Abstract
Heterocyclic ring systems are gaining attention due to their pivotal role in drug design and medicinal chemistry. Quinazolines are nitrogen-containing heterocyclic pharmacophoric units found in abundance in natural and pharmaceutical products. Imidazoquinazolines and benzimidazoquinazolines are fused tricyclic and tetracyclic heterocyclic moieties, respectively. Different isomeric forms of imidazoquinazolines and benzimidazoquinazolines exhibited a plethora of biological applications such as antitumor, antimicrobial, antioxidant, anti-inflammatory, antitubercular, anticancer, antihypertensive, anticonvulsant, antiviral, antimalarial, antiapoptotic, anti-proliferative activities, etc. This chapter addressed the recent synthetic strategies for medicinally privileged scaffolds; imidazoquinazolines and benzimidazoquinazolines. The synthetic routes of various isomeric forms of above-mentioned heterocycles have also been discussed.
Keywords
- Quinazoline
- Imidazoquinazoline
- Benzimidazoquinazoline
- synthetic methodologies
- medicinal importance
1. Introduction
Heterocycles gain much attention because of their vast applications in biology [1, 2, 3] and material sciences [4, 5]. Heterocycles possessed a lot of medicinal benefits [6, 7, 8, 9] and played an important role in drug design and development. Among the heterocycles, quinazoline is a bicyclic
Imidazole and benzimidazole are heterocyclic moieties and important pharmacophores in medicinal chemistry. Imidazole is a five-membered aromatic heterocycle that exhibits a number of biological applications, such as antibacterial [18], anticancer [19], antiepileptic [20], antitubercular [21] activities, etc. Benzimidazoles are privileged structures related to their roles in medicinal chemistry, e.g., they play their role in antibacterial [22], antidiabetic [23], antiviral [24], antiulcer [25] activities, etc. Drugs containing quinazoline, imidazole, and benzimidazole are given in Figure 1.
Imidazoquinazoline and benzimidazoquinazoline moieties play significant roles as active biological agents. Imidazoquinazoline I is a PI3K inhibitor [26], imidazoquinazoline II is antiapoptotic [27] and benzimidazoquinazoline III is antitumor [28] in its action (Figure 2).
2. Imidazoquinazoline
Imidazoquinazoline is a fused tricyclic structure containing imidazole and quinazoline and has three nitrogen atoms in its molecular architecture. It is an important scaffold in drug molecules such as antithrombotic and anticardiotonic agents [29]. Certain derivatives of imidazoquinazolines show a plethora of biological applications, i.e., antitumor [30], anticonvulsant [31], antihypertensive [32], etc.
Structures of imidazoquinazoline and their different isomeric forms are given in Figures 3 and 4.
2.1 Imidazo[1,2-c ]quinazoline
Imidazo[1,2-
2.1.1 One-pot exhaustive dehydrogenation
An expedient way to synthesize imidazo [1,2-
2.1.2 Ullmann coupling reaction
Imidazo[1,2-
2.1.3 Conventional or oxidative coupling
Imidazo[1,2-
2.1.4 Intramolecular C-N coupling
Imidazo[1,2-
2.2 Imidazo[1,5-c ]quinazline
Imidazo[1,5-
2.2.1 Oxidative domino sp3 C–H amination
An oxidative domino synthesis pathway for the formation of imidazo [1,5-
2.3 Imidazo[2,1-b ]quinazoline
Imidazo[2,1-
2.3.1 Cascade reaction
A cascade microwave-promoted reaction involving Claisen-Schmidt, aza-Michael, and cyclization reactions in the formation of imidazo[2,1-
2.4 Imidazo[4,5-g ]quinazoline
Imidazo[4,5-
2.4.1 Reductive coupling
Imidazo[4,5-
3. Benzimidazoquinazoline
Benzimidazoquinazoline is a fused tetracyclic structure containing benzimidazole and quinazoline, having three nitrogen atoms in its molecular architecture. Benzimidazoquinazoline derivatives act as potent immunosuppressors [54] and possess promising antitumor activity [55] as well.
Structures of benzimidazoquinazoline and their different isomeric forms are given in Figures 5 and 6.
3.1 Benz[4,5]imidazo[1,2-c ]quinazoline
Benzo[4,5]imidazo[1,2-
3.1.1 Conventional or oxidative coupling
Benz[4,5]imidazo[1,2-
A domino synthetic approach for benz[4,5]imidazo[1,2-
3.1.2 Intramolecular C-N coupling via C-X activation
Another approach to synthesize benz[4,5]imidazo[1,2-
3.1.3 Ullmann N -arylation reaction
An easy way found to synthesize benz[4,5]imidazo[1,2-
3.1.4 Directed arylic C-H amidation
Another efficient protocol reported to synthesize benz[4,5]imidazo[1,2-
3.1.5 Double C-H functionalization
A metal-free pathway for the synthesis of benz[4,5]imidazo[1,2-
3.2 Benz[4,5]imidazo[1,2-a ]quinazoline
Here are the methods for the synthesis of another isomer, benz[4,5]imidazo[1,2-
3.2.1 Transition metal-free tandem process
One-pot regioselective synthesis of benz[4,5]imidazo[1,2-
3.2.2 Intramolecular C: N bond formation
Benz[4,5]imidazo[1,2-
4. Conclusion
The fused heterocyclic moieties tend to achieve the top of the list position in drug design because of their wide range of pharmacological applications. In this context, this chapter is focused on the different synthetic approaches of imidazoquinazolines and benzimidazolquinazolines. These biological scaffolds were prepared by a large number of synthetic routes that are summarized in this chapter. These routes include Ullmann cross-coupling reaction, Claisen–Schmidt reaction, Aza–Michael reaction, cyclization reaction, Cu(OTf)2 catalyzed reaction, Iodine-mediated oxidative annulation reaction, oxidative and nonoxidative C-N coupling reaction, photoredox catalyzed synthesis, metal-free synthesis, green synthesis, microwave mediated synthesis, oxidative domino synthesis, transition metal-free and transition metal-catalyzed tandem processes. These synthetic strategies will be helpful in bringing novelty to the synthesis of bioactive compounds and in exploring a new area of medicinal research.
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