Abstract
In the twenty-first century, the most significant task for scientists has been the development of novel and effective therapeutic agents. Many natural chemicals have been discovered and identified as having biological functions in the body that are beneficial to maintaining a healthy lifestyle. In addition, various classes of compounds are synthesized and assessed for their potential use in biological systems. Schiff bases have the capacity to react and form complexes with metals and proteins due to their superior basic characteristics; hence, the synthesis and pharmacological application of Schiff bases has recently become one of the most popular chemical and pharmaceutical science study subjects. As a result, this review discusses some of the novel Schiff bases and chelated complexes, as well as some proven pharmacological applications.
Keywords
- Schiff bases
- complexes
- biological functions
- structures
- chelates
1. Introduction
Schiff bases are known with the structural formula R2C=NR1. They are prepared by the condensation of
![](http://cdnintech.com/media/chapter/89575/1724837370-330315732/media/F1.png)
Figure 1.
Schiff’s base structure.
Unsaturated -C=N- compounds that are easily coordinated to various metal ions are known as Schiff bases [4, 5, 6]. They have a flexible mode due to coordination (no set pattern) and a larger number of applications [7, 8, 9, 10]. Due to the ease of their synthesis by condensing carbonyl compounds with amines, Schiff bases are frequently referred to as favored ligands [11]. They can also coordinate to a variety of metal ions with varying oxidation states. R2C=NR1 compounds are considered as a vital class of organic compounds in the current development surge of medicinal compounds due to their vast applications in numerous disciplines [12]. This is because the azomethine groups attached to the nitrogen atom form a strong hydrogen bond [13] with different labile moieties and functional groups of various constituents of cell, interfering with or distorting vital cell metabolism and resulting in various desired effects [14]. The design of R2C=NR1 compounds was critical to the advancement of coordination (complex compound) chemistry, as well as inorganic biochemistry and many optical materials [15]. Metal complexes derived from chelating agents and R2C=NR1 compounds have been synthesized by a number of scientists [16, 17, 18, 19, 20, 21, 22, 23, 24, 25].
1.1 Antimicrobials
Due to a number of factors, such as continuous emergence of contagious ailments and the explosion of various microbial infections that are multidrug resistant in nature, the search for an effective treatment material for various forms of contagious diseases has been and continues to be one among the most important challenges of the century. Despite the existence of a significant number of natural, synthetic antibiotics and chemotherapeutic substances for battling these diverse diseases, there has been an explosion of both old and new microorganisms that have become highly resistant to antibiotics developed in recent decades. This illustrates the critical need for classes of novel antimicrobial agents to be discovered and developed through diverse research and advancements [26].
The Knoevenagel condensate of keto esters (phenyl 2-(2-hydroxybenzylidene)-3-iminobutanoate) was used to prepare a Schiff base by reacting 2-[3-(3-hydroxynaphthalen-2-yl)-phenyl-3-imino-2-methylidenebutanoate]benzoic acid, and
![](http://cdnintech.com/media/chapter/89575/1724837370-330315732/media/F2.png)
Figure 2.
Compounds showing antimicrobial activity.
The condensation of equivalent molar ratio of 1-(4-methylaniline)acetaldehyde oxime and 4-(diethylamino)-2-hydroxybenzaldehyde yielded a novel bidentate R2C=NR1 compound (E)-1-(4-((E)-(4-(
The condensation of tricyclo[3.3.1.13,7]decan-1-amine with three aromatic alkanals (2-hydroxybenzaldehyde, 3-ethoxy-4-hydroxybenzaldehyde, and 4-(diethylpropan-2-amine)2-hydroxybenzaldehyde) yielded a collection of novel R2C=NR1 compounds [30]. The divalent cadmium chelates of these R2C=NR1 compounds were also prepared [30]. The reaction of 4-methylaniline with 3-ethoxysalicyaldehyde yielded four Schiff bases [30] (Compound 10, Figure 2). The four Schiff base ligands that were synthesized had active antibacterial action against the tested bacterial strains, with
1.2 Antidepressant
Depression is a common psychiatric condition that affects about 21% of the world’s population [32]. The physiological causes of depression are yet to be fully proven, despite repeated attempts to do so. However, some evidence suggested that depressive disorders could be caused by a lack of serotonin and noradrenaline. The majority of synthetic chemotherapeutic agents used to treat disorders like depression work by influencing the brain’s biogenic amine system, resulting in the activation of a process capable of boosting biogenic amine concentration in the brain [33]. Oxidases of monoamine are an enzyme family that catalyzes the oxidation or inactivation of biogenic amines in the central nervous system (CNS) [34]. In addition to the wide range of antidepressant formulations and chemicals presently in the market, there are considerable proportions of patients who do not react to treatment or only have a partial response to drugs, necessitating the development of new antidepressants [34].
