Q.66 - Organic Chemistry

Question

Draw a stepwise mechanism for the following reaction. The four-membered ring in the starting material and product is called a β-lactam. This functional group confers biological activity on penicillin and many related antibiotics, as is discussed in Chapter 22. (Hint: The mechanism begins with β elimination and involves only two steps.)

Step-by-Step Solution

Answer

The mechanism is shown in the following:

Definition of β elimination

The type of reaction where simultaneously the proton of $\mathbf{\text{β}}$ carbon is abstracted, and removal of leaving group from $\mathbf{\text{α}}$ carbon takes place to give an elimination product is known as $\mathbf{\text{β}}$ elimination.

Type of base used

DBN is a strong base that abstracts a proton from  $\text{β}$ carbon. The negative charge developed in the $\text{β}$carbon then attacks the $\text{α}$  carbon containing the leaving group, and that results in a cyclopropane intermediate, as shown.

Representation of the  beta elimination reaction

Attack on the cyclopropane ring by the lone pair over sulfur

The sulfur-containing lone pair attacks the cyclopropane ring to form a six-membered ring. A six-membered ring is more stable than a four-membered ring, so this mechanism is feasible.

 

The tertiary carbocation developed in the structure forms a double bond by breaking the cyclopropane ring, as shown below.

Representation of the attack of lone pair of sulfur

The overall mechanism for the formation of the product is shown below:

Representation of the detailed reaction scheme