Acetal linkages acid hydrolysis

Naturally occurring compounds called cyanogenic glycosides, such as lotau-stralin, release hydrogen cyanide, HCN, when treated with aqueous acid. The reaction occurs by hydrolysis of the acetal linkage to form a cyanohydrin, which then expels HCN and gives a carbonyl compound-fa) Show the mechanism of the acetal hydrolysis and the structure of the cyanohydrin that results. 

Glycosyl-linkages were determined by GC-EIMS of the partially methylated alditol acetates. RG-II samples (2 mg) were methylated using sodium methyl sulfmyl carbanion and methyl iodide in dimethyl sulfoxide [24] followed by reduction of the uronosyl groups with lithium triethylborodeuteride (Superdeuteride , Aldrich) [23,25]. Methylated and carboxyl-reduced samples were then submitted to acid hydrolysis, NaBIlt reduction and acetylation, partially methylated alditol acetates being analysed by EIMS on two fused-silica capillary columns (DB-1 and DB-225) [20]. 

Depolymerization of the permethylated carbohydrate is achieved by hydrolysis with acid. Under these conditions, the amino sugar residues are N-deacetylated, and the aminohexosidic linkages become resistant to hydrolysis. Stellner and coworkers29 showed that, when the acid degradation is conducted in 95% acetic acid, the amino sugar residues are also liberated, and can be analyzed by the methylation technique.29 Therefore, acetolysis followed by acid hydrolysis is now commonly used, as it allows the analysis both of hexose and hexosamine residues. 

The main cyanogenic glycoside in laurel is prunasin, the P-o-glucoside of benzaldehyde cyanohydrin. The enzymic hydrolysis of prunasin may be visualized as an acid-catalysed process, first of all hydrolysing the acetal linkage to produce glucose and the cyanohydrin. Further hydrolysis results in reversal of cyanohydrin formation, giving HCN and benzaldehyde. 

Gums are tasteless, odorless, colorless, and nontoxic. None, except the starches and starch derivatives, are broken down by human digestive enzymes. All are subject to microbiological attack. All can be depolymerized by acid- and enzyme-catalyzed hydrolysis of the glycosidic (acetal) linkages joining the monomeric (saccharide) units. 

The remaining four hydroxyl groups can be methylated in basic solution by dimethyl sulfate or by methyl iodide and silver oxide in N,N-dimethyl-methanamide, HCON(CH3)2, solution. Hydrolysis of either of these penta-methyl glucose derivatives with aqueous acid affects only the acetal linkage and leads to a tetramethylated glucose, 20, as shown in Figure 20-4. 

Figure III-16 Acid-catalyzed hydrolysis of acetal linkages in a-methyl-D-lactoside.

---