Carbohydrates, Reactions Table

Although tris(dialkylamino)sulfonium difluorotrimethylsilicates are not sufficiently reactive to fluorinate alcohols, they are able to convert trifluoromethanesulfonates to their corresponding fluorides under mild conditions. The mild conditions required to form the ester and to perform the fluorination step, has resulted in TASF (1) and related reagents finding wide application in the syntheses of fluorinated carbohydrates (see Table 10).5"76-78 It is of interest that this process is a modern development of the first reaction ever used to synthesize an organic fluoride in 1835 (see Introduction 1.2.). 

Also unsuccessful have been all attempts to decarboxylate fluoroformates of various carbohydrates, but it has been shown that an excess of reagent, solvent, temperature and time of reaction are all of great importance in obtaining high yields for the fluorination of carbohydrate chloroformates (Table l).5... 

If a similar situation were always to exist with all crystalline carbohydrates, i.e. major amounts of hydrogen, a large number of minor products and a considerable unresolved dimer fraction, the attention that this field has found would perhaps have been undeserved. Interestingly, however, radiation-induced chain reactions were observed in the case of several crystalline carbohydrates (see Tables 4 and 5 note also the dominance of the chain product (6-deoxy-D-t/rreo-2,5-hexodiulose) compared to the fragmentation products, in Table 4). The chain reaction which often leads to an isomer of the substrate molecule or its dehydration product is governed by the crystal structure, as it is no longer observed in crystals of the ssune compound, but with a different structure. 

The action of transketolase generates vicinal diols having the same stereochemistry as the products of RAMA-catalyzed condensation. The enzyme, however, has two signiHcant advantages over RAMA the reaction docs not require DHAP, and the products arc not phosphorylated. The ketose functionality can be replaced by hydroxy pyruvate, which provides a hydroxyketo equivalent after decarboxylation. No other hydroxy acid has yet been found that is accepted by transketolase. Although the enzyme is absolute in its requirement for the R configuration of the hydroxy functionality at C2 of the aldehyde, there seem to be no other stereochemical requirements. Transketolase accepts a range of aldoses as substrates, and should be a useful enzyme for carbohydrate synthesis (Table 1) (37). 

Superarmed Glycosyl Donors in Glycosylation Reactions Table 5.6 1,2-Migration in carbohydrates... 

A few examples of the applications of TLC to the monitoring of carbohydrate reactions are given in Table 215. Microchromatoplates prepared by coating microscope slides [105] are convenient for this purpose since the analysis is complete in approximately 10 min. 

A great number of other aHyl compounds have been prepared, especially aHyl ethers and aHyl ether derivatives of carbohydrates and other polymers. They are made by the reaction of hydroxyl groups with aHyl chloride in the presence of alkaU (1). Polymerizations and copolymerizations are generally slow and incomplete. Products have only limited use in coatings, inks, and specialties. Properties of a few aHyl ethers are given in Table 10. 

Sections Carbohydrates undergo chemical reactions characteristic of aldehydes and 25.17-25.24 ketones, alcohols, diols, and other classes of compounds, depending on their structure. A review of the reactions described in this chapter is presented in Table 25.2. Although some of the reactions have synthetic value, many of them are used in analysis and structure deter-mination. 

Commercial A -acetylneuraminic acid aldolase from Clostridium perfringens (NeuAcA EC 4.1.3.3) catalyzes the addition of pyruvate to A-acetyl-D-mannosamine. A number of sialic acid related carbohydrates are obtained with the natural substrate22"24 or via replacement by aldose derivatives containing modifications at positions C-2, -4, or -6 (Table 4)22,23,25 26. Generally, a high level of asymmetric induction is retained, with the exception of D-arabinose (epimeric at C-3) where stereorandom product formation occurs 25 2t The unfavorable equilibrium constant requires that the reaction must be driven forward by using an excess of one of the components in order to achieve satisfactory conversion (preferably 7-10 equivalents of pyruvate, for economic reasons). 

The oldest way to produce caramel is by heating sucrose in an open pan, a process named caramelization. Food applications require improvement in caramel properties such as tinctorial power, stability, and compatibility with food. Caramels are produced in industry by controlled heating of a rich carbohydrate source in the presence of certain reactants. Carbohydrate sources must be rich in glucose because caramelization occurs only through the monosaccharide. Several carbohydrate sources can be used glucose, sucrose, com, wheat, and tapioca hydrolysates. The carbohydrate is added to a reaction vessel at 50°C and then heated to temperatures higher than 100°C. Different reactants such as acids, alkalis, salts, ammonium salts, and sulfites can be added, depending on the type of caramel to be obtained (Table 5.2.2). 

Selected clay stabilizers are shown in Table 1-10. Thermal-treated carbohydrates are suitable as shale stabilizers [1609-1611]. They may be formed by heating an alkaline solution of the carbohydrate, and the browning reaction product may be reacted with a cationic base. The inversion of nonreducing sugars may be first effected on selected carbohydrates, with the inversion catalyzing the browning reaction. 

The most important photochemical reaction of carbon to carbon unsaturated carbohydrates is addition to the unsaturated system. Two types of addition reaction are readily recognized. The first consists of those in which the molecule adding to the carbohydrate does so by involving a 77-bond of its own. Processes of this type, listed in Table I, are those which lead to formation of a new ring-system (cycloaddition). The second class of addition reaction is one in which a cr-bond is broken in the molecule adding to the unsaturated carbohydrate. The reactions that belong to the latter category (see Tables II and III) follow two basic patterns, and comprise the majority of the addition processes reported. 

Cycloaddition reactions (see Table I) involving unsaturated carbohydrates are regio- and stereo-selective. These selectivities can be understood by assuming that the photochemical interaction between the two 7r-systems results in formation of the more stable 1,4-diradical. The reaction between 3,4,6-tri-O-acetyl-D-glucal (1) and acetone pro-... 

Cycloaddition reactions between alkenes and noncarbohydrate, carbonyl compounds have been described in discussing the reactions of alkenes (see Table I and Scheme 1). The depiction of the excited carbonyl given in Scheme 6 is useful in understanding the regiochem-istry of the cycloaddition process, as it suggests that the electron-deficient oxygen atom in the excited carbonyl will react with the alkene to produce the (more-stable) 1,4-diradical. Table VIII lists cycloaddition reactions in which the excited carbonyl is part of a carbohydrate. 

Table XXI (Continued) Photochemical Reactions of Carbohydrate Azides...

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