Question

What lactol (cyclic hemiacetal) is formed from intramolecular cyclization of each hydroxy aldehyde?

a.

b.

Short answer

a.

b.

Step-by-step solution

Cyclic hemiacetals

Acyclic hemiacetals are regarded to be unstable. Cyclic hemiacetals comprising either a five-membered or a six-membered ring are considered to be stable and can be easily isolated.

Formation of cyclic hemiacetals by intramolecular cyclization

The hydroxyl group is incorporated into the carbonyl group by nucleophilic addition, which results in hemiacetal formation. The intramolecular cyclization of a hydroxyl aldehyde leads to the creation of a cyclic hemiacetal.

Lactol formed by the intramolecular cyclization of each aldehyde

The nucleophilic addition of a hydroxyl group to the carbonyl group leads to the formation of a hemiacetal.

a. The product formed on the intramolecular cyclization of the given aldehyde is 3-methyltetrahydro-2H-pyran-2-ol. The cyclic hemiacetal of the compound a can be given as:

Lactone formed by the intramolecular cyclization of compound a

b. The product formed on the intramolecular cyclization of the given aldehyde is 5-methyltetrahydrofuran-2-ol. The cyclic hemiacetal of the compound b can be given as:

Lactone formed by the intramolecular cyclization of compound b