Interconversions, pyranose-furanose

Isbell and Pigman17 have shown that the rapid and anomalous mutarotation involves pyranose—furanose interconversion. On the basis that only D-fructofuranose (Ic) is fermented by yeast, Gottschalk18 has shown that the equilibrium mixture in aqueous solution at 0° contains 12% of D-fructofuranose. Gottschalk has calculated, from the kinetics of the mutarotation, that the aqueous solution at 20° contains about 20 % of the sugar in the furanose form. 

Although no mutarotation was observed with the first small sample of D-altrose, its [a]D value of +32.6° in water was in agreement with the equilibrium rotation —32.3° recorded by Austin and Humoller for L-altrose. When a larger amount of the sugar became available, D-altrose was found to exhibit a complex mutarotation. From calculations of the velocity coefficients it would appear that the mutarotation consists of a very rapid interconversion of furanose and pyranose modifications, followed by a slower interconversion of o and /3 pyranose modifications. 

Specific rotation [ajD —132° to —92° (2% w/v aqueous solution). Note that fructose shows rapid and anomalous mutarotation involving pyranose-furanose interconversion. 

The observation that mutarotation of monosaccharides is retarded significantly by such solvents as l r,i f-dimethylformamide and methyl sulfoxide induced Kuhn and coworkers to investigate the pyranose-furanose interconversion more closely. Indeed, the rrii mechanism (normal a-/3 interconversion) is completely suppressed in these solvents, and the Isbell conversion (the rri2 mechanism) can be followed according to the equation ... 

According to the reaction-rate theory,256 the rates of interconversion of the anomers, as well as the rates of ring isomerization, should depend on the differences in free energy between the reactants in the ground state and in the transition states. One curve of Fig. 11 depicts a pseudo-acyclic intermediate on each side of the transition state (B) governing the a jS-pyranose interconversion. The a-pyranose may be converted into a furanose form through a pyranose-furanose transition state (D) with less activation energy than that required for the... 

Fig. 11. —Hypothetical Role of Pseudo-Acyclic Intermediates in Mutarotation Reactions. [The curves qualitatively represent the changes in free energy of a sugar for which the /3-pyranose is more stable than the a-pyranose (curve 1). The pyranose-furanose interconversion (curve 2) is faster than either the a-/3-pyranose anomerization (curve 1) or the a-/8-furanose anomerization (curve 3). The relative stabilities of the isomers are represented in the decreasing order /3-pyranose > a-pyranose > /3-furanose > a-furanose.]...

Isbell and Pigman112 showed that the activation energies for a /8-pyranose anomerizations are usually higher than those for pyranose-furanose interconversions. Thus, the values of E given in Table VI (see page 53 of Part I) range from 18.6 to 14.2 kcal. mol-1 for the a-/8-pyranose anomerizations and from 15.8 to 10.7 kcal. mol-1 for... 

Orthorhombic, bisphenoidal prisms from ale, dec 103-105. Sweetest of the sugars. Shows mutarotation. Mg — 132 — —92 (c = 2). Rapid and anomalous mutarota tion involves pyranose-furanose interconversion. The final value is obtained instantly in the presence of hydroxyl ions. Ka at 18" 8.8 x l0-u. Freely sol in water. One gram dissolves in 15 ml ale, in 14 ml methanol. Slightly sol in cold, freely in hot acetone sol in pyridine, ethylamine, methyl-amine. 

Because six-membered rings aie nonnally less strained than five-membered ones, pyranose forms are usually present in greater anounts than furanose forms at equilibrium, and the concentration of the open-chain form is quite small. The distribution of carbohydrates among then- various herniacetal forms has been examined by using H and NMR spectroscopy. In aqueous solution, for exanple, D-ribose is found to contain the various a- and p-furanose and pyranose forms in the amounts shown in Figure 25.5. The concentration of the open-chain form at equilibr ium is too small to measure directly. Nevertheless, it occupies a central position, in that interconversions of a and p anorners and furanose and pyranose forms take place by way of the open-chain form as an intermediate. As will be seen later, certain chemical reactions also proceed by way of the open-chain form. 

Monosaccharides have many structural variations that correspond to local minima that must be considered. Acyclic carbohydrates can rotate at each carbon, and each of the three staggered conformers is likely to correspond to a local minimum. The shapes of sugar rings also often vary. Furanose rings usually have two major local minima and a path of interconversion. Experimental evidence shows a clear preference for only one chair form for some pyranose rings, but others could exist in several conformers. For exanqple, the and conformers must all be considered as possible structures for L-iduronate, as discussed by Ragazzi et al. in this book. 

Several monosaccharides mentioned in this Section are present in polysaccharide chains not only as pyranoses but also as furanoses. From the biogenetic point of view, a furanosidic form of a monosaccharide must be considered to be an additional component, as no ready interconversions of cyclic forms may be expected for monosaccharide residues incorporated into oligosaccharide chains, or in the activated form used for their formation. 

An extension of the simple mechanism in which ring forms are interconverted via a central, aldehydo intermediate includes direct pathways for the interconversion of the two pyranoses on one hand, and the two furanoses on the other. The existence of such direct pathways would permit a starting pyranose anomer to be converted rapidly to the other pyranose, as observed, even though the rate constant for the closure of the aldehydo form to the furanose ring were much greater than that for closure to the pyranose ring. 

Galactose-H2lsO Exchange. A possible mechanism for the interconversion of the two pyranoses (or furanoses), not involving the aide-hydo sugar, operates via a cyclic oxycarbonium ion (I). The participa-... 

The rate of the interconversion may also be followed by measuring the change in volume or in refractive index. Such measurements give rate coefficients identical with those obtained by the polarimetric method. In Table XVIII, rate coefficients for the mutarotation of a number of sugars are listed. The rates of mutarotation of several sugars (for example, D-ri-bose, D-galactose, and all the ketoses) do not obey the first-order law. Their complex mutarotations result from the presence in solution, in appreciable concentrations, of more than two species. In addition to pyranoses, there must be present either furanoses or acyclic forms, or both. 

The chair DFAs are thermodynamically favored, and can be obtained in good yield if furanose-pyranose interconversion is prevented by protecting groups. Thus, even though compound 10 is a kinetic isomer when considering the whole set of DFAs, it is thermodynamically favored in the difuranose DFA subset. The boat conformers are neither kinetically nor thermodynamically favored. The term... 

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