Fructopyranose sweetness

D-Fructose is the sweetest sugar known in naturally occurring carbohydrates, and its intense sweetness is produced only by ) -D-fructopyranose. "... 

As it has been revealed that replacement of the ring-oxygen atom in a pyranoid sugar by a CH2 group is not detrimental to its sweetness, 6a-carba- -D-fructopyranose may have the same intense sweetness as D-fruc-tose. To substantiate this prediction, the following two reaction routes have been successfully developed for 6a-carba-y -DL-fructopyranose, as well as for the enantiomers. 

Support for this result was obtained from the taste of 1,5-anhydrohexitols, which, only for purposes of comparison, can be regarded as 1-deoxyal-dopyranoses. 1,5-Anhydro-D-glucitol (that is, the incorrectly named 1-deoxy-D-glucopyranose ) (14), 1,5-anhydro-D-mannitol ( 1-deoxy-D-mannopyranose or 2-deoxy-D-fructopyranose ) (15), and 1,5-anhydro-D-galactitol ( l-deoxy-o-galactopyranose ) (16) are all purely sweet, without any trace of bitterness. Furthermore, the complete absence of bitterness of 1,5-anhydromannitol (16) clearly indicates that the anomeric... 

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. 

When Shallenberger and coworkers attempted to explain the sweetness of /8-D-fructopyranose, they intuitively assigned the anomeric 2-hydroxyl group as AH and the oxygen atom of the 2-(hydroxymethyl) substituent as B. This assignment has since been supported by Lindley and Birch. It was shown that 1,5-anhydro-D-mannitol (15, 2-deoxy-D-fructopyranose ) and jS-D-arabinopyranose (22) (in both of which, one of the AH or B units... 

Among the most interesting examples of varying sugar sweetness is the fact that crystalline or freshly dissolved / -D-fructopyranose is about twice as sweet as sucrose, but after mutarotation or during thermal mutarotation, sweetness diminishes markedly (3). a-D-Glucopyranose is about two-thirds as sweet as sucrose, but the mutarotated solution is even less... 

When crystalline / -D-fructopyranose is newly dissolved in water, it is twice as sweet as sucrose, but shortly thereafter it is only slightly sweeter. Fructose mutarotates rapidly, and such phenomena have been associated by Isbell (4) with the formation of furanose forms of the sugars. Using a gas chromatographic procedure (5), we have shown (6) that the mutarotation primarily results from the formation of that isomer present in the sucrose molecule or -D-fructofuranose. 

At equilibrium in water at 20°, gas-liquid chromatography indicates that there is 76% -D-fructopyranose, 20% -D-fructofuranose, and 4% of an unknown compound, which has a specific rotation of about +122° (if the value of +17° assigned by Hudson (7) to / -D-furariose is correct). We deduced that the furanose form is void of sweetness for at least two reasons. As an example of hydrogen bonded hydroxyl groups, both hydroxy-methyl substituents are so dispersed as to be (perhaps) completely bonded to the ring oxygen atom (8). 

Pseudo-p-DL-fructopyranose 180 was found to be nearly as sweet as D-fructose. The D-enantiomer 196 and the L-antipode 207 were also as sweet as D-fructose, but 196 was somewhat sweeter than 207. The small but observable difference in sweetness of 196 and 207 might be due to a stereogeometrical deformation of interrelations between the sweetness eliciting tripartite and a sweet receptor. 

Fructose (= (3-I> Fructopyranose) (hexose monosaccharide) Universal Phoenix dactylifera (Arecaceae), Cichorium intybus (Asteraceae), Allium cepa (Liliaceae) Sweet (2 X >glucose)... 

No pronounced biological effects were reported for 6-thio-D-fructopyranose 36. Nonetheless, this compound was found to be unusually sweet and essentially nontoxic to mice [86]. In addition to its chemical synthesis [87], an enzymatic preparation from 6-thio-D-glucose employing glucose isomerase (EC 5.3.1.5) was reported [88]. In keeping with the general properties of pyranoid ring... 

T. Suami, S. Ogawa, M. Takata, K. Yasuda, A. Sugar, K. Takei, and Y. Uematsu, Synthesis of sweet tasting pseudo (S-fructopyranose, Chem. Lett., (1985) 719-722. 

The interactions of D-glucopyranose, D-fructopyranose, and sucrose with a proteinaceous receptor have been examined by use of CPK models to validate the steieomolecular inteipretation of sweet taste proposed for these sugars. In continuation of earlier studies (see Vol. 24, Chapter 2, ref. 7) the kinetic effects of 17 free sugars on the hydrolysis of l-benzoyl-3-phenyl-l,2,4-triazole have been measured. Analysis of the results in terms of hydration and stereochemical features led to the conclusion that the relative positions of 2-OH and 4-OH in the sugars are critical. ... 

For example, D-fructose solutions are sweeter than sucrose solutions, but in pastries and hot coffee both sugars show the same sweetness. The sweetest form is P-D-fructopyranose, which has about 180% of sucrose sweetness, but as a result of mutarota-tion, the sweetness of solutions decreases to about 150% of the sucrose sweetness because the individual anomers each have a... 

The relatively low degrees of sweetness of carbohydrates when compared with the sweetness of some noncarbohydrate compounds such as cyclamates, saccharin, certain aminoacids, and so on (see Fig. 5.3) can be explained by the relatively weak hydrophobic character of the C-6 hydroxymethyl group found in many pyranoses. However, the presence of a hydrophobic sweetness intensifier can explain why certain carbohydrates are much sweeter than other carbohydrates for example, D-fructose and D-xylose are much sweeter than D-glucose and sucrose (see Table 5.1). Both D-fructopyranose and D-xylopyranose (the predominant forms of D-fructose and D-xylose in solution and in the crystalline state) have methylene groups that are not substituted by a hydroxyl group, and hence are more hydrophobic and produce a sweeter taste (see Fig. 5.5). 

Figure 5.5. Proposed AH-B-X glucophores for A, P-D-fructopyranose B, P-o-xylopyra-nose C, 1,5-anhydro-D-glucitol D, 1,5-anhydro-D-mannitol, showing the involvement of a hydrophobic methylene group sweetness intensifier.

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