Chapter 27: Pericyclic Reactions

Question: What product is formed when each compound in Problem 27.6 undergoes photochemical electrocyclic ring-opening or ring closure? Label each process as conrotatory or disrotatory and clearly indicate the stereochemistry around tetrahedral stereogenic centers and double bonds.

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 ${\mathbf{D}}_3$.