Question

What is cyclic acetal?

Answer 1

A cyclic acetal is formed from the reaction of an aldehyde with a diol. You see these as protecting groups in carbonyl chemistry.

Suppose you had an acid catalyst. Then you have the following mechanism:

1. With the aldehyde already protonated by the hydronium, the diol is a good enough nucleophile to backside-attack the carbonyl carbon.
2. Proton transfer Pt1.
3. Proton transfer Pt2.
4. Tetrahedral collapse.
5. INTRAmolecular backside-attack of the carbonyl carbon (faster than INTERmolecular acid/base reaction).
6. Regeneration of the acid catalyst.