Question

Question: Vitamin ${\mathbf{D}}_3$, the most abundant of the D vitamins, is synthesized from 7-dehydrocholesterol, a compound found in milk and fatty fish, such as salmon and mackerel. When the skin is exposed to sunlight, a photochemical electrocyclic ring-opening forms provitamin ${\mathbf{D}}_{\mathbf{3}}$, which is then converted to vitamin ${\mathbf{D}}_3$ by a sigmatropic rearrangement (Section 27.5). Draw the structure of provitamin ${\mathbf{D}}_3$.

Short answer

Answer

Step-by-step solution

Electrocyclic ring-opening reactions

The reaction that involves the conversion of a cycloalkene to a polyene by the movement of electrons under thermal or photochemical conditions is referred to as the electrocyclic ring-opening reaction.

Sigmatropic rearrangement reactions

The intramolecular pericyclic reaction that involves the cleavage of a sigma bond, rearrangement of a pi bond followed by the formation of a sigma bond yielding the product is named the sigmatropic rearrangement reaction.

Conversion of 7-dehydrocholesterol to provitamin D3

The conjugated system undergoes electrocyclic ring-opening under photochemical conditions.

Electrocyclic ring-opening of 7-dehydrocholesterol