Chapter 31, Problem 31.28P

Draw the products formed when cholesterol is treated with each reagent. Indicate the stereochemistry around any stereogenic centers in the product.

a. ${\text{CH}}_{\text{3}}\text{COCl}$

b. ${\text{H}}_{\text{2}}$, $\text{Pd}$

c. PCC

d. oleic acid, ${\text{H}}^{+}$

e. [1] ${\text{BH}}_{\text{3}}$ THF; [2] ${\text{H}}_{\text{2}}{\text{O}}_{\text{2}}$, ${\text{OH}}^{-}$

Interpretation Introduction

(a)

Interpretation: The product formed on treatment of cholesterol with ${\text{CH}}_{\text{3}}\text{COCl}$ is to be predicted.

Concept introduction: In the presence of base, the hydroxyl group loses its proton to form alkoxide. The alkoxide attacks on the carbonyl carbon of acyl chloride or anhydride to leading to the formation of acyl group and removal of chloride. This process is known as acylation.

Answer to Problem 31.28P

The product formed on treatment of cholesterol with ${\text{CH}}_{\text{3}}\text{COCl}$ is,

Figure 1

Explanation of Solution

In presence of base and acetyl chloride, the hydroxyl group of cholesterol undergoes acetylation. The corresponding chemical reaction is shown below.

Figure 2

Conclusion

The product formed on treatment of cholesterol with ${\text{CH}}_{\text{3}}\text{COCl}$ is shown in Figure 1.

Interpretation Introduction

(b)

Interpretation: The product formed on treatment of cholesterol with ${\text{H}}_{\text{2}}$, $\text{Pd}$ is to be predicted.

Concept introduction: The addition of ${\text{H}}_{\text{2}}$ in presence of catalyst is known as catalytic hydrogenation reaction. This is a syn addition. Alkene is reduced to alkane. In this reaction weak pi bond of alkene and sigma bond between ${\text{H}}_{\text{2}}$ is broken and two $\text{C}-\text{H}$ bonds are formed.

Answer to Problem 31.28P

The product formed on treatment of cholesterol with ${\text{H}}_{\text{2}}$, $\text{Pd}$ is,

Figure 3

Explanation of Solution

In presence ${\text{H}}_{\text{2}}$ with $\text{Pd}$ catalyst the pi bond in cholesterol is reduced. The corresponding chemical reaction is shown below.

Figure 4

Conclusion

The product formed on treatment of cholesterol with ${\text{H}}_{\text{2}}$, $\text{Pd}$ is shown in Figure 3.

Interpretation Introduction

(c)

Interpretation: The product formed on treatment of cholesterol with PCC is to be predicted.

Concept introduction: Alcohols are oxidized to different carbonyl compounds depending upon the reagents and alcohol used. In presence of strong oxidizing reagents such as ${\text{KMnO}}_{\text{4}},{\text{K}}_{\text{2}}{\text{Cr}}_{\text{2}}{\text{O}}_{\text{7}}$ and ${\text{CrO}}_{\text{3}}$, primary alcohols are oxidized to carboxylic acids and secondary alcohols are oxidized to ketones. In presence of mild oxidizing agents such as $\text{PCC}$, primary alcohols are oxidized to aldehydes and secondary alcohols to ketones.

Answer to Problem 31.28P

The product formed on treatment of cholesterol with PCC is,

Figure 5

Explanation of Solution

On treatment of cholesterol with PCC, the secondary alcohol is oxidized to ketone. The corresponding chemical reaction is given below

Figure 6

Conclusion

The product formed on treatment of cholesterol with PCC is shown in Figure 5.

Interpretation Introduction

(d)

Interpretation: The product formed on treatment of cholesterol with oleic acid, ${\text{H}}^{+}$ is to be predicted.

Concept introduction: In presence of acid, the carboxylic acids are converted to ester using alcohols. This process is known as esterification.

Answer to Problem 31.28P

The product formed on treatment of cholesterol with oleic acid, ${\text{H}}^{+}$ is,

Figure 7

Explanation of Solution

In the presence of acid, the hydroxyl group of cholesterol reacts with oleic acid to form ester. The corresponding chemical reaction is shown below.

Figure 8

Conclusion

The product formed on treatment of cholesterol with oleic acid, ${\text{H}}^{+}$ is shown in Figure 7.

Interpretation Introduction

(e)

Interpretation: The product formed on treatment of cholesterol with 1. ${\text{BH}}_{\text{3}}$ THF; 2. ${\text{H}}_{\text{2}}{\text{O}}_{\text{2}}$, ${\text{OH}}^{-}$ is to be predicted.

Concept introduction: In the presence of borane, hydroperoxide and base the alkenes are converted to alcohols. In this reaction hydrogen atom and hydroxyl groups are added to the double bond. This reaction takes place in anti-markovnikov addition manner. This is a syn addition.

Answer to Problem 31.28P

The product formed on treatment of cholesterol with 1. ${\text{BH}}_{\text{3}}$ THF; 2. ${\text{H}}_{\text{2}}{\text{O}}_{\text{2}}$, ${\text{OH}}^{-}$ is,

Figure 9

Explanation of Solution

In the presence of borane, hydroperoxide and base the double bond of cholesterol is oxidized to alcohol. The corresponding chemical reaction is shown below.

Figure 10

Conclusion

The product formed on treatment of cholesterol with 1. ${\text{BH}}_{\text{3}}$ THF; 2. ${\text{H}}_{\text{2}}{\text{O}}_{\text{2}}$, ${\text{OH}}^{-}$ is shown in Figure 9.