Question

Question: The biosynthesis of lanosterol from squalene has intrigued chemists since its discovery. It is now possible, for example, to synthesize polycyclic compounds from acyclic or monocyclic precursors by reactions that form several C-C bonds in a single reaction mixture.

1. Draw a stepwise mechanism for the following reaction.
2. Show how X can be converted to 16, 17-dehydroprogesterone. (Hint: See Figure 24.5 for a related conversion.

Short answer

Answer

a.

b.

Step-by-step solution

Biosynthesis reaction

Biosynthesis reactions involve complex structures in living organisms, and larger rings are synthesized from smaller rings by the ring-expansion reactions.It involves oxidative cleavage of alkenes and intermolecular aldol reactions.

Stepwise mechanism for the reaction 

a. A stepwise mechanism involves the reaction with water and forming a carbocation. Again, on reaction with water, the required compound is formed.

The stepwise mechanism for the reaction

Conversion of X to 16,17-dehydroprogesterone

b. X compound underwent oxidative cleavage of an alkene with ozone $\left(O_3\right)$, followed by Zn, $H_{2}O$. After that, an intermolecular aldol reaction occurs in the dilute $O{H}^{-}$ and $H_{2}O$; it forms 16,17-dehydroprogesterone.

Conversion of X to 16,17-dehydroprogesterone