KILIANI FISCHER SYNTHESIS PDF

Reactions of Sugars: Glycosylation and Protection. The Kiliani-Fischer Synthesis is a method for extending a carbohydrate chain by a single carbon. The Ruff Degradation is a method for shortening a carbohydrate chain by a single carbon. The Kiliani-Fischer Synthesis involves addition of cyanide ion to an open-chain aldehyde in the case of aldoses which is then partially reduced and then hydrolyzed to give a new aldehyde. In the absence of chiral reagents, a mixture of diastereomers will be produced. The Ruff Degradation is a method for peforming the reverse reaction.

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Overview Emil Fischer made use of several key reactions in the course of his carbohydrate studies. These are described here, together with the information that each delivers. As noted above, sugars may be classified as reducing or non-reducing based on their reactivity with Tollens' , Benedict's or Fehling's reagents.

The Tollens' test is commonly used to detect aldehyde functions; and because of the facile interconversion of ketoses and aldoses under the basic conditions of this test, ketoses such as fructose also react and are classified as reducing sugars. When the aldehyde function of an aldose is oxidized to a carboxylic acid the product is called an aldonic acid.

If both ends of an aldose chain are oxidized to carboxylic acids the product is called an aldaric acid. By converting an aldose to its corresponding aldaric acid derivative, the ends of the chain become identical this could also be accomplished by reducing the aldehyde to CH 2 OH, as noted below.

Such an operation will disclose any latent symmetry in the remaining molecule. Thus, ribose, xylose, allose and galactose yield achiral aldaric acids which are, of course, not optically active. The ribose oxidation is shown in equation 2 below. Other aldose sugars may give identical chiral aldaric acid products, implying a unique configurational relationship.

The examples of arabinose and lyxose shown in equation 3 above illustrate this result. Thus, allitol and galactitol from reduction of allose and galactose are achiral, and altrose and talose are reduced to the same chiral alditol. A summary of these redox reactions, and derivative nomenclature is given in the following table.

HOBr Oxidation. HNO 3 Oxidation. NaBH 4 Reduction. The osazone reaction was developed and used by Emil Fischer to identify aldose sugars differing in configuration only at the alpha-carbon. The upper equation shows the general form of the osazone reaction, which effects an alpha-carbon oxidation with formation of a bis-phenylhydrazone, known as an osazone.

Application of the osazone reaction to D-glucose and D-mannose demonstrates that these compounds differ in configuration only at C These two procedures permit an aldose of a given size to be related to homologous smaller and larger aldoses.

The importance of these relationships may be seen in the array of aldose structures presented earlier, where the structural connections are given by the dashed blue lines.

Thus Ruff degradation of the pentose arabinose gives the tetrose erythrose. Working in the opposite direction, a Kiliani-Fischer synthesis applied to arabinose gives a mixture of glucose and mannose. An alternative chain shortening procedure known as the Wohl degradation is essentially the reverse of the Kiliani-Fischer synthesis. To determine which of these epimers was glucose, Fischer made use of the inherent C2 symmetry in the four-carbon dissymmetric core of one epimer B.

This is shown in the following diagram by a red dot where the symmetry axis passes through the projection formula. Fischer looked for and discovered a second aldohexose that represented the end group exchange for the epimer lacking the latent C2 symmetry A.

The remaining epimer is therefore mannose. Steven Farmer Sonoma State University. Oxidation As noted above, sugars may be classified as reducing or non-reducing based on their reactivity with Tollens' , Benedict's or Fehling's reagents.

Osazone Formation 1. Chain Shortening and Lengthening 1. Fischer's train of logic in assigning the configuration of D-glucose Ribose and arabinose two well known pentoses both gave erythrose on Ruff degradation. As expected, Kiliani-Fischer synthesis applied to erythrose gave a mixture of ribose and arabinose.

Oxidation of erythrose gave an achiral optically inactive aldaric acid. This defines the configuration of erythrose. Oxidation of ribose gave an achiral optically inactive aldaric acid.

This defines the configuration of both ribose and arabinose. Ruff shortening of glucose gave arabinose, and Kiliani-Fischer synthesis applied to arabinose gave a mixture of glucose and mannose. Glucose and mannose are therefore epimers at C-2, a fact confirmed by the common product from their osazone reactions.

A pair of structures for these epimers can be written, but which is glucose and which is mannose? Contributors Prof.

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Kiliani–Fischer synthesis

Overview Emil Fischer made use of several key reactions in the course of his carbohydrate studies. These are described here, together with the information that each delivers. As noted above, sugars may be classified as reducing or non-reducing based on their reactivity with Tollens' , Benedict's or Fehling's reagents. The Tollens' test is commonly used to detect aldehyde functions; and because of the facile interconversion of ketoses and aldoses under the basic conditions of this test, ketoses such as fructose also react and are classified as reducing sugars. When the aldehyde function of an aldose is oxidized to a carboxylic acid the product is called an aldonic acid. If both ends of an aldose chain are oxidized to carboxylic acids the product is called an aldaric acid. By converting an aldose to its corresponding aldaric acid derivative, the ends of the chain become identical this could also be accomplished by reducing the aldehyde to CH 2 OH, as noted below.

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The Ruff Degradation and Kiliani-Fischer Synthesis

It proceeds via synthesis and hydrolysis of a cyanohydrin , thus elongating the carbon chain of an aldose by one carbon atom while preserving stereochemistry on all the previously present chiral carbons. The new chiral carbon is produced with both stereochemistries, so the product of a Kiliani—Fischer synthesis is a mixture of two diastereomeric sugars, called epimers. For example, D - arabinose is converted to a mixture of D - glucose and D - mannose. The original version of the Kiliani—Fischer synthesis proceeds through cyanohydrin and aldonic acid lactone intermediates. The first step is to react the starting sugar with aqueous cyanide typically NaCN ; the cyanide undergoes nucleophilic addition to the carbonyl group of the sugar while sugars tend to exist mainly as cyclic hemiacetal , they are always in chemical equilibrium with their open-chain aldehyde or ketone forms, and in the case of these aldoses it is that aldehyde form that reacts in this synthesis.

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22.8: Lengthening the Chain- The Kiliani-Fischer Synthesis

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