Protection of Alcohols by Acetal Formation

If alcohols can be used to protect carbonyl compounds by formation of an acetal (or ketal), then the same method can protect an alcohol using a carbonyl compound. For example, victual diols can be protected by forming a cyclic five-membered ketal with acetone. Similarly, 1,3-diols react with 

2-Dimethoxypropane drives such reactions to completion. It is an unstable ketal that reacts with the water generated by the reaction of the diol with acetone. 

Alcohols that contain only one hydroxyl group can also be protected by forming an acetal. Alcohols react regiospedficaUy with dihydropyran in an acid-cataly2ed addition reaction. The carbocation formed is resonance stabihzed by the lone pair electrons of the ring oxygen atom. Subsequent attack of the carbocation by the nucleophilic oxygen atom of the alcohol followed by deprotonation yields an alkoxy-substituted tetrahydropyran known as aTHP derivative. 

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