Cyclic structures of sugars

Reagents that specifically oxidize vicinal glycols [e.g., NaI04, Pb-(02CCH3)4, and NaBi03 Section 16-9A] are quite helpful in determining the cyclic structures of sugars. With periodate, the numbers of moles of oxidant consumed and the moles of methanoic acid and methanal produced are... 

The most abundant five-carbon sugars are L-arabinose, D-ribose, 2-deoxy-D-ribose,1 and D-xylose, which all are aldopentoses. Both the open-chain and cyclic structures of the D-aldoses up to Cfi are shown in Figure 20-1. 

The building blocks for polysaccharides are monosaccharides (sugars) such as glucose or fructose (QH O ). Two cyclic structures of D-glucose are reproduced below (a third, open-chain structure is not shown). 

If you like, you can derive the cyclic structure of an L-sugar by doing the D-sugar first and then writing its mirror image. 

Fructose is a ketohexose sugar. The cyclic structure formed is called a hemiketal, and the five membered ring that is formed is called furanose ring. The straight chain structure and the cyclic structures of fructose are shown in Figure 29-5. 

Pyranose and Furanose Names Cyclic structures of monosaccharides are named according to their five- or six-membered rings. A six-membered cyclic hemiacetal is called a pyranose, derived from the name of the six-membered cyclic ether pyran. A five-membered cyclic hemiacetal is called a furanose, derived from the name of the five-membered cyclic ether ran. The ring is still numbered as it is in the sugar, not beginning with the heteroatom as it would be in the heterocyclic nomenclature. These structural names are incorporated into the systematic names of sugars. 

The carbohydrate dienophiles in the above examples are all derived from cyclic structures. Acyclic sugar dienophiles have also been studied. These substrates arc accessible through the reaction of a carbohydrate with a stabilized Witdg reagent or by fragmentation reactions. Two examples of the use of acyclic carbohydrate dienophiles in Diels-Alder reactions are presented below. 

Nucleosides are compounds formed when a base is linked to a sugar via a gly-cosidic bond (Figure 11.10). Glycosidic bonds by definition involve the carbonyl carbon atom of the sugar, which in cyclic structures is joined to the ring... 

The following cyclic structure is that of allose. Is this a furanose or pyranose form Is it an a or /3 anomer is it a D or L sugar ... 

Cyclic adenosine monophosphate (cyclic AMP), a modulator of hormone action, is related to AMP (Problem 29.24) but has its phosphate group linked to two hydroxyl groups at C3 and C5 of the sugar. Draw the structure of cyclic AMP. 

There is another aspect of the structure of glucose and fructose. They, like other simple sugars, can exist as a straight chain but this form is in equilibrium with a cyclic structure. In solutions the latter form prevails. Reaction (2) shows both forms of glucose. 

Owing to flexibility in the substrate, the TycATE was also used to synthesize a variety of novel cyclic structures. Inclusion of a propargylated amino acid into the linear substrate allowed the synthesis of over 247 macrocyclic glycopeptides, where azido-sugars were coupled onto the cyclized alkyne via copper-catalyzed 1,3-dipolar cycloaddition [44] (Figure 13.12). 

One of the more difficult problems encountered in obtaining a valid assay of formic acid is that of formate ester formation. The formate is derived from the cyclic, hemi-acetal structure which is an equilibrium form of many free sugars in solution. For example, the oxidation of one of the cyclic forms of D-glucose can readily be seen to give a formate ester (as well as a C-formyl group) on the atom originally denoted as C5. It... 

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