PROTECTION FOR THE CARBONYL GROUP

During a synthetic sequence, a carbonyl group may have to be protected against attack by various reagents such as strong or moderately strong nucleophiles including organometallic reagents; acidic, basic, catalytic, or hydride reducing agents; and some oxidants. The most useful protective groups are the acyclic and cyclic acetals or ketals, and the acyclic or cyclic thio acetals or ketals. The protective group is introduced by treating the carbonyl compound in the presence of acid with an alcohol, diol, thiol, or dithiol. If one carbonyl is conjugated with a double bond, the unconjugated carbonyl is selectively protected. This generalization appears to be independent of ring size. Simple aldehydes are generally selectively protected over simple ketones. The most common method to prepare a ketal is to treat the carbonyl compound with ethylene glycol and an acid at reflux with a solvent that will azeotrope water using a Dean–Stark trap.