Carbohydrates carbonyl group, addition

Aldoses incorporate two functional groups C=0 and OH which are capable of react mg with each other We saw m Section 17 8 that nucleophilic addition of an alcohol function to a carbonyl group gives a hemiacetal When the hydroxyl and carbonyl groups are part of the same molecule a cyclic hemiacetal results as illustrated m Figure 25 3 Cyclic hemiacetal formation is most common when the ring that results is five or SIX membered Five membered cyclic hemiacetals of carbohydrates are called furanose forms SIX membered ones are called pyranose forms The nng carbon that is derived... 

Alditol (Section 25 18) The polyol obtained on reduction of the carbonyl group of a carbohydrate Aldol addition (Section 18 9) Nucleophilic addition of an aldehyde or ketone enolate to the carbonyl group of an aide hyde or a ketone The most typical case involves two mole cules of an aldehyde and is usually catalyzed by bases... 

Cellobiose was prepared first by Skraup and Konig by the saponification of the octaacetate with alcoholic potassium hydroxide, and the method was improved by Pringsheim and Merkatz.3 Aqueous barium hydroxide also has been employed for the purpose, and methyl alcoholic ammonia has been used extensively for the hydrolysis of carbohydrate acetates. The method of catalytic hydrolysis with a small quantity of sodium methylate was introduced by Zemplen,i who considered the action to be due to the addition of the reagent to the ester-carbonyl groups of the sugar acetate and the decomposition of the addition compound by reaction with alcohol. The present procedure, reported by Zemplen, Gerecs, and Hadacsy, is a considerable improvement over the original method (see Note 2). 

Biochemistry is carbonyl chemistiy. Almost all metabolic pathways used by living organisms involve one or more of the four fundamental carbonvl-group reactions we ve seen in Chapters 19 through 23. The digestion and metabolic breakdown of all the major classes of food molecules—fats, carbohydrates, and proteins—take place by nucleophilic addition reactions, nucleophilic acyl substitutions, a substitutions, and carbonyl condensations. Similarly, hormones and other crucial biological molecules are built up from smaller precursors by these same carbonyl-group reactions. 

If the carbonyl and the hydroxyl group are in the same molecule, an intramolecular nucleophilic addition can take place, leading to the formation of a cyclic hemiacetal. Five- and six-membered cyclic hemiacetals are relatively strain-free and particularly stable, and many carbohydrates therefore exist in an equilibrium between open-chain and cyclic forms. Glucose, for instance, exists in aqueous solution primarily in the six-membered, pyranose form resulting from intramolecular nucleophilic addition of the -OH group at C5 to the Cl carbonyl group (Figure 25.4). The name pyranose is derived from pyran, the name of the unsaturated six-membered cyclic ether. 

This section presents selected examples of the use of zeolites and related porous materials for transformations of carbohydrates, that fall beyond the scope of the previous paragraphs. They include the use of zeolites in click chemistry and in a variety of reactions, including the synthesis of anhydro sugars, cyclization, elimination, and addition reactions to the carbonyl group. 

In a study of the reactivity of organolithium reagents with selected carbohydrate enones, such as 2.3.6-trideoxy- -L-hex-2-cnopyranosid-4-ulose (152) and 2,3-dideoxy-a-D-hex-2-enopyranosid-4-ulose, addition to the carbonyl group has been found to occur with increasing stereoselectivity in the order butyl, benzyl, and 2,5-dimethoxy-4-methylphenyllithium. By contrast, 2,5-dimethoxybenzyllithium underwent preferential and completely stereoselective 1,4-addition (the 1,4- to 1,2-addition ratio was 1.7 1 in this instance).109... 

Carbonyl groups are not always present in their usual double-bond form, but also exist as the tautomeric enols or as hydrates upon addition of water, which was demonstrated by means of differently oxidized carbohydrate model compounds by NMR [ 102] (Scheme 6). A recent study revealed a much higher percentage of hydrated aldehyde groups compared to the pure acyclic form for all aldohexoses in water [44]. 

In the classic carbohydrate chemistry addition reactions of carbonyl groups served as valuable tools for sturctural studies of carbohydrates. For example, hydroxyl amine, hydrazine, and phenyl hydrazine react with carbonyl groups to yield oximes and hydrazones. In the presence of an excess... 

The addition of a dialkyl phosphite to the carbonyl group of a carbohydrate provides further insight into the stereochemistry of the reaction. When l,6-anhydro-3,4-0-isopropyli-dene-/f-D-/> A ohexopyranos-2-ulose or l,6-anhydro-2,3-0-isopropylidene-j8-D-/yAo-hexopyra-nos-4-ulose are treated with dialkyl phosphite under Abramov conditions, the predominant products are the ta/o-2-C-dialkoxyphosphinyl derivatives 5 or m/o-4-C-dialkoxyphosphinyl derivatives 6, respectively30. 

Many natural compounds include heterocyclic systems constructed by hemiacetal formation between a hydroxyl group and a keto group of the chain in a 1,4- or 1,5-relative disposition. It is advantageous in a retrosynthetic analysis to take this point into account and to devise a disconnection next to the carbonyl group. Moreover, the synthetic connection does not involve chiral center formation. An obvious translation of this principle in carbohydrate chemistry is the formation of a carbon-carbon bond at the anomeric center by nucleophilic addition to lactones. However, other methods have also been devised to reach this goal. 

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