Fructofuranose ring

Nucleophilic acyl substitution and tautomerization lead to the formation of glucosamine 6-phosphate from fructose 6-phosphate. The mechanism of opening of the fructofuranose ring was shown in the previous problem. 

From a two-step sequence, the yield was calculated here based on the amount of the first step product that reacted. yThe main product involved substitution of the fructofuranose ring, at 0-3 (72% yield). 

There are a number of ways of using the regressional analysis method and the simplest, and for many purposes the most powerful of these can be illustrated with respect to the determination of the relaxation contributions which the anomeric proton of thegalactopyranose residue of V receives from the protons of the fructofuranose ring. The non-selective Ri-values... 

Thus the Ri-value of H-2 gives us immediately an estimate of Pg (0.570 sec- ) and since these interproton relaxation contributions are mutual, the additional relaxation experienced by H-l can only come from the protons of the fructofuranose ring, estimated as being 0.280 sec-, which is ca. 40% of the total relaxation experienced by H-l. Although this simple arithmetic is founded on an extremely simplistic premise it is interesting to note that the above estimate of pg from the H-2 resonance is in excellent accord with the value from H-4 (0.595 sec- ). Furthermore, the ratio of Pq/Paa = 0.582/0.358 1.63 is in excellent... 

They do not occur naturally but the disaccharidc sucrose (cane-sugar) contains a j8-fructofuranose ring condensed with an a-glucopyranose ring ... 

Turning now to the data for th6 fructofuranose ring of (A) we note that once again the methylene protons relax more rapidly than the methine protons. The fact that the latter relax rather slowly certainly is... 

As shown in Table I (54), there is considerable difference in the ease of hydrolysis of oligosaccharides sucrose, with its fructofuranose ring, is particularly labile. The ease of hydrolysis of the sucrose linkage in comparison with that of the glycopyranosides makes it possible to hydrolyze preferentially the sucrose linkage in trisaccharides with the formation of a resistant disaccharide. Thus, turanose, a disaccharide, is prepared by the partial hydrolysis of the parent trisaccharide melezitose. 

It is interesting to note that the formation of the dioxane ring stabilizes the anhydrides as to their pyranose or furanose structures, difructose anhydride I as 2,1 l,2 -di-D-fructofuranose and dihetero-levulosan as 2,1 l,2 -di-D-fructopyranose. 

In a previous work, using D-fructose pyran- and furan- forms as inhibitors of D-fructose transport in CHO (Chinese Hamsters Ovary)-GLUT5 cells, Rollin, Holman and co-workers established that both ring forms were tolerated. The approach used was to block each hydroxyl function with allylic ether it was concluded that two sites, 0-2 (pyranose and furanose) and 0-6 (furanose) could be modified and addressed a visualization of vital interactions with the protein. These interactions were considered to occur because the D-fructofuranose form is relatively symmetrical for that reason, the binding site can arise either in anomeric center side or on the other side of the molecule. Hence D-fructopyranose appears to present to GLUT5 transporter by hydroxyl 3, 4, 5 recognition (Fig. 3). 

Figure 2. Puckering angles (< )) for perfect envelope (E) and symmetrical twist (T) forms of fructofuranose. The non-planar ring atoms in symmetrical twists are displaced equally above and below the ring. The anplitude of puckering (Q) is the radius of the circle.

The energies of the different ring conformations are affected by the rotational orientations of the two primary alcohol groups of fructofuranose. Therefore, all 9 combinations of likely orientations of these groups must be considered before the energy differences inherent in different ring conformations can be understood (French, A.D./ Tran, V.H. Biopolvmers In Press). 

When the original methyl D-fructofuranoside sirup was fermented with yeast, the unstable beta isomer was selectively eliminated and the residue yielded a crystalline methyl D-fructoside melting at 81° and with [a] D +93° in water. The ring structure of this new isomer was proved to be furan by methylation to the liquid tetramethyl derivative, of [a] °D +129.4°, and subsequent hydrolysis to 1,3,4,6-tetramethyl-D-fructofuranose (structure IX) with the correct specific rotation of +29.8° in water. Both the methyl D-fructoside and its fully methylated derivative were therefore of the alpha configuration, since the latter was more dextrorotatory than the tetramethyl-D-fructose and also since the former was more dextrorotatory than the isomer, of [a] D —51°, unstable to invertase. Similar work with the benzyl D-fructofuranoside sirup produced the crystalline alpha isomer, melting point 89°, [a] D +45.7° in water, the liquid tetramethyl derivative, [a] D +83.3° in chloroform and, after acid hydrolysis of the latter, 1,3,4,6-tetramethyl-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). 

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-... 

A five-membered cyclic sugar ring is called a furanose. Fructose prefers a fu-ranose ring system, and is formally named fructofuranose. Like glucose, fructose can cyclize and can form either an alpha anomer or a beta anomer. Notice that,... 

Figure 9.24 Traditional Fisher and Haworth projections of the ketose, fructose. Fructose can form a five-C ring called a furanose when the C-2 keto group reacts with the hydroxyl on C-5, as shown in a-D-fructofuranose.

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