Furanose sugars

A number of examples involving the stereochemistry of five membered rings are met in furanose sugars. An interesting example is that of 2, 5 dimethylcyclopentane 1, 1 dicarboxylic acid. This acid can exist in two geometrically isomeric forms which can be distinguished by decarboxylation. The cis xxvii isomer forms two monocarboxylic acids which are meso because they possess a vertical plane of symmetry. The trans isomer xxviii forms only one monocarboxylic acid and since it possesses no elements of symmetry, therefore, exists in optically active forms and a meso variety. 

Reduced furan rings occur in many important anhydrides, lactones, hemiacetals and ethers. Maleic anhydride is frequently used as a dienophile in Diels-Alder reactions and it is a component of alkyd resins. Several unsaturated y-lactones are natural products, while the furanose sugars are cyclic hemiacetals. 

Scheme 2 Carbocyclic compounds from furanose sugars.

Any one of the five atoms of the ring can be either above or below the plane defined by the other four in the envelope conformation. The energy barriers separating them are very low, and in cyclopentane or in proline all of the envelope conformations are freely interconvertible through intermediate skew forms.32 Furanose sugar rings are very flexible but the presence of the bulky substituents reduces the number of possible conformations.33-363 See Chapter 5 for further discussion. 

Certain furanose sugars have been transformed into epoxy derivatives by a similar process. Thus, l,2- M opropy)id nn 0-... 

Until recently the lactol ring structure of 2-desoxy-D-ribose in nucleic acid had been proved conclusively only for the thymidine nucleoside component and in this case it was furanose in form.26 Subsequently Brown and Lythgoe,27 by application of the periodate oxidation procedure to the 2 -desoxy ribosides of guanine, hypoxanthine, cytosine and thymine, afforded proof of the presence of a furanose sugar in each compound. 

Borate complexing appears to involve the generation of a substantial proportion of furanose sugars in solution,85 although virtually none of the furanose forms may be present before the addition of borate. 

An extension of the foregoing to include acid-catalyzed transacetalation re-su ted in the formation of furanose sugars. By treating the protected xyloside 115... 

The marked stereoselectivity of these radical reactions must be ascribed to the effect of the bulk of the protecting groups. However, the radicals that were manipulated may have also shown an anomeric effect due to the vicinal oxygen. In a furanose sugar it is not easy to evaluate the anomeric effect without ESR, and also, the molecules concerned have two fused five-membered rings. This fixes the conformation into a V-shape. Such molecules are well known to have exo-reactivity and the formation of endo-bonds is difficult. All these effects may be acting together to make this nucleoside chemistry especially stereospecific. 

Structural similarity with furanose sugars was the common idea for two independent groups which published the synthesis of optically active 11-oxa-prostanoids93 9S ... 

Isomerism at the acetal carbon atom is theoretically possible in five-membered acetal rings formed when pyranose or furanose sugars are condensed with aldehydes. For furanose derivatives, the isomerism is exo-endo, and the superior stability of exo isomers ensures that one isomer only will be obtainable, and this of the exo configuration. " 1,2-0-Benzylidene-a-D-glucofuranose " therefore has the complete configuration XLV. If the... 

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