GENERAL ARTICLE Carbohydrate Chemistry from Fischer to Now N R Krishnaswamy The story of carbohydrate chemistry from its embryonic stage to the present day high profile research bridging organic chemistry and the life sciences is like a fascinating travelogue through space and time. In this brief article, this intriguing field of natural products chemistry is presented with appro- priate illustrations, with the hope that it will kindle further N R Krishnaswamy interest in the young readers to whom this is primarily ad- was initiated into the dressed. We begin our journey with Emil Fischer and quickly world of natural products traverse some areas of classical and modern organic chemis- by T R Seshadri at try. In the process we come across some familiar landmarks University of Delhi and has carried on the glorious as well as visit a few exotic places before ending on the borders traditions of his mentor. of biology. Beyond this is a region full of promise inviting He has taught at further exploration. Bangalore University, Calicut University and Introduction Sri Sathya Sai Institute of Higher Learning. Among organic compounds the most well known, even to lay- men, are the carbohydrates, produced by plants. Green leaves produce glucose using atmospheric carbon dioxide and water with the help of chlorophyll and sunlight. Several molecules of glucose are then condensed together to form cellulose, which serves as a structural material, and starch which acts as a source of food. Glucose, sucrose, cellulose and starch are household names even if the common man may not know that glucose is a constituent of Keywords the other three, two of which are polymers! Within this group, one Carbohydrates, mutarotation, comes across a wide range of molecular sizes (from monomers to Fischer–Kiliani synthesis, cyclo- oligomers to polymers), and shapes. The predominant functional dextrins, end-group analysis, group is the hydroxyl, several of which occur in a carbohydrate. oligosaccharides, glycosidation reaction, glycocode and glyco- Another key functional group is the carbonyl group, which plays therapy. a pivotal role in the chemical behavior of carbohydrates. 620 RESONANCE July 2011 GENERAL ARTICLE Emil Fischer and his students were responsible for elucidating the The synthesis of structures and stereochemistry of the monosaccharides. The syn- glucose achieved by thesis of glucose achieved by them in 1890 is considered as one of Fischer and his the important milestones in the development of organic chemis- students in 1890 is try. This was preceded by the discovery of phenyl hydrazine by considered as one of Fischer in 1875. He used this reagent to explore the chemistry of the important glucose and related compounds. In the course of these studies, milestones in the Fischer developed the mode of molecular representation now development of known as the Fischer projection formula. organic chemistry. This was preceded With the discovery of complex oligosaccharides and polysaccha- by the discovery of rides of natural origin, the focus shifted to the biological impor- phenyl hydrazine by tance of these compounds. Secrets of this aspect of the carbohy- Fischer in 1875. drates are being gradually revealed and active research to unravel the role of carbohydrates in living organisms is in progress. For example, it is now known that in eukaryotic organisms, oligosac- charides occurring as conjugates with proteins and lipids on cell surfaces have a key role in cellular communications. For the elucidation of the structures of such complex oligosaccha- rides, classical conventional chemical methods proved inad- equate. Progress in this area became possible only after instru- mental methods such as GC-MS and NMR spectroscopy became more powerful and effective as a consequence of advances in these techniques. For confirmation of the structures thus deduced it also became imperative to develop synthetic methods similar to those used for the synthesis of polypeptides. In the following paragraphs, these developments beginning with the pioneering It is now known that in eukaryotic studies of Fischer and others and culminating in present day research, are briefly described organisms, oligosaccharides Classification occurring as conjugates with Carbohydrates are primarily classified according to their molecu- proteins and lipids lar size. Monosaccharides are monomers. The most important on cell surfaces member of this group is glucose, which is an aldohexose as it has have a key role in six carbon atoms, five hydroxyl groups (one primary and the other cellular four secondary) and an aldehyde function at one end, as in the communications. RESONANCE July 2011 621 GENERAL ARTICLE Oligosaccharides are Fischer representation. Fructose, which is an