Pyranose monosaccharides, ring torsion

As we saw in Chapter 6, six-membered rings are generally more stable than five-membered rings, primarily because of increased torsional strain in the latter. Therefore, it is not surprising that the pyranose form of a monosaccharide is usually more stable than the furanose form. At equilibrium, glucose exists primarily as die pyranose (>99.8%), with little, if any, furanose (<0.2%) present. There is also a trace amount (0.02%) of the uncyclized aldehyde present. Of course, this equilibrium depends on the structure of the monosaccharide, and some other sugars have larger amounts of the furanose form. 

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