Carbohydrates open-chain

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 m d erythrose To visualize furanose nng formation more clearly redraw the Fischer projection m a form more suited to cyclization being careful to maintain the stereochemistry at each chirality center... 

Structural drawings of carbohydrates of this type are called Haworth formulas, after the British chemist Sir Walter Norman Haworth (St Andrew s University and the University of Birmingham) Early m his career Haworth contributed to the discovery that carbohydrates exist as cyclic hemiacetals rather than m open chain forms Later he col laborated 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 prepa ration on a commercial scale Haworth was a corecipient of the Nobel Prize for chem istry m 1937... 

Because six membered rings are normally less strained than five membered ones pyranose forms are usually present m greater amounts than furanose forms at equilib rium and the concentration of the open chain form is quite small The distribution of carbohydrates among their various hemiacetal forms has been examined by using H and NMR spectroscopy In aqueous solution for example d ribose is found to contain the various a and p furanose and pyranose forms m the amounts shown m Figure 25 5 The concentration of the open chain form at equilibrium is too small to measure directly Nevertheless it occupies a central position m that mterconversions of a and p anomers and furanose and pyranose forms take place by way of the open chain form as an inter mediate As will be seen later certain chemical reactions also proceed by way of the open chain form... 

Up to this point all our attention has been directed toward aldoses carbohydrates hav ing an aldehyde function in their open chain form Aldoses are more common than ketoses and their role m biological processes has been more thoroughly studied Nev ertheless a large number of ketoses are known and several of them are pivotal inter mediates m carbohydrate biosynthesis and metabolism Examples of some ketoses include d nbulose l xylulose and d fructose... 

Although carbohydrates exist almost entirely as cyclic hemiacetals m aqueous solution they are m rapid equilibrium with their open chain forms and most of the reagents that react with simple aldehydes and ketones react m an analogous way with the carbonyl functional groups of carbohydrates... 

Carbohydrates are marvelous molecules In most of them every carbon bears a functional group and the nature of the functional groups changes as the molecule mterconverts between open chain and cyclic hemiacetal forms Any approach to understanding carbohydrates must begin with structure... 

Aldonic acid (Section 25 19) Carboxylic acid obtained by oxi dation of the aldehyde function of an aldose Aldose (Section 25 1) Carbohydrate that contains an aldehyde carbonyl group in its open chain form Alicyclic (Section 2 15) Term describing an a/iphatic cyclic structural unit... 

Ketose (Section 25 1) A carbohydrate that contains a ketone carbonyl group in its open chain form Kiliam-Fischer synthesis (Section 25 20) A synthetic method for carbohydrate chain extension The new carbon-carbon bond IS formed by converting an aldose to its cyanohydnn Reduction of the cyano group to an aldehyde function com pletes the synthesis... 

Aldose (Section 25.1) Carbohydrate that contains an aldehyde carbonyl group in its open-chain form. 

We ll see numerous instances in future chapters where the chemistry of a given functional group is strongly affected by being in a ring rather than an open chain. Because cyclic molecules are so commonly encountered in all classes of bioniolecules, including proteins, lipids, carbohydrates, and nucleic acids, it s important that the effects of their cyclic structures be understood. 

If the carbonyl and the hydroxyl group are in the same molecule, an intramolecular nucleophilic addition can take place, leading to the formation of a cyclic hemiacetal. Five- and six-membered cyclic hemiacetals are relatively strain-free and particularly stable, and many carbohydrates therefore exist in an equilibrium between open-chain and cyclic forms. Glucose, for instance, exists in aqueous solution primarily in the six-membered, pyranose form resulting from intramolecular nucleophilic addition of the -OH group at C5 to the Cl carbonyl group (Figure 25.4). The name pyranose is derived from pyran, the name of the unsaturated six-membered cyclic ether. 

The same authors also published the synthesis of two nucleosides 135 and 136 by application of this process <20030L2067>. In these cases, the reagent iminium salts were generated in situ by adding the appropriate sugar and ammonium chloride to the reaction mixture which allowed the intermediate ring closure of the cyclic carbohydrate to an open-chained iminium salt. 

From a structural point of view, the carbohydrate template can have either furan or pyran rings although in some cases open chain structures can be formed. A large variety of aldopentoses (e.g. d- and L-arabinose, D-ribose, D-xylose), aldohexoses (e.g. D-glucose, D-mannose, D-galactose) as well as ketohexoses (e.g. D-fructose, L-sorbose) can be used as scaffolds. 

Most carbohydrates exist as closed rings rather than open chains. 

The reaction of carbohydrate-derived imines with the Danishefsky diene, ( )-1 -methoxy-3-(trimethylsilyloxy)butadiene, to form heterocycles via the open-chain adducts (for assignment, see pp 456 and 478)130. 

Sharpless asymmetric epoxidation of open-chain allylic alcohols, an important discovery of the previous decade, was extensively used in carbohydrate synthesis [9-12], Presently, many aliphatic substrates can be elaborated into sugars [5,6 see also Chap. 9). 

Because six-membered rings are normally less strained than five-membered ones, pyranose forms are usually present in greater amounts than furanose forms at equilibrium, and the concentration of the open-chain form is quite small. The distribution of carbohydrates among their various hemiacetal forms has been examined by using H and... 

Another important group of heterocycles present in wine is the Carbohydrates, polysubstituted pyrans and furans, which can exist in both open-chain and cyclic structures. The most important of these sugars are glucose 23 and fructose 24, and they represent the basis for the fermentation process giving the three main products CO2, glycerol, and ethanol ... 

Aldoses carbohydrates with an aldehyde in the open-chain form. 

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