Dihexulose dianhydrides

V. Dihexulose Dianhydrides by Protonic and Thermal Activation of Saccharides. 216... 

The literature in this field is confusing because of a somewhat haphazard method of nomenclature that has arisen historically. This is compounded by some mistakes in structure determination, reported in early papers, and which are occasionally quoted. The first part of this chapter deals with nomenclature and with a brief overview of early work. Subsequent sections deal with the formation and metabolism of di-D-fructose dianhydrides by micro-organisms, and the formation of dihexulose dianhydrides by protonic and thermal activation. In relation to the latter topic, recent conclusions regarding the nature of sucrose caramels are covered. Other sections deal with the effects of di-D-fructose dianhydrides upon the industrial production of sucrose and fructose, and the possible ways in which these compounds might be exploited. An overview of the topic of conformational energies and implications for product distributions is also presented. 

Dihexulose dianhydrides are intermolecular cyclic acetals formed by the condensation of two hexulose molecules with the elimination of two water molecules, and the formation of two linkages involving the 2- and one other position of each component. They are named4 by placing the word dianhydride after the names of the two parent monosaccharides. The order of precedence for these... 

Table 1 contains a list of the dihexulose dianhydrides currently in the literature, together with some mixed fructose-glucose dianhydrides. Trivial and IUPAC names are included. Each entry has a proposed abbreviation. Because of the great similarity of structure between all the compounds in Table I, these abbreviations are used, rather than numbers, in the context of this chapter. Thus, 1 is named as ot-D-Fru/-l,2 2,1 - 3-D-Frup, 2 as (3-D-Fru/-l,2 2,3 - 3-D-Fru/, and 3 as 3-D-Frup-1,2 2,l -ot-L-Sor/>. 

When D-fructose and L-sorbose are refluxed with aqueous HC1, dihexulose dianhydrides are formed.91 If the water is replaced by N./V-dimethylformamide, substantially increased yields are obtained and 1,2-linked disaccharides are detected. Higher yields of dianhydrides were obtained from fructose, rather than sorbose, under comparable conditions. Treatment of levan with dilute H2S04 at 60°C yielded92 a-D-Fru/-l,2 2,1 -fi-D-Fru/(5). Presumably, any products that contain 2,6-linkages with large central rings would rapidly isomerize to the more stable 1,2-linked product. 

VI. Conformational Energies in Dihexulose Dianhydrides and the Control of Product Distributions... 

In 1991, an important paper was published by Bock et a/.84 that described the steric and electronic effects on the formation of the dispiroketal dihexulose dianhydrides. The authors described the conformation of six dihexulose dianhydrides, as determined by X-ray crystallography or NMR spectroscopy. They concluded that these conformations are dictated by the anomeric and exo-anomeric effects. Thus, the dihexulose dianhydrides are disposed to adopt conformations that permit operation of these effects—even if this results in the dioxane ring having a boat conformation or all three substituents on one pyranose ring being axial. 

In the case of the dihexulose dianhydrides containing three spino-linked rings, it is perhaps inappropriate to use the terms endo- and exo- without any fixed point of reference—especially because both effects have the same origin. An appreciation of the outcome of these electronic effects can be obtained by taking the 1,4-dioxane ring as the point of reference. This ring adopts a conformation... 

The products of the dimerization of l,6-dihydroxy-2-hexanone118 are the simplest models for the dihexulose dianhydrides. Under thermodynamic conditions, the isomer (20) was the sole product. Under kinetic conditions, compounds 20 and 21 are formed as a 1 1 mixture. Whereas 20 has the conformation illus-... 

Optical Rotations and Melting Points of Dihexulose Dianhydrides and Related Compounds Compound [a]D (degrees) (T°C, c, solvent) Melting Point (°C) Ref. 

Optical Rotations and Melting Points of Per-0-acetyl Dihexulose Dianhydrides and Related... 

Diheterolevulosans, 209-211, 240 Dihexulose dianhydrides, 207 -266, see also Caramels Di-D-fructose dianhydrides 13C NMR spectra, 245-246 conformation, electronic control, 224-228 conformational rigidity, energetic outcomes, 228 hexulopyranose rings, 226 historical overview, 210-213 H NMR spectra, 248 -249 intramolecular hydrogen-bonds, 227 isomerization, 231 -232 1,2-linked, ero-anomeric effect, 224-225 listing, 240-241 nomenclature, 208-210 optical rotations and melting points, 242-243 protonic activation... 

X-ray diffraction pattern, 334-335 Mass spectra, partial, per-O-trimethylsilylated dihexulose dianhydrides, 253 meso forms... 

Per-O-acetyl dihexulose dianhydrides l3C NMR spectra, 247 H NMR spectra, 250-251 optical rotations and melting points, 244 Per-O-acetyl fructose glucose, H-NMR spectra, 252... 

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