By using a standard approach, a novel Schiff base of indole-carrying azetidinone derivatives was synthesized with high purity and higher yields [35]. The compounds’ derivatives (Compounds 14, Figure 3) showed antidepressant activity that was comparable to that of fluoxetine, a well-known antidepressant. The ortho-position of the phenyl ring system was replaced with nitrogen and chlorine, resulting in active compounds with immobility times of 66.82 and 65.61%, respectively. The results of the evaluation of molecular docking studies were in agreement with the pharmacological studies with a docking score of −2.8474 kcal/mol [35].
![](http://cdnintech.com/media/chapter/89575/1724837370-330315732/media/F3.png)
Figure 3.
Compounds showing antidepressant properties.
1.3 Antitubercular
The condensation cyclorization of 2-amino-5-phenyl-5H-thiazolo[4,3-b]-l,3,4-thiadiazole with various aromatic alkanals resulted in a collection of R2C=NR1 complexing agents [40]. The antitubercular activity (
![](http://cdnintech.com/media/chapter/89575/1724837370-330315732/media/F4.png)
Figure 4.
Compounds with antitubercular activities.
By reacting ethyl-2-amino-4,5,6,7-tetrahydrobenzo-1-carboxylato-1
1.4 Anthelmintic
The anthelmintic activity of some substituted 1,3,4-thiadiazole Schiff bases synthesized from benzimidazole (Compound 22, Figure 5) was investigated [42]. The new compounds had a mean paralyzing time (min) of 9.10–18.23 and 13.11–22.31 min against
![](http://cdnintech.com/media/chapter/89575/1724837370-330315732/media/F5.png)
Figure 5.
Compounds that showed anthelmintic activity.
1.5 Antioxidant and antitumor
The azomethine compound, 2-(2-oxoacenaphthylen-1(2
![](http://cdnintech.com/media/chapter/89575/1724837370-330315732/media/F6.png)
Figure 6.
Compounds having antioxidant and antitumor properties.
1.6 Anticonvulsant
Epilepsy is a neurological condition characterized by abnormal electrical impulses in the CNS. It causes seizures that occur spontaneously and repeatedly, with loss or gain of consciousness [45]. Six Schiff bases (Compound 27, Figure 7) were synthesized from 1
![](http://cdnintech.com/media/chapter/89575/1724837370-330315732/media/F7.png)
Figure 7.
Compounds having anticonvulsant properties of the SARS CoV 2 (severe acute respiratory syndrome coronavirus 2) inhibitor.
The SARS-CoV-2 virus, which produced the unique coronavirus epidemic known as Coronavirus 2019 (COVID-19), quickly expanded throughout the world, with Wuhan, China, serving as its epicenter by the end of 2019 [47, 48, 49, 50]. The virus, called coronavirus, has a diameter of 60–140 nm and resembles spike glycoproteins in the form of a crown when viewed under an electron microscope [50, 51, 52, 53]. On March 11, 2020, the World Health Organization (WHO) declared the COVID-19 epidemic to be a pandemic due to its high rate of infection, intensity, and rate of spread [54]. Currently, COVID-19 is a problem in over 225 countries, and the number of infections is still growing quickly. This coronavirus epidemic has not only put people’s health at danger but has also had a significant negative impact on the global economy [55, 56].
Vaccines, however, are being researched on children and have been found to be safe for adults [57]. Once more, virus mutations or alterations could render vaccinations useless [58]. Drugs to treat those infected with SARS-CoV-2 are still required, even though vaccination rates have decreased. Nevertheless, a number of local and international research organizations are still working to find a COVID-19 therapy medicine or vaccine that works while taking into account the risk factors related to viral infection. New chemotherapeutics have caught the interest of medicinal chemists in recent years. Isatin’s schiff base ligands have been shown to exhibit antiviral properties against the SARS, vaccinia, rhino, and Moloney leukemia viruses [59]. Considerable antiviral activity was demonstrated by Schiff bases made from 5-acyl-1,2,4-triazines containing oximes, hydrazones, semicarbazones, and thiosemicarbazones [60]. Some 1,6-hexanediamine tetra-dentate symmetrical bis-Schiff bases were predicted
![](http://cdnintech.com/media/chapter/89575/1724837370-330315732/media/F8.png)
Figure 8.
Proposed SARS CoV 2 inhibitors.
2. Conclusion
Schiff bases are a type of chelating agent that can be found in a variety of drugs. There has been a lot of research on Schiff bases. This study emphasized the importance of R2C=NR1 compounds in the advancement of novel compounds with medicinal applications. Regardless that study in this field is still in its infantile phase, there have recently been surges in reports demonstrating the impact of R2C=NR1 compounds on therapeutically important microorganisms. For decades, because of this bioactive core, researchers have been interested in obtaining several R2C=NR1 compounds and chelates of biological significance. The goal of this study was to look at all of the biological information available.
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