Fructose, anomers pyranose form

The effect of the solvent on the proportions of the anomers for sugars in solution has also been studied by Perlin and coworkers by nuclear magnetic resonance measurements (see p. 46, Part I). These workers suggested that, in aqueous solutions of D-fructose, the pyranose form is stabilized by hydrogen-bonding with water molecules. Additional data supplied by Perlin are given in Table XVII. 

Previous assignments of bands characteristic of furanose and pyranose forms (Mathlouthi, Carbohydr. Res., 1980, 7, 225) were shown to be incorrect since these bands co-occur in the spectra of crystalline -D-fructopyranose. Prom the intensity of the carbonyl absorption in fructose solutions (the 0 -deuterated form in D-O) relative to that of diethyl ketone, it was concluded that 0.9 of fructose was present as the open-chain form. From a study of the i.r. spectra of peracetylated aryl glycosides, the value of bands in the 800-1000 cm region for differentiating a- and 6-anoraers has been reassessed. The relative intensities of C-O-C stretching vibrations in the 1000-1100 cm region and a band near 300 cm for -anomers only were suggested as criteria for... 

Figure 25.5 Pyranose and furanose forms of fructose in aqueous solution. The two pyra-nose anomers result from addition of the C6 -OH group to the C2 carbonyl the two furanose anomers result from addition of the C5 -OH group to the C2 carbonyl.

In sucrose, fructose is present as the P anomer. Now, one of these sugars has acted as an alcohol to make a bond to the other sugar. We can look at this in two ways. Either frnctose acts as an alcohol to react with the hemiacetal glucose to form an acetal, or alternatively, glucose is the alcohol that reacts with the hemiketal fructose to form a ketal. In sucrose, the pyranose ring is an acetal, whilst the fnranose ring is a ketal. This all seems rather... 

Figure 20-2 Structure and configuration of the o-ketoses from C3 to C6. As with the aldoses (Figure 20-1), the cyclic form is predominantly an oxacyclohexane (pyranose) ring in the free sugar, but the oxacyclopen-tane (furanose) form is shown for fructose because it occurs widely in this form as the disaccharide, sucrose. Only the a anomers are shown (see Section 20-2B).

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. 

Figure 11.6. Ring Structures of Fructose. Fructose can form both five-membered furanose and six-membered pyranose rings. In each case, both a and P anomers are possible.

At mutarotational equilibrium in water, D-fructose (51) exists preponderantly as the j8-D-pyranose anomer in the 1C(d) conformation. A 1,2-alkylidene acetal (52) is formed in the same way as for L-sorbose, but this monoacetal has cts-disposed hydroxyl groups at C-4 and C-5 that react readily, forming a l,2 4,5-di-0-alkylidene-)8-D-fructopyranose (53). No evidence is available to indicate that the 1,2-alkylidene acetal might rearrange to a 1,3-alkylidene acetal, and it is to be expected that the activation energy for this isomerization would exceed that for formation of an acetal at 0-4 and 0-5. 

Ans. The previous discussion was an oversimplification. Hemiketal formation in o-fructose and other ketohexoses results in reaction at both C-5 and C-6 to give a mixture of 5- and 6-membered rings. Pure fructose in solution consists of a mixture of the a- and /3-anomers of both furanose and pyranose structures, as well as a very small amount (< 0.2 percent) of the open-chain form. However, all combined forms of o-fructose, e.g., fructose combined with glucose in sucrose, involve only the 5-membered ring. For this reason, most texts simplify matters for the student by ignoring the pyranose structures. The exclusive use of the 5-membered ring in combined forms of o-fructose is not understood. 

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