Glycopyranosides hydrolysis

In their study of Bronsted acid induced cleveage of a [with axial C(l)-OMe] and (3 [with equatorial C(l)-OMe] glycopyranosides, Fraser-Reid et al.46 demonstrated that the (3/a rate ratios for hydrolysis of methyl pyranosides (Table 5) can be explained by the different intermediates and transition-state structures through which proceed the heterolysis of a and [3 isomers (Fig. 18). 

The Rates of Hydrolysis of Some Glycopyranosides in 2 M Hydrochloric Acid at 803... 

The accepted mechanism of hydrolysis of glycopyranosides involves intermediate, oxocarbonium-ion formation at C-l, with attainment of a half-chair conformation,32 and it has been postulated33 that the rate of hydrolysis of glycopyranosides will be affected by the relative ease of attainment of this conformation, in which C-2, C-l, the ring O, and C-5 lie in one plane. Bulky 5-substituents may be ex-... 

The relatively rapid hydrolysis of glycofuranosides by acids was the first property used for differentiating these compounds from the more normally encountered, more stable glycopyranosides. In Tables VIII and IX are assembled data from the literature the rate constants are given in sec.-1 and common logarithms.118 The conditions of acid concentration and temperature varied in the data reported,48 48 the conditions were the same, and so the rate constants can be compared. Heidt and Purves188 used a term, fc = fc/[H], to eliminate differences in the concentration of acid used. 

Acid Hydrolysis of Some Alkyl Glycofuranosides and Glycopyranosides... 

Relative Rates of Acid-catalyzed Hydrolysis of Some Methyl Glycopyranosides ... 

Acid hydrolysis of the acetal linkages in wood polysaccharides follows the normal hydrolysis of glycopyranosides with fission of the... 

Under alkaline conditions hydrolysis of glycopyranosides is much slower and proceeds only at higher temperatures. The mechanism may involve intramolecular displacement of the exocyclic glycosidic oxygen with formation of a cyclic 1,6-anhydroglycopyranose (5). 

Tables IV and V provide a comparison between the rates of hydrolysis of the above methyl glycopyranosides and the degree of instability or strain in the chair conformations of their pyranose rings. Using the Reeves method, the instability factors are denoted by the position of the axial substituents and by that arrangement in which an axial hydroxyl group on C2 bisects the oxygen valences of Cl (XXII), denoted by A2.

Bols and coworkers reported linear free energy relationships for the hydrolysis of glycopyranosides.48 Axial and equatorial substituent effects, determined from amine basicity of similarly configured compounds, were employed.49 The two reactions studied were the spontaneous hydrolysis of 2,4-DNP glycosides, and the specific acid-catalyzed hydrolysis of methyl glycosides (Scheme 21). 

Obviously, few of the results presented thus far can be taken as unequivocal evidence for the cyclic mechanism (see equation 1). However, there is little, if any, evidence for the acyclic mechanism (equation 2) (see, however. Ref. 1), and it is generally agreed that the acid-catalyzed hydrolysis of glycopyranosides proceeds through a cyclic, carbonium-ion intermediate. The following discussions will, therefore, be based on the first mechanism. 

Feather and Harris gave a slight variation of Edward s explanation, which appears to correlate satisfactorily the relative hydrolysis-rates of various glycopyranosides. Rates for groups of methyl aJdopyranosides compared by them are given in Table X. In this Table, the rates used to determine relative rates were measured under a variety of conditions. The effects of the type of acid and Ho have been noted in Section 11,1 (see p. 27), and the effect of temperature is obvious. The differences introduced by the variety of conditions probably do not affect the conclusion, however. 

The differences in rates of hydrolysis between methyl glycopyranosides... 

Rate Constants and Kinetic Parameters for the Hydrolysis of D-Glycopyranosides ... 

Snyder and Link had investigated the acid hydrolysis of some o- and p-nitrophenyl glycosides (see Table XXVII), and found that the ease of hydrolysis is in the same order as has been found for alkyl glycopyranosides ... 

The introduction into a sugar of a halogen atom in place of a hydroxyl group might influence the general reactivity either by steric or electronic effects. Such effects have been extensively discussed with respect to the acid hydrolysis of methyl glycopyranosides, and it is generally considered that, except for substituents at C-2, steric factors predominate. 

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