Sugar ring size

In the course of their structural studies of polysaccharides, the Aspinall group isolated several oligosaccharide fragments and characterized them by comparison with specifically synthesized samples. Disaccharides made for this purpose were 2-0-/ -D-xylopyranosyl-D-xylose, its monomethyl derivative with the substituent at 0-3 of the nonreducing moiety, and 2-0-/ -D-xylopyranosyl-L-arabinose. A biosynthetic investigation led to the establishment of the sugar ring size of natural uridine 5 -(L-arabinofuranosyl pyrophosphate). 

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. 

In solution, most simple sugars and many of their derivatives occur as equilibrium mixtures of tautomers. The presence of a mixture of two anomers of the same ring size may be indicated in the name by the notation a,P-, e.g. a,P-D-gIucose. In formulae, the same situation can be expressed by separating the representation of the ligands at the anomeric centre from the a and P bonds [see examples (a) and (c)], or by use of a wavy line [(b) and (d)] (particularly if hydrogen atoms are omitted). 

The chelate ring size principle can have structural effects as well as effects on thermodynamic stability in aqueous solution. An example is coordination of metal ions by sugars (44). The cyclic polyol cts-inositol can coordinate metal ions in two distinct ways (Fig. 14) (45). In ax-ax-ax bonding (Fig. 14), the metal ion is part of three fused six-membered chelate rings. Alternatively, in ax-eq-ax coordination, the metal ion is part of two fused five-membered and one six-membered chelate rings. Angyal has noted that metal ions of radius more than 0.8 A adopt the ax-eq-ax structure (44), whereas with an ionic radius... 

In carbohydrate chemistry, the most described method for the preparation of saccharidic thionocarbamates involves preliminary introduction of the amine function on a partially or non-protected saccharidic template. The condensation of amino sugars with carbon disulfide or thiophosgene leads to cyclization in 1,3-oxazolidine- or l,3-oxazine-2-thiones. This reaction involves the formation of an intermediate isothiocyanate, which reacts further with a 3- or y-located hydroxyl group. The viability and facility of this process depends on the saccharidic ring size and the inherent strain. Some major rules can be put into light from the cases studied 30... 

A general method for the preparation of carbasugars with various ring size consists of the metathesis of properly prepared sugar diolefins. 

Both six-membered pyranose and five-membered furanose structures are encountered, a particular ring size usually being characteristic for any one sugar. Thus, although glucose has the potential to form both six-membered and five-membered rings, an aqueous solution consists almost completely of the six-membered hemiacetal form five-membered rings... 

Ring size and anomenc form of common sugars... 

Sugars are polyhydroxy compounds, and it is not always easy to predict which of the hydroxyls will react in this way. There are other complicating factors too. The ring size (pyranose/furanose) of the product may differ from that of the starting sugar. It may be... 

Fig. 3. Position of the nitrogen atom in various basic sugar analogues of proven glycosidase inhibitory activity (disregarding conformational freedom as well as other ring sizes).

Uracil is removed from the DNA by a uracil glycosidase which excises the base from the sugar ring. This activity is analogous to the hydrolytic activity of the isoleucyl-tRNA synthetase toward Val-tRNAIle. In both cases the hydrolytic site is too small by the size of one methylene group to accommodate the substrate that is to be left intact. In DNA synthesis, the editing is performed by a separate enzyme, since the editing can wait until after polymerization. As this luxury is not permitted in protein synthesis, the hydrolytic function is on the synthetase, so that correction can occur before the misacylated tRNA leaves the enzyme. 

The oxide ring is six-membered in some sugars and five-membered in others, and it is helpful to use names that indicate the ring size. The five- and six-membered oxide rings bear a formal relationship to oxa-2,5-cyclohexadiene and oxa-2,4-cyclopentadiene that commonly are known as pyran and furan, respectively ... 

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