Furanose isomers

Fructose is unique among known sugars in being sweeter than sucrose. In solution, fructose can exist as four or five isomers, and the relative sweetness of a solution is dependent upon the equilibrium between the sweeter pyranose isomers and the less sweet furanose isomers, which is in turn dependent on such conditions as pH and temperature. In cold conditions the pyranose form predominates and, therefore, fructose solutions are sweeter (Danisco Sweeteners, 2003). Fructose has a clean, sweet taste it is also synergistic with many bulk and intense sweeteners and is often used at low levels to improve the taste profile of some intense sweeteners. It is very soluble and also relatively hygroscopic, compared with sucrose (Danisco Sweeteners, 2003). 

Zinner98 has studied the acetylation of D-ribose in pyridine solution with acetic anhydride at various temperatures. At 0° the pyranose tetraacetate is the sole product, but as the temperature at which acetylation is carried out is increased, the furanose isomer is also formed, the proportions of the two isomers formed at 100° being nearly equal. These findings parallel the earlier work by Schlubach and Prochownick99 in the D-galactose series. Zinner also observed that the acetylation of D-ribose with sodium acetate and acetic anhydride at higher temperatures gives rise to the furanose tetraacetate. 

Since almost no furanose form of D-glucose is observed in solution, it is expected that the energies of the furanose conformational isomers will be much greater than those of the pyranose form. In fact, the four furanose isomers examined are predicted to be about 4-5 kcal mol higher than the lowest pyranose conformers 6a and 6c at HF/6-31G. Quite remarkable, however, was the B3LYP/6-31G ... 

It is interesting that 2,3-O-isopropylidene-D-erythrose forms only the )3-furanose isomer. 

Dehydration Reactions. Detailed analysis of the pyrolysis tar as discussed previously (Figure 12 and Scheme 3) shows the presence of levoglucosan, its furanose isomer (1,6-anhydro-p-D-glucofuranose) and their transglycosylation products as the main components. In addition to these compounds, the pyrolyzate contains minor amounts of a variety of products formed from dehydration of the glucose units. The dehydration products detected include 3-deoxy-o-erythrohexo-sulose, 5-hydroxymethyl-2-furaldehyde, 2-furaldehyde (furfural), other furan derivatives, levoglucosenone (l,6-anhydro-3,4-dideoxy-P-D-glycerohex-3-enopyranos-2-ulose), l,5-anhydro-4-deoxy-D-hex-l-ene-3-ulose, and other pyran derivatives. The dehydration products are important as intermediate compounds in char formation. 

The /3-furanose isomer is best suited for silicon ligation because it exhibits a torsion angle close to 0° for the most acidic diol function, thus assuring a flat chelate ring. The same structural principles are also found in the anions [PhSi(a-D-Rul/2,3H 2)2] 157, [PhSi(/3-D-Ara/1,2H 2)2] 158, [PhSi-(a-D-Rib/l,2H 2)2] 159 and [PhSi(a-D-Xyl/l,2H 2)2] 160. 

In general, only one of these four isomers crystallizes at a given condition (e.g., a-glucopyranose from water at high temperature or p-glucopyranose at low temperature). It is either the predominant species that crystallizes or the least soluble one in the solvent used. Crystallization then gives rise to optical mutarotation, which is caused by establishment of the equilibrium mixture in a solution from which one component is removed. In the literature crystal structures of pyranoses predominate with only a few samples of crystalline fura-noses. The furanose isomers of D-ribose and 2-deoxy-D-ribose, for example, have never been isolated and crystallized since configurational interconversion of cyclic compounds in solution tends to inhibit crystallization. For this reason, the 1-0-methyl acetals, which cannot isomerize, crystallize more readily than half-acetals. 

The 2,4-di-C-methyl-D-galactopyranose derivative (16) has been rearranged to the furanose isomer (17) and hence to the derivative (18)(Scheme 4) required for the synthesis of an erythronolide ring fragment. 

C4 Hj4O30 1066.757 In equilib. with the furanose isomer. A major tannin in a number of species of Myrtaceae, Fagaceae and Lythraceae. Pale brown amorph. powder + 6H2O. M -33 (c, 1.2 in 50% McjCO aq.). Xmax 220 280 (MeOH) (Berdy). I-Deglycosyl, la-hydroxy Castalagin [24312-00-3]... 

From Achiral Non-carbohydrates. — 3-Deoxy-3-guanidino-D-threose 48 equilibrates with 49. a transition state inhibitor for galactosidase. It was synthesized as shown in Scheme 12 from epoxide 47, which was obtained by porcine pancreatic lipase catalysed enantioselective esterification of the racemic epoxy-alcohol precursor. 6-Deoxy-L-talonolactone 50 was synthesized by an asymmetric aldol condensation - dihydroxylation sequence (Vol.24, p.lS2) in improved diastereoselectivity and was converted into 2-acetamido-2,6-dideoxy-L-fucose (shown as its furanose isomer 51 in Scheme 13), 3-acetamido-3,6Hlideoxy-L-idose and 5-acetamido-S,6-dideoxy-D-allose by S 2 displacements of triflate with azide ion. 4-Amino-4-deoxy-DL-erthrose 53 was obtained from the hetero-Diels-Alder adduct 52 by a sequence of reactions including cis-dihydroxylation (OSO4, NMNO) of the alkene moiety (Scheme 14). The synthesis of a racemic branched-chain lactam is covered in Chapter 16. 

Preparation of the Adduct with Pseudouridine. The p-azidophen-acyl derivative of pseudouridine (4 ) is prepared by incubation of a 65% MeOH solution containing 0.09 M NaHCOs (pH 9), 14 mM 4 o (the natural 8-furanose isomer), and 32 mM APA-Br at 37° for 16 hr in the dark. By this time, most of the 4- has reacted. The reaction mixture is streaked out on Merck 2 mm-thick silica gel TLC plates, 1 ml per plate,... 

In the case of 5-0-methyl-D-ribose [17] a-furanose isomer consist of 33% and P-furanose 67 % in the solution. After the addition of calcium chloride the proportion was changed to 70 30 respectively. It is due to the possibility of a-furanose to adopt conformation in which hydroxyl groups Ol, 02, 03 are quasi axial, quasi equatorial, quasi axial which is close to typical arrangement ax-eq-ax. 

D-Fractose and L-sorbose are ketoses. The first one forms weak complexes in the P-pyranose 1C4 conformation with ax-eq sites for binding of calcium and weak complexes by a- and P-furanose isomers. L-Sorbose forms also very weak complexes. It is possible only in the P-4C1 conformer due to two sets of cis ax-ax. 

Reducing oligosaccharides show many of the properties of simple sugars such as mutarotation, reduction of alkaline solutions of salts of heavy metals, existence of and p)ranose and furanose isomers, and ready oxidation and reduction. Nonreducing sugars behave as alcohols unless the linkages are hydrolyzed by acids. The linkages are usually stable to the action of alkalies, but a few (e.g., turanose) are alkali-labile. 

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