Glycosides acid-catalyzed hydrolysis

This reaction has been used m an imaginative way to determine the ring size of glycosides Once all the free hydroxyl groups of a glycoside have been methylated the glycoside is subjected to acid catalyzed hydrolysis Only the anomeric methoxy group IS hydrolyzed under these conditions—another example of the ease of carbocation for matron at the anomeric position... 

Because of its importance to carbohydrate technology, biochemistry, and physical organic chemistry, the hydrolytic cleavage of glycosides has been extensively studied with respect to both acid and enzymic catalysis. Reviews on the acid-catalyzed hydrolysis have been presented by BeMiller, Capon, ... 

Scheme I.—Acid-catalyzed Hydrolysis of) -Glycoside (Hydroxyl substituents omitted).

The implications of the above observations may be important, especially if similar trends are observed in pyranose anomers. For example, with respect to the mechanism of acid-catalyzed hydrolysis of pyranosides, endocyclic C-0 bond cleavage (preceeded by 05 protonation) may be assisted in P-anomers in which the Cl-01 bond is equatorial, since the 04-Cl bond may already be extended in these anomers. By a similar argument, exocyclic C-0 scission (preceeded by 01 protonation) may be assisted in the hydrolysis of a-pyranosides in which the Cl-01 is axial and extended, thus resembling the transition state. Post and Karplus have recently suggested that enzyme-catalyzed glycoside hydrolysis of P-pyranosides may indeed take place by ring oxygen protonation, followed by endocyclic C-0 bond scission. 

The descriptive term alkali-sensitive glycoside has special significance to one concerned with a study of the chemistry of carbohydrate substances, for experience often refutes the generalization that the gly-cosidic linkage is stable to alkalis. The acid-catalyzed hydrolysis of glycosides is well known, and is characteristic of the acetal structure they possess.1 However, it is not practical to assume that most glycosides have... 

Figure 16. Acid-catalyzed hydrolysis of a glycosidic linkage...

N( 1 j,AY7)-diplatinated species exhibit a significant pH-independent depu-rination. With dlno and dAdo, these reactions become faster than the acid-catalyzed hydrolysis of the uncomplexed nucleoside at pH > 5 and at pH > 3, respectively [45], Addition of aquated Pdn(dien) to the solutions of A(7),AT(7)-diplatinated dlno and dAdo markedly accelerates depurination reactions. It has been proposed that this rate enhancement is due to the binding of Pdn(dien) to the N(3) site of the purine moiety, which destabilizes the AT-glycosidic bond [45],... 

An opposite reaction to the acid-catalyzed hydrolysis is the above-mentioned reversion. Acids can also catalyze the formation of anhydro sugars (see Section 2.3.4). Reversion tends to result in formation of (1—>6)-glycosidic bonds. The degradation of pentoses and uronic acids into furfural and of hexoses into hydroxymethylfurfural, levulinic, and formic acids are also important acid-catalyzed reactions, which, however, require concentrated acid and higher temperatures (Fig. 2-31). 

Very mild acid, or dilute acid at or below room temperature, has little effect on nucleic acids. Indeed, dilute acids (e.g., trichloroacetic, HCIO4, HC1) at 0 to 25°C are routinely used to precipitate (isolate) nucleic acids and other polar macromolecules. Somewhat stronger acidic conditions (prolonged exposure to dilute acid or dilute acid at increased temperature or acid strength) cause depurination, i.e., acid-catalyzed hydrolysis of purine N-glycosides. Thus, 15 min of treatment with 1 N HC1 at 100°C removes most of the purines from DNA and RNA. Much harsher acidic conditions (several hours at 100°C or more concentrated acid) are required to remove pyrimidine N-glycosides. Some limited... 

Beelik, Andrew, Kojic Acid, 11,145-183 Bell, D. J., The Methyl Ethers of D-Galactose, 6,11-25 BeMiller, J. N., Acid-catalyzed Hydrolysis of Glycosides, 22, 25-108 BeMiller, J. N. See also, Whistler,... 

A second property of many of the monoterpene glycosides is their high reactivity in weakly acid media ( ). Acid-catalyzed hydrolysis at pH 3.0-3.5 of these non-volatile and flavorless glycosidic compounds, can give volatile monoterpenes, sane of vdiich have significant sensory properties. Accordingly, the presence of many monoterpenes found in grapes and wines can be accounted for by non-enzymatic reactions (] ). 

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