Carbohydrates chain lengthening

Much of the chemistry of monosaccharides is the familiar chemistry of alcohols and aldehydes/ketones. Thus, the hydroxyl groups of carbohydrates form esters and ethers. The carbonyl group of a monosaccharide can be reduced with NaBH4 to form an alditol, oxidized with aqueous Br2 to form an aldonic acid, oxidized with HNO3 to form an aldaric acid, oxidized enzymatically to form a uronic acid, or treated with an alcohol in the presence of acid to form a glycoside. Monosaccharides can also be chain-lengthened by the multistep Kiliani-Fischer synthesis and can be chain-shortened by the Wohl degradation. 

Palladium on barium sulfate, Pd/BaS04 Acts as a hydrogenation catalyst for nitriles in the Kiliani-Fischer chain-lengthening reaction of carbohydrates (Section 25.6). 

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. 

Chain lengthening Kohlehydrate Carbohydrates Kohlendioxyd Carbon dioxide Kohlensaure Carbonic acid Kohlenwasserstoffe Hydrocarbons Kugelmiihlenreaktor Ball mill reactor Kunstharz-Base Resin base Kupfer Copper Kupplung Coupling Mangan Manganese... 

Chain lengthening Carbohydrates Carbon dioxide Carbonic acid Hydrocarbons Ball mill reactor Resin base Copper Coupling... 

Chain-lengthening reactions can be applied to carbohydrates. As the term implies, the carbohydrate chain is extended by one carbon. One important method uses a transformation called the Nef reaction, after John U. Nef (Switzerland/ United States 1862-1915). When d-threose (13) reacts with nitromethane in the presence of sodium methoxide as a base, the enolate anion of nitromethane (90) reacts with the aldehyde unit of 13 (an enolate-aldehyde condensation reaction see Chapter 22, Section 22.2, for related reactions) to give two products 91 and 92. 

Nature forms, stores, and uses chemical energy. Carbohydrates are the central chemical in this dance of life. Chemists manipulate saccharides many ways, as well. The Kdiani-Fischer synthesis lengthens a carbohydrate chain by one carbon atom. The Ruff degradation shortens the chain by one carbon. Both reactions occur without disturbing the remaining stereogenic carbons. 

Another method for lengthening the carbohydrate chain is the addition of a nitroalkane to the aldehyde group. The nitroalkane is treated with sodium methox-ide to form a carbanion that adds to the aldehyde group to give a 1-nitro-l-deoxyalditol. The alditol can be converted into an aldehyde by treatment with sodium hydroxide, followed by acid this is the Nef reaction [41] (reaction 4.42). 

In the field of the nitrogen-free sugars, syntheses with nitromethane are used for the lengthening of the carbohydrate chain and for the production of C-nitro-deoxyinositols, etc. Recently s mtheses with nitromethane have also been carried out in the field of the amino sugars. Sowden and Fischer had already succeeded in synthesizing 3,4,5,6-tetra-O-acetyl-... 

The nib o-aldol reaction using l,l-diethoxy-2-nib oethane is useful for lengthening of the carbon chain of carbohydrates. The reaction of Eq. 3.86 proceeds in a stereoselective way (ds 75%) to give the syn-niho alcohol in 58% isolated yield.The product is converted into 2-amino-2-deoxyaldoses by reaction with Ha/Raney Ni. 

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. 

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