Question

Question. Under the right conditions, the following acid-catalyzed double cyclization proceeds in remarkably good yields. Propose a mechanism. Does this reaction resemble a biological process you have seen?

Short answer

Answer

This process resembles the cyclization of squalene oxide to lanosterol.

Step-by-step solution

Acid-catalyzed double cyclization

In double cyclization, the oxygen in epoxide abstracts the proton from the acid and acquires positive charge and the ring opens up to release strain and carbocation gets formed. By neutralization of charge, a final product formed by double cyclization which is intramolecular in nature.

Acid-catalyzed double cyclization

In the first step of double cyclization, the oxygen in epoxide abstracts the proton from the acid and acquires positive charge which is unstable and also the strain on three-membered ring also increases, thus ring opens up to release strain and carbocation gets formed. After that, alkene carbon acts as nucleophile as it is electron rich and intramolecularly attack at the carbocation which in turn opens up possibility for another cyclisation reaction to occur at alkene carbon. Then after neutralization of charges, we get our final product which is formed by double cyclization.

Double cyclization reaction

Resemblance of the reaction with biological process

This process of double cyclization reaction resembles the cyclization of squalene oxide to lanosterol. Pharmaceutical synthesis of steroids uses the similar type of reaction which is known as biomimetic cyclization.