Question 31.16 - Organic Chemistry

Question

Treatment of cholesterol with mCPBA results in formation of a single epoxide A, with the stereochemistry drawn. Why isn’t the isomeric epoxide B formed to any extent?

Step-by-Step Solution

Answer

Answer

The epoxide in B is above the plane on the same side as ${\mathrm{CH}}_3$ groups. In B, the epoxidation on the double bond is more sterically hindered and is less accessible. 

Step-by-Step Solution Step 1: Epoxidation

Epoxidation is converting the C-C double bond into epoxides by using reagents like hydrogen peroxide and mCPBA.

A three-membered ring is formed when oxygen is joined to each of two C-C double bonds, and a double bond is converted to a single bond.

Step 2: Reason for isomeric epoxide B cannot be formed to any extenta

The four rings of cholesterol are all in the same plane. ${\mathrm{CH}}_3$ groups are bulkier and are present above the plane at 18 and 19^th positions.

The epoxide in A is favored because epoxidation occurs from below the plane, which is on the opposite side of the ${\mathrm{CH}}_3$ groups. These groups shield the top of the molecule by the attack of reagents.

The epoxide in B is above the plane on the same side as ${\mathrm{CH}}_3$ groups. In B, the epoxidation on the double bond is more sterically hindered and is less accessible. So, this path is disfavored.