Aldoses carbon chain, lengthening

In Summary Step-by-step one-carbon chain lengthening or shortening, in conjunction with the symmetry properties of the various aldaric acids, allows the stereochemical assignments of the aldoses. 

The Kiliani-Fischer synthesis lengthens the carbon chain of an aldose by one carbon at the aldehyde end and forms a new aldose with its corresponding epimers. When glucose and its epimer are produced from the corresponding pentose via the Kiliani-Fischer synthesis, and then both epimers are reacted with dilute nitric acid, both form optically active compounds. 

Killani-Fischer synthesis (Section 27.10B) A reaction that lengthens the carbon chain of an aldose by adding one carbon to the carbonyl end. 

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

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 Technische 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. Sec. 20.15) can be controlled to yield the corresponding aldose. In Fig. 34.2, the entire Kiliani-Ffscher synthesis is illustrated for the conversion of an aldopentose into two aldohexoses. 

In 1885, Heinrich Kiliani (Freiburg, Germany) discovered that an aldose can be converted to the epimeric aldonic adds having one additional carbon through the addition of hydrogen cyanide and subsequent hydrolysis of the epimeric cyanohydrins. Fischer later extended this method by showing that aldonolactones obtained from the aldonic acids can be reduced to aldoses. Today, this method for lengthening the carbon chain of an aldose is called the Kiliani-Fischer synthesis. 

Any of the chain-lengthening synthetic methods are, of course, applicable for the introduction of isotopic carbon. Of these, the cyanohydrin (260) and nitromethane (261) syntheses have proved useful for the preparation of 1-labeled aldoses. The addition of hydrogen cyanide to an aldose is essentially quantitative even with only stoichiometric amounts, whereas the... 

Methods for lengthening the carbon chains of sugars may involve the formation of aldonic acids as intermediates. The Kiliani cyanohydrin synthesis (see p. 106) creates two new aldonic acids with one more carbon atom than in the original aldose. The configuration of the new asymmetric atom can be assigned by use of the lactone rule discussed below. 

If the carbon chain is lengthened by the addition of another -CHOH unit between the asymmetric carbon atom and the aldehyde group this extra carbon atom will also be asymmetric and two new isomeric compounds will be formed. Thus the number of possible aldose sugars increases by a factor of 2 with each additional C atom so that the number of possible aldose... 

Just as the Kiliani-Fischer synthesis lengthens an aldose chain by one carbon, the Wohl degradation shortens an aldose chain by one carbon. The Wohl degradation is almost the exact opposite of the Kiliani-Fischer sequence. That is, the aldose aldehyde carbonyl group is first converted into a nitrile, and the resulting cyanohydrin loses HCN under basic conditions—the reverse of a nucleophilic addition reaction. 

Two common procedures in carbohydrate chemistry result in adding or removing one carbon atom from the skeleton of an aldose. The Wohl degradation shortens an aldose chain by one carbon, whereas the Kiliani-Fischer synthesis lengthens it by one. Both reactions involve cyanohydrins as intermediates. Recall from Section 21.9 that cyanohydrins are formed from aldehydes by addition of the elements of HCN. Cyanohydrins can also be re converted to carbonyl compounds by treatment with base. 

Just as the Kiliani—Fischer synthesis can be used to lengthen the chain of an aldose by one carbon atom, the Ruff degradation can be used to shorten the chain by a similar unit. The Ruff degradation involves (1) oxidation of the aldose to an aldonic acid using bromine water and (2) oxidative decarboxylation of the aldonic acid to the next lower aldose using hydrogen peroxide and ferric sulfate. d-(—)-Ribose, for example, can be degraded to D-(—)-erythrose ... 

The Kiliani-Fischer synthesis lengthens a carbohydrate chain by adding one carbon to the aldehyde end of an aldose, thus forming a new stereogenic center at C2 of the product. The product consists of epimers that differ only in their configuration about the one new stereogenic center. For example, the Kiliani-Fischer synthesis converts o-arabinose into a mixture of D-glucose and D-mannose. 

The Kiliani-Fischer chain extension is a method for lengthening an aldose chain by one carbon, giving two new aldoses that differ in stereochemistry at C2. D-Erythrose, for instance, yields a mixture of D-rihose and D-arahinose on Kiliani-Fischer chain extension. 

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