Chapter 28, Problem 28.25P

Interpretation Introduction

Interpretation:

When previtamin D₂ (which is identical to previtamin D3, p. 1478, except for the R-group) is isolated and irradiated, ergosterol is obtained along with a stereoisomer, lumisterol. The origin of lumisterol is to be explained mechanistically.

Concept introduction:

Electrocyclic reactions are a pericyclic reaction which occur intramolecularly. These reactions will result in the formation of ring compounds under the influence of heat or light. Notably, in this process one new sigma bond is formed and one old π-bond is consumed. Intriguingly, the reverse ring opening electrocyclic reaction can also be possible to occur under the same reaction mechanism but in reverse manner. In phase orbital overlap results in symmetry allowed electrocyclic reactions. Selection rules of electrocyclic reactions are;

No. of electrons	Activation mode	Stereochemistry of rotation
4n	Thermal Photochemical	Con Dis
4n + 2	Thermal Photochemical	Dis Con