Hemiacetals sugars

O-Benzylated hemiacetal sugars as furanose and pyranose can he used as telogens in palladium-catalyzed butadiene telomerization reactions too (Scheme IS)." These substrates were much less reactive than their free or acetylated corresponding congeners in DMF. 

An alcoholic component needed to participate in the formation of a hemiacetal need not be a foreign molecule it may be a part of the same molecule as the carbonyl function. In such a case one speaks of an internal hemiacetal. Sugars can form internal hemiacetals, which are then present in a ring structure (Fig. 44). 

Oxidation with Benedict s reagent (Section 25 19) Sugars that con tain a free hemiacetal function are called reducing sugars They react with copper(ll) sulfate in a sodium citrate/sodium carbonate buffer (Benedict s reagent) to form a red precipitate of copper(l) oxide Used as a qualitative test for reducing 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. 

An alternative approach to the use of a-aminoketones involves acetals (72JOC221) and pyrazine-2,3-diones have been synthesized by this route (Scheme 58). The acetals are readily available from the phthalimido derivatives via the a-chloroketones. Hemiacetals have also served as a starting point for pyrazine synthesis, although in most cases hemiacetals are too labile to be easily prepared examples are common in the 2-amino-2-deoxy sugar series 2-amino-2-deoxy-D-glucose for example dimerizes to the pyrazine (101) when generated in situ from the hydrochloride salt (68JAP6813469). 

Maltose and cellobiose are both reducing sugars because the anomeric carbons on the right-hand glucopyranose units have hemiacetal groups and are in equilibrium with aldehyde forms. For a similar reason, both maltose and cellobiose exhibit mutaiotation of a and /3 anomers of the glucopyranose unit on the right. 

Anomers (Section 25.5) Cyclic stereoisomers of sugars that differ only in their configuration at the hemiacetal (anoineric) carbon. 

Variants on this theme can be seen in methods for the protection of amines (16), alcohols (17), including sugar hemiacetals (18), and pyrrole (19). 

Towards the end of the nineteenth century it was realized that the free sugars (not only the glycosides) existed as cyclic hemiacetals or hemiketals. Mutarotation, discovered in 1846 by Dubrunfaut, was now interpreted as being due to a change... 

Cyclic hemiacetals or hemiketals of sugars with a five-membered (tetrahydrofuran) ring are called furanoses, those with a six-membered (tetrahydropyran) ring pyran-oses. For sugars with other ring sizes see 2-Carb-5. 

Monosaccharides in which an alcoholic hydroxy group has been replaced by an amino group are called amino sugars (see 2-Carb-14). When the hemiacetal hydroxy group is replaced, the compounds are called glycosylamines. 

Glycosides are mixed acetals formally arising by elimination of water between the hemiacetal or hemiketal hydroxy group of a sugar and a hydroxy group of a second compound. The bond between the two components is called a glycosidic bond. 

The c.d. spectra of these 21 monosaccharides that were studied contain a wealth of information, although proper analysis of the data is not always obvious. However, c.d.-difference spectra between pairs of sugars that differ at only one carbon atom can be used to simplify the analysis. Each of the chromophores in a monosaccharide (hydroxyl, methoxyl, hydroxymethyl, hemiacetal, and acetal) are symmetric and obtain their c.d. by interaction... 

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