**6. Use of carbenes in organic synthesis**

### **6.1 Example 1**

Preparation of tetracyclic cyclopropapyrroloindole, an analogue of the mitosene antitumor agent, involves an intramolecular carbene addition to an alkene (**Figure 25**).

**Figure 25.** *Intramolecular carbene addition in the synthesis of tetracyclic indole.*

## **6.2 Example 2**

Cyclopentanes are prepared using intramolecular C-H insertions. The reactions involve the rhodium (II) catalyzed decomposition of α-diazo-β-keto esters as shown in **Figure 26**.

**Figure 26.** *Synthesis of cyclopentanones by intramolecular C-H insertion reaction.*

## **6.3 Example 3**

The β-lactam antibiotics are synthesized by using carbene reactions. This example as shown in **Figure 27** involves a Wolff rearrangement of a diazoketone to effect a ring contraction of five-membered ring.

**Figure 27.** *Carbene reaction in the synthesis of β-lactam antibiotics.*

#### **6.4 Example 4**

This example involves intramolecular O-H insertion reaction to synthesize heterocyclic compound (**Figure 28**).

*Basic Information about Carbenes DOI: http://dx.doi.org/10.5772/intechopen.100425*

**Figure 28.** *Intramolecular O-H insertion.*

### **6.5 Example 5: Arndt-Eistert reaction**

Chain extension in carboxylic acid. Diazomethane is one of the reagents used to extend the chain in carboxylic acids. Carboxylic acids are converted to acid chlorides and treated with diazomethane without isolation to form α-diazoketones. The formed diazoketone easily turns into ketenes when heated. Finally, ketene reacts with water and gives carboxylic acid (**Figure 29**).

**Figure 29.** *Chain extension in carboxylic acid by using diazomethane.*
