Sorbose conformation

These spectra have detected only one species of L-sorbose in aqueous solution. However, after prolonged storage in methyl sulfoxide 5-10% of a second component is generated. Although it has not as yet been properly identified, there is no counterpart in spectra of the heptulose, and most likely it is the /f-pyranose—i.e., the existence of furanose forms of 1 and 2 should have about equal probability, whereas the / -pyranose form of 2 in a chair conformation should be much less stable than that of 1. 

There are several reasons for reservations about applying the computer extrapolation of crystal structure data for carbohydrates. One is that much of the crystal structure data refer to unsubstituted sugars which are only soluble in hydroxylic or polar solvents where the conformational analysis may be complicated by hydrolysis, isomerism (muta-rotation) (12), or stereospecific solvent interactions which require a more sophisticated model. However, assuming that such chemical changes do not occur or can be suppressed, there still remain questions to be answered before the conformation observed in the crystal can be accepted as a close enough approximation to that of one or more of the rotomers which may predominate in the solution state. (a-L-Sorbose gives an example of the coexistence of two primary alcohol rotameric... 

S) Ketose Structures.—The equilibria for ketoses differ markedly from those of aldoses, because the hydroxymethyl group attached to the anomeric carbon atom tends to assume an equatorial position. Maintenance of the anomeric hydroxymethyl group in the equatorial position keeps the anomeric hydroxyl group in the axial position. With a-D-sorbose (a-D-xylo-hexulose) and a-D-gluco-heptulose in the CA conformation, all large groups except the anomeric hydroxyl... 

The CD and conformational properties of acyclic derivatives of fructose, sorbose, tagatose, and four deoxy derivatives of fructose have been reviewed [14],... 

The diacetal 46 may cyclize reversibly to give a tautomer (47). In one conformation (46a) of 46, the carbonyl group at G-2 and the hydroxyl group at C-5 of the L-sorbose chain can readily achieve... 

At mutarotational equilibrium in water, D-fructose (51) exists preponderantly as the j8-D-pyranose anomer in the 1C(d) conformation. A 1,2-alkylidene acetal (52) is formed in the same way as for L-sorbose, but this monoacetal has cts-disposed hydroxyl groups at C-4 and C-5 that react readily, forming a l,2 4,5-di-0-alkylidene-)8-D-fructopyranose (53). No evidence is available to indicate that the 1,2-alkylidene acetal might rearrange to a 1,3-alkylidene acetal, and it is to be expected that the activation energy for this isomerization would exceed that for formation of an acetal at 0-4 and 0-5. 

Proton magnetic resonance spectroscopy has also been used to study the conformations of several pyranoid derivatives of D-fructose and L-sorbose. Once more, data obtained in this study are limited, because of the lack of a proton at the anomeric center the only coupling constants useful in conformational studies of acetals of ketopyranoses are 73,4, 7s.b. and Js-s. and the conclusions that... 

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. 

By use of the semi-empirical molecular orbital method AMI, the optimised geometries and energies of the pyranoid forms of D-fructose (14), L-sorbose (15), 5-deoxy-D-rhreo-hexulose (16), and carba-D-fmctose (17) have been determined. The findings that a low-energy conformation exists for each of the three sweet compounds (14), (16), and (17) [but not for the non-sweet analogue (15)] in which HO-2 is hydrogen-bonded to 0-3, is thought to provide an explanation for the relative sweetness of these compounds. ... 

A Co(III) complex containing L-sorbose and two molecules of 1,10-phenanthro-line has been investigated by CD, o.r.d. and H-NMRspectroscopy, all H-NMR signals have been assigned. The complete H-NMR assignments of the aminoglycoside antibiotics butirosin A and kanamycins A and B, as well as conformational studies on butirosin A, have been reported. The conformation of adriamycin has been examined by 500 MHz H-NMR spectroscopy.The H-and C-NMR spectra of the tumour antibiotic aclacinomycin A have been fully assigned and its solution conformation analysed. A conformational analysis of... 

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