Aldopyranosides, methyl

Nucleophilic displacements at carbon atoms adjacent to the ano-meric center of aldopyranosides do not normally proceed satisfactorily, but, surprisingly, treatment of methyl 3,4-0-isopropylidene-2-0-(p-nitrophenylsulfonyl)-a-D-glucoseptanoside with tetrabutylammon-ium fluoride in acetonitrile afforded the corresponding mannoside, from which 2-deoxy-2-fluoro-/3-D-mannopyranose was obtained by subsequent deprotection.153 Whether this is a general property of sep-... 

Cope Degradation of Methyl 3,4-Dideoxy-3-(dimethylamino)-DL-aldopyranoside... 

The ratio of the rate coefficient for the hydrolysis of the methyl aldopyranoside (fc) to that of methyl a-D-mannopyranoside, under the reaction conditions indicated. 

These data extend to the pyranoside series the relationship between steric strain and reactivity, and indicate that the conformational instability and non-bonded interaction of the axial substituents in the favored chair conformation result in a higher rate of hydrolysis. Another aspect of these theoretical considerations is that the methyl /3-d and fi-L aldopyranosides are only hydrolyzed faster than their corresponding a anomer when the normal chair conformation is favored, and thus, when the glycosidic group of the (3 anomer assumes an exposed, equatorial position (see page... 

Ratio of the rate constant (k) for the hydrolysis of the methyl aldopyranoside to that of methyl a-D-glucopyranoside (k = 1.98 X 10 min ). The numbers represent the position on which the substituent is axial. In the terminology suggested by Reeves, the normal chair conformation is denoted by Cl for d sugars and IC for L sugars. Indirectly obtained value. 

In their chemical properties, both these types of compound resemble to a certain degree the methyl /3-D-aldopyranosides or the methyl a- or /3-D-aldofuranosides. They do not have reducing properties, and are relatively stable in alkaline media, but are hydrolyzed by acids to the free hexoses. 

Ratio of the rate constant for the hydrolyas of the methyl aldopyranoside to that of methyl t ecified. Obtained by extrapolation. 

His investigations on the auto-oxidation of alcohols - have been extended by Wells to various a- and iS-n-aldopyranoses, methyl a- and /3-D-aldopyranosides, and polysaccharides. The author is indebted to Dr. C. F. Wells for making his results available for this article prior to their publication.During the auto-oxidation of alcohols in aqueous solutions photosensitized by sodium 9,10-dihydro-9,10-dioxo-2-anthracene-sulfonate, the energy of the photo-excited, quinonoid sensitizer is degraded by the following two processes. 

A notable exception has been the application of i.r. spectroscopy by Isbell and Tipson to the analysis of the conformations of methyl aldopyranosides," acetylated methyl aldopyranosides," " and fully acetylated aldopyranoses in the solid state. Their approach was based on (J) the assumption that the pyranoid sugar derivatives predicted by Reeves to exist in only one of the chair conformations in solution would adopt that same conformation in the crystalline state, and (2) the observation that axially and equatorially attached groups at the anomeric carbon atom give rise to different vibrations. 

The first such approach to the interpretation of optical rotation in terms of the conformational properties of carbohydrates was made by Whifien in 1956. He proposed that the observed rotation of an optically active molecule can be regarded as an algebraic summation of partial rotatory contributions of various conformational elements of asymmetry. For these contributions, empirical values were determined that allowed estimation of the net rotations of various cyclic sugars and cyclitols with fair accuracy. A more extensive treatment was presented by Brewster,and it was applied to a wide range of optically active, acyclic and cyclic compounds. The best correlations between the calculated and the experimental values were obtained with compounds that do not absorb in the near-ultraviolet and have predictable, fixed conformations, or for which the conformational populations can be reliably estimated. In the carbohydrate field, the calculations are quite simple for the poly-hydroxycyclohexanes, - and differences between the calculated and observed values for the rotation have been interpreted in terms of conformational equilibria. Similar comparisons have been made for the methyl D-aldopyranosides, although the lack of precision in the correlation does not allow a detailed treatment in terms of conformational populations. 

Neglecting contributions from boat or skew forms to the conformational population. Reeves predicted the favored chair conformation for a wide range of methyl D-aldopyranosides in aqueous solution. Only methyl a-D-idopyranoside, methyl 4,6-O-benzylidene-a- and -j8-D-idopyranoside, and methyl a- and jS-D-arabinopyranoside appeared to exist in the 1C(d) conformation. The methyl D-altro- and D-lyxo-pyranosides exhibited behavior intermediate between that expected for the two chair forms, and it was, therefore, concluded that these glycosides exist in solution in an equilibrium composed of appreciable proportions of each chair conformer. However, it was later acknowledged that boat or skew conformations might be possible were the nonbonded interactions between the axial groups in the chair conformation to become too large. 

A new method for the conversion of aldopyranosides to 5-thioaldopyranosides is exemplified in Scheme 3 the isomeric acyclic intermediates, e.g. compound 33, could be separated, inverted at C-5 and cyclized stereoselectively, thus offering access to several different products. 5-Thio-D-arabinose (34) has been synthesized from D-arabinose in six standard reaction steps. Synthesis of 5-thio-L-fucopyranose, as the tetraacetate 35, from D-arabinose required chain-extension at the non reducing end which was achieved by diastereoselective reaction of aldehyde 36 with methyl lithium the D-aftro-product 37 was then treated sequentially with tosyl chloride and potassium thioacetate. A number of aryl a-5-thiofucosides, as well as p-nitrophenyl a-l,5-dithiofucoside, were prepared from 1-acetate 35 directly or via a trichloroacetimidate. These compounds were used to demonstrate the importance of sulfur in the ring and oxygen at the... 

Table 3.1 Relative rates of acid-catalyzed hydrolysis of methyl a- and p-aldopyranosides [6] in 0.01-0.5M HCl or H2SO4 at 58-100°...

Computation of the conformational energies of 2-methoxytetrahydropyran, a model for the methyl aldopyranosides, gave results in agreement with experimental observations. A preference for the axial conformer was apparent, but the nature of the stabilizing factor (anomeric effect) was not resolved. Force-field calculations have been extended to include alcohols, ethers, simple acetals, and 2-methoxytetrahydropyran. 2 The preference of 2-methoxytetrahydropyran for the axial conformer was attributed to interaction of the axial lone-pair orbital on the ring-oxygen atom with the axial C—O bond of the methoxy-group, which can be represented by the resonance structure (501) (see also, Vol. 6, p. 163). 

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