**3. Microwave irradiation**

One of the potential green chemistry method using microwaves (MW) have emerged during the recent years [22, 23]. A review outlines the use of MW technique for the synthesis of N-containing heterocycles [24]. Novel 2-aryl-3,4-dihydro-2*H*thieno[3,2-*b*]indole derivatives (**Figure 10**) has been synthesised regioselectively in good yields from the reaction of 5-aryldihydro-3(2*H*)-thiophenones and arylhydrazine hydrochloride under microwaves [25]. The group also did a comparison of the efficacies of the thermal and microwave reactions, by carrying out the reaction under microwave for the synthesis of **34a** (Ar = C6H5, R = H) at the reflux temperature of 80°C. At 80°C, the reaction using microwaves was completed in 5 min, that is ten times faster than that under thermal condition (50 min) using the same solvent.

A rapid solvent-less synthesis of 5-hydroxy-benzo[*g*]indole scaffolds (**Figure 11**) is accomplished from Lewis acid-catalyzed one-pot reaction of naphthoquinone, ωmorpholinoacetophenone, and urea under microwave irradiation [26]. To investigate the role of Lewis acid, the researchers also carried microwave-mediated reaction using Lewis acids like TiCl4, AlCl3, ZnCl2, SmCl2, and InCl3 under similar reaction conditions. They found that in comparison to BF3OEt2 catalyzed reaction, all the other Lewis acids gave poor yields. Also the reaction failed to proceed in the absence of the Lewis acid (**Figure 12**).

Researchers have developed a novel, convenient, environmentally friendly one-pot synthesis of imidazo[1,2-a]pyridines (**Figure 13**) using 2-aminopyridines and in-situ generated phenacyl bromides under microwave irradiation in polyethylene glycol (PEG-400) and water (1:2) [27]. This protocol is a better alternative to the existing method as it involves use of in-situ-generated α-bromoacetophenones, utilization of

**Figure 10.**

*Construction of novel 2-aryl-3,4-dihydro-2*H*-thieno[3,2-*b*]indole derivatives.*

**Figure 11.** *Solvent-less synthesis of 5-hydroxy-benzo[*g*]indole scaffolds.*

**Figure 12.** *Proposed mechanism for the synthesis of benzo[*g*]indole derivative (38a).*

**Figure 13.** *One-pot synthesis of imidazo[1,2-a]pyridines.*

*Greener Approach towards the Synthesis of Nitrogen Based Heterocycles DOI: http://dx.doi.org/10.5772/intechopen.108489*

#### **Figure 14.** *Synthesis of a series of substituted quinolines.*

lachrymatric α-haloketones and volatile toxic organic solvents is avoided. There is reduction in the reaction time also to obtain excellent yield.

A series of substituted quinolines (**Figure 14**) was developed via the Friedländer reaction employing microwave irradiation (MW), in the presence of a catalytic amount of hydrochloric acid [28]. The products were tested in vitro against the parasites causative of malaria, leishmaniasis, sleeping sickness and Chagas' disease (TDR, WHO). Some of these compounds exhibited activity against *Plasmodium falciparum* and others resulted moderately active against *Trypanosoma cruzi*. The MW syntheses were carried out in a domestic oven adapted for the use of a reflux condenser, with constant power of (400 W). All the reactions were completed between 1.5 and 12 min.

In search for green reactions leading to the Formation of N-Heterocycles, an excellent review was identified entitled "More Sustainable Approaches for the Synthesis of N-Based Heterocycles", published in 2009 [29]. A recent review on multicomponent green synthesis of N-containing heterocycles using mixed oxides as heterogeneous catalysts is also reported [30]. A group of workers had recently published review on "Eco-friendly and sustainable synthetic approaches to biologically significant fused N-heterocycles" [31]. Other groups had also presented well-documented work on the synthesis of heterocyclic compounds using greener methods [32–34].
