Acetals nucleophilic addition reactions

Acetal and hemiacetal groups are particularly common in carbohydrate chemistry. Glucose, for instance, is a polyhydroxy aldehyde that undergoes an internal nucleophilic addition reaction and exists primarily as a cyclic hemiacetal. 

Each of the following substances can be prepared by a nucleophilic addition reaction between an aldehyde or ketone and a nucleophile. Identify the reactants from which each was prepared. If the substance is an acetal, identify the carbonyl compound and the alcohol if it is an imine, identify the carbonyl compound and the amine and so forth. 

Nucleophilic addition reactions to oxathiazinane N,0-acetals proceed smoothly with al-lylsilane, alkynylzinc, silyl enol ether, and silylketene acetal reagents (Scheme 17.22)... 

Chiral acetals can be used as auxiliaries in the diastereoselective reactions of Grignard reagents with acyclic as well as cyclic a-keto acetals. Nucleophilic addition to the monoprotected diketone (69 equation 18) occurs with excellent stereoselectivity to generate the corresponding tertiary alcohol (70) as the major product, usually with greater than 95 5 selectivity. Removal of the ketal yields a-hydroxy ketones of high optical purity. In most examples, enantiomeric excesses of 95% and higher are observed in the resultant keto alcohols. Table 17 represents the results of additions to cyclic and acyclic substrates. 

Aldehydes can undergo reactions with alcohols under acidic conditions. The product of this nucleophilic addition reaction is called hemiacetal. Hemiacetal is formed by the nucleophilic addition of alcohol to the carbonyl carbon. Hemiacetal can react with one more mole of alcohol to form an acetal. 

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