Pyranose isomers

Fructose is unique among known sugars in being sweeter than sucrose. In solution, fructose can exist as four or five isomers, and the relative sweetness of a solution is dependent upon the equilibrium between the sweeter pyranose isomers and the less sweet furanose isomers, which is in turn dependent on such conditions as pH and temperature. In cold conditions the pyranose form predominates and, therefore, fructose solutions are sweeter (Danisco Sweeteners, 2003). Fructose has a clean, sweet taste it is also synergistic with many bulk and intense sweeteners and is often used at low levels to improve the taste profile of some intense sweeteners. It is very soluble and also relatively hygroscopic, compared with sucrose (Danisco Sweeteners, 2003). 

Equilibrates to a more stable furanosc or pyranose isomer. 

It has been shown that tetrahydrouridine (77), and other ribo-furanosides of fully reduced cyclic ureas, undergo a rapid acid-catalysed isomerization to the -D-ribopyranosyl isomers, which predominate at equilibrium. In the case of tetrahydrouridine, the pyranose isomer was synthesized by the Vorbruggen procedure,... 

Syrup. [a]D -84 (c, 0.9 in CHCI3). Cont. some pyranose isomer. 

C11H15NO4 225.244 Mp 135-136°. [a]o +129 (c, 1 in Py). [a]o +178 (Py). The usual reaction of ribose and aniline produces crystalline anomeric mixtures of the pyranose isomer one crystalline isolate Mp 115° was formerly erroneously assigned a furanose struct. 

Typical applications of the DHAP aldolases indude the synthesis of monosaccharides and derivatives of sugars from suitable functionalized aldehyde precursors. High conversion rates and yields are usually achieved with 2- or 3-hydroxyaldehydes, because for these compounds reaction equilibria benefit from the cyclization of the products in aqueous solution to give more stable fiiranose or pyranose isomers (Figure 5.32). For example, enantiomers of glyceraldehyde are good substrates, and stereoselective addition of dihydroxyacetone phosphate produces enantiomerically pure ketohexose... 

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