Question

What carbonyl compound and CBS reagent are needed to prepare X, an

intermediate in the synthesis of ezetimibe (trade name Zetia), a drug that lowers cholesterol levels by inhibiting its absorption in the intestines.

Short answer

Answer

The carbonyl compound needed to prepare X is shown below:

(R)-CBS reagent is used since it provides hydrides that can attack the carbonyl carbon center position from the back and yields alcohol having an S-configuration.

Step-by-step solution

Step-by-Step Solution Step 1: Function of CBS

CBS is a chiral reducing agent. It is a boronic compound that functions as an asymmetric reduction and forms enantiomeric alcohols.

Stereoselectivity of CBS

(S)-CBS reagent provides hydrides that can attack the carbonyl carbon center from the front and yields alcohol having an R-configuration, where the alcohol group lies in the dash position.

The hydrides from (R)-CBS attacks the carbonyl carbon center from the back and yields alcohol having an S-configuration, where the alcohol group lies in the wedge position.

Hence, based on the explanations, the carbonyl compound needed to prepare X is shown below:

(R)-CBS reagent is used since it provides hydrides that can attack the carbonyl carbon center from the back and yields alcohol having an S-configuration.