Aldose chain-lengthening

Discovery of the chain-lengthening sequence was initiated by the observation of Heinrich Kiliani in 1886 that aldoses react with HCN to form cyanohydrins (Section 19.6). Emil Fischer immediately realized the importance of Kiliani s discovery and devised a method for converting the cyanohydrin nitrile group into an aldehyde. 

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. 

Cyanide ions react with aldehydes and ketones to yield cyanohydrins (Kiliani) (Fig. 2-28). Hydrolysis of the cyanohydrins gives aldonic acids, which can be reduced to aldoses. Kiliani reaction thus opens the possibility for chain lengthening of aldoses. Because of the formation, of a hydroxyl group in place of the aldehyde group a new asymmetric center is generated. It is to be observed, however, that the reaction is subject to so-called asymmetric induction, which means that the diastereoisomers are formed in unequal proportions. 

Kiliani-Fischer synthesis (Section 25.6) a multistep sequence for chain-lengthening an aldose into the next higher homolog. 

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. 

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... 

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. 

Larger sugars can be made from smaller ones and vice versa, by chain lengthening and chain shortening. These transformations can also be used to structurally correlate various sugars, a procedure applied by Fischer to prove the relative configuration of all the stereocenters in the aldoses shown in Figure 24-1. 

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. 

The Kiliani-Fischer synthesis lengthens a carbohydrate chain hy adding one carhon 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 D-arabinose into a mixture of D-glucose and D-mannose. 

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. 

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