2,25:5-anhydro mutarotation

It is to be expected, on these grounds, that the predominating component of the mutarotation equilibrium of 3,6-anhydro-glucose will be 3,6-anhydro-glucofuranose whereas that of 3,6-anhydro-galactose will be the open-chain form. Neither will include pyranose forms. 

In contrast to other 2,5-anhydroaldoses (which exhibit mutarota-tion, possibly due to the formation of hemiacetals28), 2,5-anhydro-D-glucose does not show any mutarotation.27 The importance of this compound as a potentially useful precursor to C-nucleosides warrants a reinvestigation of the deamination reaction, and the definitive proof of the structure of the compound. The readily accessible 2,5-anhydro-D-mannose (11) does not possess the cis-disposed side-chains at C-2 and C-5 that would be required of a synthetic precursor to the naturally occurring C-nucleosides, with the exception of a-pyrazomycin (8). The possibility of an inversion of the orientation of the aldehyde group in 11 by equilibration under basic conditions could be considered. 

To accommodate these facts, the earliest mechanisms proposed for degradation of D-fructose assumed that it was present in the furanose form, and that the ring remained intact. It was assumed that the initial reaction was the elimination of water, to form the 1,2-enolic form of 2,5-anhydro-D-mannose, and that further dehydration resulted in 2-furaldehyde. The necessity for D-glucose to isomerize to D-fructose was assumed to account for the much lower reaction-rate of D-glucose. This mechanism does not account for the observation that 2,5-anhydro-D-mannose is less reactive than D-fructose, nor is there any evidence that 2,5-anhydro-D-mannose is present in reacting D-fructose solutions. Nevertheless, similar mechanisms have since been proposed.13-16 Because of the ease of mutarotation of D-fructose... 

Mutarotation of Various Free or Partially Substituted 2,5-Anhydro-aMefce/do-aldoses... 

In.view of these results, the apparent absence of mutarotation for 2,5-anhydro-aZde/iydo-D-glucose, attributed18,22 to the impossibility of its forming an internal hemiacetal, remains to be confirmed. 

Grant74 conducted a detailed study of the properties of 2,5-anhydro-D-mannose (chitose), and prepared several new derivatives that confirmed the anhydrohexose structure. The mutarotation of 2,5-anhydro-D-mannose, its conversion under mild conditions into a... 

The reported lack of mutarotation for 2,5-anhydro-D-glucose, which was tentatively identified as the product of the reaction of 2-amino-2-deoxy-D-mannose with mercuric oxide, has not been confirmed.77,78... 

Formation of neither a furanose nor a pyranose form can occur for 2,5-anhydro-D-talose, and the mutarotation reported for an aqueous solution must be due to hydration of the aldehyde group, or other processes,6,75 or both (see p. 20). 

Since the trimethylglucose has been shown to form a 6-lactone, characterized by its rapid mutarotation in aqueous solution,161-163 and to give rise when methylated and hydrolyzed to 2,3,4,6-tetramethyl-D-gluco-pyranose,189 it follows that C5 is unsubstituted. While it is clear that the third methoxyl group must be present on C6, little direct evidence is available to support this view. The syntheses of the trimethylglucose from methyl 2,3-anhydro-/3-D-mannopyranoside (known to condense with benzaldehyde and therefore to carry free hydroxyl groups on C4 and C6)168 and also from 3-benzyl-D-glucose162 leave, however, no doubt that C6 is methylated. 

Lyxose, mutarotation of, 23 —, 5-acetamido-5-deoxy-D-, 170 —, 2,5-anhydro-5-seleno-D-, dimethyl acetal, 232... 

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