Q.58. - Organic Chemistry

Question

Question: Epoxidation of the following allylic alcohol using the Sharpess reagent (-)-DET gives two epoxy alcohols in a ratio of 87:13.

1. Assign structures to the major and minor product.
2. What is the enantiomeric excess in this reaction?

Step-by-Step Solution

Answer

Answer

a.

b. The enantiomeric excess product is shown below.

Step 1: Epoxidation

Epoxidation of double bonds can be done in many ways. 

Sharpless epoxidation is a method that favors only one product of the possible stereoisomers based on whether the reagent DET is dextrorotatory or leavorotatory during the reaction.

Step 2: Epoxidation

The addition of oxygen to an alkene forms a cyclic ether known as epoxide (ethylene oxide). 

During Sharpless epoxidation, the stereochemistry of the reaction plays a major role.

• (+)-DET causes a bottom face attack on the double bond.
• (-)-DET causes a top face attack on the double bond of the alkene.

Step 3: Products of the given reaction

The products of the given reaction are shown below.

a.

Sharpless reaction

 

b. The first product is enantiomeric excess.

Enantiomeric excess epoxide