isomer of glucose, made up of two or has a keto carbonyl function and is known as a ketohexose. more monosaccharide Monosaccharides having fewer carbon atoms are also known. For units; for example, example, arabinose and ribose are aldopentoses, that is, they are disaccharides, such C5 compounds with an aldehyde group and four hydroxyls. as sucrose, lactose Oligosaccharides are made up of two or more monosaccharide and maltose, are units; for example, disaccharides, such as sucrose, lactose and hydrolysable to yield maltose, are hydrolysable to yield two monosaccharide units. In two monosaccharide the case of sucrose, the monomers obtained are glucose and units. fructose. Raffinose, which can be isolated from molasses, is a trisaccharide. This compound on hydrolysis yields one molecule each of glucose, galactose, another aldohexose, and fructose. As already mentioned, cellulose and starch are polysaccharides, being polymeric compounds. Another example of a polysaccha- ride is glycogen, commonly known as animal starch. Carbohydrates which do not conform to the general formula Cn(H2O)m include deoxy sugars and amino sugars. Monosaccharides The optical activity exhibited by (+)-glucose was first observed by Biot in the year 1817. Two years earlier he had recorded that sucrose was optically active. However, the stereochemistry of glucoseand other monosaccharides remained obscureuntil Fischer began his pioneering studies. The molecular formula, formation of a pentaacetate and reduction of Tollen’s reagent established that glucose is a pentahydroxy aldehyde having six carbon atoms. The presence of the aldehyde group could be confirmed by Glucose cyanohydrin, oxidation with bromine water, the product being gluconic acid. on hydrolysis followed Glucose cyanohydrin, on hydrolysis followed by reduction with by reduction with hydriodic acid gave n-heptanoic acid showing that glucose is a hydriodic acid gave n- straight-chain aldohexose. On catalytic hydrogenation over a heptanoic acid nickel catalyst glucose yielded glucitol or sorbitol, which is showing that glucose 1,2,3,4,5,6-hexahydroxyhexane. However, structure (1) that is a straight-chain emerged from the above mentioned reactions. could not account aldohexose. for all the known properties of glucose. On the basis of structure 622 RESONANCE July 2011 GENERAL ARTICLE (1), which has only historical significance, for glucose, gluconic One property which acid can be formulated as (2) and glucitol as (3). could not be explained by structure (1) is the CHO COOH CH2OH mutarotation exhibited (CHOH)4 (CHOH) (CHOH) 4 4 by aqueous solutions CH2OH CH OH CH OH 2 2 of glucose. A thorough 1 2 3 investigation of this phenomenon, One property which could not be explained by structure (1) is the discovered by mutarotation exhibited by aqueous solutions of glucose. The Dubrunfaut, showed initial specific rotation of ordinary glucose in water is [] = D that all mono- +112°. However, it changes over a period of time and finally saccharides exhibit reaches the value of +52.3°. A thorough investigation of this this property phenomenon, discovered by Dubrunfaut, showed that all monosac- charides exhibit this property which could be attributed to the existence of two stereoisomers which are interconvertible. These were designated as - and - forms. To account for this phenom- enon, Tollens suggested a five-membered cyclic oxide structure (cyclic hemiacetal) (4) for glucose involving the aldehyde group at position 1 and the hydroxyl at position 4. However, when Tollens made this proposition in 1883, there was no experimental evidence available to support it. Only 12 years later, Tanret could provide this crucial evidence by isolating the two forms of (+)- glucose. Several years later, as a result of the studies of Haworth and others, the ring structure of (+)-glucose was corrected to a six-membered cyclic hemiacetal structure (5), which incorpo- rates the correct configurations at all the chiral centres, which had earlier been determined by Fischer and his coworkers. In this 1 Haworth projection1 formula, the hydroxyl group at position 1, Haworth Projection: The mode of two-dimensional repre- which is known as the anomeric carbon atom, is on the top in the sentation of the cyclic structures -form, whereas it is oriented downwards in the -form, as of sugar molecules is known as shown in 6 and 7 respectively. the Haworth projection, and was developed by Sir Walter Norman Haworth. The method devel- CH OH CH2OH CH OH HO 2 2 oped by him for the preparation O H OH H O O O OH OH OH of methyl ethers of sugars using H CHOH-CH2OH OH dimethyl sulphate was an im- HO H OH HO OH OH OH OH OH OH portant early step in structural 4 5 6 7 studies on carbohydrates. RESONANCE July 2011 623 GENERAL ARTICLE Fischer