hydrolyzed by water. In aqueous solution anomer is converted via the open-chain form into an equilibrium mixture containing both cyclic isomers. Thus, mutarotation results from the ready opening and closing of the hemiacetal ring

• The typical aldehyde reactions of D-(-f-)-glucose osazone formation, and perhaps reduction of Tollens' and Fehling's reagents are presumably due to a small amount of open-chain compound, which is replenished as fast as it is consumed. The concentration of this open-chain structure is, however, too low (less than 0.5%) for certain easily reversible aldehyde reactions like bisulfite addition and the Schiff test.

• Although formed from only one mole of methanol, they are nevertheless full acetals, the other mole of alcohol being D-( + )-glucose itself through the C-5 hydroxyl group. The glucosides do not undergo mutarotation since, being acetals, they are fairly stable in aqueous solution. On being heated with aqueous acids, they undergo hydrolysis to yield the original hemiacetals (II and III). Toward bases glycosides, like acetals generally, are stable. Since they are not readily hydrolyzed to the open-chain aldehyde by the alkali in Tollens' or Fehling's reagent, glucosides are non-reducing