Mutarotation, of D-fructose

To accommodate these facts, the earliest mechanisms proposed for degradation of D-fructose assumed that it was present in the furanose form, and that the ring remained intact. It was assumed that the initial reaction was the elimination of water, to form the 1,2-enolic form of 2,5-anhydro-D-mannose, and that further dehydration resulted in 2-furaldehyde. The necessity for D-glucose to isomerize to D-fructose was assumed to account for the much lower reaction-rate of D-glucose. This mechanism does not account for the observation that 2,5-anhydro-D-mannose is less reactive than D-fructose, nor is there any evidence that 2,5-anhydro-D-mannose is present in reacting D-fructose solutions. Nevertheless, similar mechanisms have since been proposed.13-16 Because of the ease of mutarotation of D-fructose... 

In the D-fructosides the convention is that the more dextro-rotatory anomer is the a form. Attempts have been made by Boeseken and Couvert,82 Verschuur83 and MacPherson and Percival84 to apply Boese-ken s boric acid method to determine the configuration of D-fructose at C2. The problem is much more complex than for aldoses because there are three hydroxyl groups near the reducing center, and also because the mutarotation of D-fructose involves the conversion of some pyranose to furanose form with the loss of a pair of cis hydroxyls (on C4-C5). This work has been discussed by Boeseken88 but no conclusion has been reached. 

Fig. 17. —Mutarotation of D-Fructose in N V-Dimethylformamide-Water Mixtures at 24°. [(1) Change following addition of an equal volume of water to a previously equilibrated solution of D-fructose in N,N-dimethylformamide (2) change following addition of an equal volume of water to a freshly prepared solution of crystalline D-fructose in N V-dimethylformamide.1,7]...

By application of first-order, kinetic equations, B. Anderson and Degn claimed that an equilibrated (25°) aqueous solution of D-fructose contains 31.56% of jS-D-fructofuranose and 68.44% of -D-fructopyranose. N.m.r. studies, however, showed that, at equilibrium, a solution of D-fructose contains /3-D-fructopyranose, -D-fructofuranose, a-D-fructofuranose, and a trace of a-D-fructopyranose the distribution of these isomers was shown by gas-liquid chromatography to be 76,19.5, and 4%, respectively. Based on Anderson and Degn s result, Shallenberger reasoned that, as 0.68 X 1.8 = 1.22 (which approximates the reported sweetness of mutarotated D-fructose ), the furanose form(s) must possess very little sweetness. 

Mutarotation changes and rates of change for D-fructose show that / -D-fructofuranose is a plentiful component at equilibrium in water and that both of the a-anomers are relatively unimportant (2). The recently-described 13C NMR spectrum of D-fructose (14) confirms and amplifies this early polarimetric evidence, giving an equilibrium composition of... 

This is one example of the second criterion mentioned previously. However, the other OH substituents, depending upon the furanose ring conformation, are either eclipsed or in the anti conformation. In the former they are disposed to form a strong intramolecular hydrogen bond in the latter they are incapable of such bonding. Further evidence to support the contention that free / -D-fructofuranose is nearly tasteless is seen in the thermal mutarotation (3) of D-fructose. As the temperature of D-fruc-... 

P- pyranose form of D- fructose (this form is present in crystals of this sugar) to the a- pyranose, a, P- furanose and to the non-cyclic fructose. In ehemistry this reaction is known as a mutarotation. 

Some workers have reported a cation dependence and others an independence. Any differences, if noted, were attributed usually to the strength of the base. Thus, Lobry de Bruyn and Alberda van Ekenstein (78) reported the reaction products of lead hydroxide to be different from those of numerous other bases which they studied. Notably, with lead hydroxide the ketose of the 1,2-enediol equilibrium was missing. This was attributed to its very rapid conversion to the supposed 3-ketose. Kusin (97) recorded a similar observation with calcium hydroxide as compared to sodium hydroxide, but he believed the ketose was never formed. Under the conditions studied, lime acting on glucose gave mannose but no detectable amounts of fructose, whereas sodium hydroxide gave a measurable amount of fructose but only a trace of mannose. Sowden and Schaffer (81) found no differences in the initial mutarotation of D-mannose in the presence of 0.035 N sodium and calcium hydroxides at 35°, but differences in the direction of... 

Fig. 4.5. Temperature dependence of the mutarotation equilibrium of D-fructose, (— p-D-fructo-pyranose, -— P-D-fructofuranose,-------a-D-fructofuranose) (ac-...

The most familiar of all the carbohydrates is sucrose—common table sugar. Sucrose is a disacchar ide in which D-glucose and D-fructose are joined at then anomeric carbons by a glycosidic bond (Figure 25.7). Its chemical composition is the same ine-spective of its source sucrose from cane and sucrose from sugar beets are chemically identical. Because sucrose does not have a free anomeric hydroxyl group, it does not undergo mutarotation. 

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