Carbonyl groups of sugars

The stmcture of monosaccharides is often written in the acycHc form although only very minor amounts of it ever occur in that form. Because the interconversions are rapid, the carbonyl groups of sugars can and do react both as if they are free and as if they are in a hemiacetal ring form. 

However, it is important to keep in mind that the carbonyl groups of sugars usually are combined with one of the hydroxyl groups in the same molecule to form a cyclic hemiacetal or hemiketal. These structures once were written as follows, and considerable stretch of the imagination is needed to recognize that they actually represent oxacycloalkane ring systems ... 

The reaction of phenylhydrazine, and particularly substituted phenylhydrazines, with the carbonyl group of sugars is extensively utilized in their identification. The derivatives are called osazones if two molecules of phenylhydrazine add to one molecule of sugar, as in the case of glucose, and phenylhydrazones when one molecule... 

Hara et al. [117] demonstrated an excellent separation of the enantiomers of nine aldoses by GC as their diastereomeric thiazolidine derivatives. The sugars were derivatized with L-cysteine methyl ester (49) and subsequently silylated. This simple derivatization, including the acylation, was carried out in pyridine at 60 C. The rapid reaction of thiols with the carbonyl group of sugars was also exploited by Little [118], who used (-k)-l-phenylethanethiol (50) to form acyclic dithioacetals. 

Inversion a to the masked carbonyl group of sugar thiazoles by oxidation -reduction, as exemplified in Scheme 11, promises to be a very useful extension of... 

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). 

Alditol (Section 25.6) The polyaicohol resulting from reduction of the carbonyl group of a sugar. 

Hydrogenation of the carbonyl group of a mono- or disaccharide gives the corresponding sugar alcohol. On paper, this reaction is simple, and it can schematically be written as O... 

The addition of organometallic reagents to the carbonyl group of conveniently substituted aldonolactones constitutes a viable chain-extension method. The reaction leads to the formation of hemiacetals of glyculoses, 1-methylene sugars, and C-glycosyl compounds, which are precursors of, or occur as subunits of, a variety of natural products. 

The addition of an amine to the carbonyl group of an aldehyde yields—after removal of water—an aldimine (not shown see p. 178). Aldimines are intermediates in amino acid metabolism (see p. 178) and serve to bond aldehydes to amino groups in proteins (see p. 62, for example). The addition of an alcohol to the carbonyl group of an aldehyde yields a hemiacetal (R-O-C(H)OH-R). The cyclic forms of sugars are well-known examples of hemi-... 

The synthesis of type III branched-chain sugars is based mainly on the use of ketosugars treated under Wittig-type conditions (see path a, Scheme 4) [13]. Several other methods, such as aldolization-crotonization or direct alkylidenation at the a-position of the carbonyl group of a keto sugar have been developed (path b). 

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