A-Furanose

The anomeric carbon of a furanose or pyranose form of a ketose bears both a hydroxyl group and a carbon substituent In the case of 2 ketoses this substituent is a CH2OH group As with aldoses the anomeric carbon of a cyclic hemiacetal is readily identifi able because it is bonded to two oxygens... 

Figure 3 The underlying tree of a furanose ring in nucleic acids. Atoms are numbered 1,. . . , 5 corresponding to the natural tree ordering. All bond lengths are fixed. Arrows illustrate five internal coordinates that determine the ring conformation.

In a similar manner, ketones can react with alcohols to form hemiketals. The analogous intramolecular reaction of a ketose sugar such as fructose yields a cyclic hemiketal (Figure 7.6). The five-membered ring thus formed is reminiscent of furan and is referred to as a furanose. The cyclic pyranose and fura-nose forms are the preferred structures for monosaccharides in aqueous solution. At equilibrium, the linear aldehyde or ketone structure is only a minor component of the mixture (generally much less than 1%). 

The use of tetra-n-butylammonium fluoride (54) in an aprotic solvent such as acetonitrile may be more advantageous. Foster and colleagues (19, 37) have effected an SN2 type of reaction using this reagent in the conversion of l,2 5,6-di-0-isopropylidene-3-0-p-tolylsulfonyl-D-allofura-nose into the C-3 epimeric fluorodeoxy derivative. Note that whereas potassium fluoride is ineffective in displacing secondary sulfonate esters in sugars, tetra-n-butylammonium fluoride is capable of effecting a displacement with Walden inversion even in a furanose drivative. 

Once again no molecular-ion peak is seen, but the M-CH3 peak is prominent at m/e 189 in Figure 7. An important peak at m/e 159 from C-4-C-5 cleavage with charge retention on C-4 establishes the presence of a furanose ring and of a 6-deoxy function. Charge retention on C-5 leads to the ion at m/e 45 (see Equations 19 and 20). 

Some monosaccharides also exist in a five-mem be red cyclic hemiacetal form called a furanose form. D-Fructose, for instance, exists in water solution as 70% /Tpvranose, 2% a-pyranose, 0.7% open-chain, 23% /3-furanose, and 5% a-furanose. The pyranose form results from addition of the -OH at C6 to the carbonyl group, while the furanose form results from addition of the —OH at C5 to the carbonyl group (Figure 25.5). 

Problem 25.11 Ribose exists largely in a furanose form, produced by addition of the C4 -OH group to the Cl aldehyde. Draw D-ribose in its furanose form. 

The following cyclic structure is that of allose. Is this a furanose or pyranose form Is it an a or /3 anomer is it a D or L sugar ... 

A possible electron circuit in this complex reaction is shown in LXVIII and the products of the completed reaction in LXIX. The h-ans-arrangement of the hydroxyl on C4 and the aldehyde group (Cl) in LXIX makes the establishment of a furanose ring sterically unlikely and consequently 2,5-anhydro-D-mannose (chitose) probably exists as an dldehydo-mga,T. 

A furanose-fused c/.,(3-unsaturated 8-lactone was also prepared from ester 11 as a key chiral intermediate for the synthesis of the enantiomer of (+ )altholactone, a natural product with cytotoxic and antitumor activities.18 A Reformatsky reaction with ethylbromoacetate or a Wittig... 

Actually, on oxidation of the carbohydrate with sodium periodate, three moles of periodate are consumed and one mole of formic acid is formed. These data agree with the assumption that the disaccharide contains a pyranose and a furanose ring. The possibility that the disaccharide is made up of glucofuranose and sorbopyranose can also be eliminated on the basis of the periodate oxidation data. Glucofuranose would contain two pairs of adjacent hydroxyls, on carbon atoms 2 and 3 and on 5 and 6, and the sorbopyranose would have three adjacent hydroxyls, on carbon atoms 3, 4 and 5. In oxidizing such a disaccharide, a total of four moles of periodate would thus be used, giving rise to one mole of formic acid. This is inconsistent with the experimental data. 

It is noteworthy that D-fructose, which has a pyranose structure in the free crystalline state, assumes a furanose configuration whenever it combines with another sugar to form an oligosaccharide or polysaccharide. Apparently the ketohexose L-sorbose shows the same behavior. 

D-glucose and its lactone from 2,3,6-trimethyl-D-glucose provided conclusive proof that the ring system was not of the hexylene oxide type.142 188 The final evidence necessary to characterize the tetramethylglucose in question as a furanose derivative was provided by Haworth, Hirst and Miller,176 who demonstrated that oxidation of the tetramethylglucose with bromine water and of the resulting lactone with nitric acid yielded dimethoxysuccinic acid and oxalic acid, but not i-zyZo-trimethoxyglutaric acid, the absence of which ruled out a pyranose structure. 

Aldohexose a-Pyranose form (%) P-Pyranose form (%) a-Furanose form (%) P-Furanose form (%)... 

The selective introduction of fluorine into a furanose ring was demonstrated by the reaction of TASF with l,2 5,6-di-0-isopropylidene-3-O-trifluoromethanesulfonyl-a-D-allofuranose (12) which gave a 66% yield of 3-deoxy-3-fluoro-l,2 5,6-di-0-isopropylldene-a-D-glucofuranose (2. The isomeric triflate 13,... 

Fig. 1. Higher-order sugars L-g/yceroD-manno-Heptose (1, open-chain and 2, a-pyranose) and A -acetylneurami ilic acid (3, a-pyranose form). Structures 4-8 depict D-glucose (4) as its open-chain Fischer projection, and as the a-oxetose (5), a-furanose (6), a-pyranose (7), and a-septanose (8) ring forms.

An alternative explanation for the formation of monoacetamides has also been suggested by Hockett and Chandler, who point out that the monoamide could have been formed from the diamide by the loss of acetamide. However, this explanation seems unlikely in view of the fact that Niemann and Hays have reported the preparation of AT-acetyl-D-glucofuranosylamine by the action of ammonia on pentaacetyl-/3-D-glucose, a reaction which involves the conversion of a pyranose to a furanose ring. 

Scheme 41 Synthesis a furanose-fused butyrolactone from a glycal-derived cyclopropane...

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