Reactive Oxygen Species (ROS) refers to the highly reactive substances which contain oxygen radicals. Hypochlorous acid, peroxides, superoxide, singlet oxygen, alpha-oxygen, and hydroxyl radicals are the major examples of ROS. Generally, the reduction of oxygen (O2) in molecular form produces superoxide (•O2 −) anion. ROS are produced during a variety of biochemical reactions within the cell organelles, such as endoplasmic reticulum, mitochondria, and peroxisome. Naturally, ROS are also formed as a byproduct of the normal metabolism of oxygen. The production of ROS can be induced by various factors such as heavy metals, tobacco, smoke, drugs, xenobiotics, pollutants, and radiation. From various experimental studies, it is reported that ROS acts as either a tumor-suppressing or a tumor-promoting agent. The elevated level of ROS can arrest the growth of tumors through the persistent increase in cell cycle inhibition. The increased level of ROS can induce apoptosis by both intrinsic and extrinsic pathways. ROS is considered to be a tumor-suppressing agent as the production of ROS is due to the use of most of the chemotherapeutic agents in order to activate cell death. The cytotoxic effect of ROS provides impetus towards apoptosis, but in higher levels, ROS can cause initiation of malignancy that leads to uncontrolled cell death in cancer cells. In contrast, some species of ROS can influence various activities at the cellular level, including cell proliferation. This review highlights the genesis of ROS within cells by various routes and their role in cancer therapies.

Polycyclic aromatic hydrocarbons (PAHs) are aromatic compounds with two or more fused benzene rings in their structural configurations. PAHs do not contain heteroatoms and substituents on the ring system. PAHs containing up to four rings are called light PAHs while those that contain more than four rings are considered as heavy PAHs. Heavy PAHs are more stable and more toxic than the light PAHs. Generally, the increase in the size and angularity of a PAH molecule results in an increase in hydrophobicity and electrochemical stability. Ring linkage patterns in PAHs may occur in such a way that the tertiary carbon atoms are centers of two or three interlinked rings. The examples of PAHs are naphthalene, anthracene, phenanthrene, acenaphthylene, acenaphthene, fluorene, fluoranthene, pyrene, benz[a]anthracene, chrysene, benzo[b]fluoranthene, etc. PAHs can be produced either naturally or anthropogenically and have toxic properties. Due to the health risk posed by their exposure, there is a need to control the release of PAHs through air quality management. Refinery industries are required to monitor and regulate their discharges. There is an urgent need for the considerable efforts to be applied in the field of research to degrade and monitor potentially hazardous substances to control, predict and avoid negative effects of PAHs pollution.

N-heterocyclic scaffolds represent a privileged architecture in the process of drug design and development. It has widespread occurrence in synthetic and natural products, either those that are established or progressing as potent drug candidates. Additionally, numerous novel N-heterocyclic analogues with remarkable physiological significance and extended pharmaceutical applications are escalating progressively. Hence, the classical synthetic protocols need to be improvised according to modern requirements for efficient and eco-friendly approaches. Numerous methodologies and technologies emerged to address the green and sustainable production of various pharmaceutically and medicinally important N-heterocyclic compounds in last few years. In this context, the current review unveils greener alternatives for direct access to categorically differentiated N-heterocyclic derivatives and its application in the establishment of biologically active potent molecules for drug design. The green and sustainable methods accentuated in this review includes microwave-assisted reactions, solvent-free approaches, heterogeneous catalysis, ultrasound reactions, and biocatalysis.

During the last two decades, tetrabutylammonium bromide (TBAB) has gained significant attention as an efficient metal-free homogeneous phase-transfer catalyst. A catalytic amount of TBAB is sufficient to catalyze various alkylation, oxidation, reduction, and esterification processes. It is also employed as an efficient co-catalyst for numerous coupling reactions. It has also acted as an efficient zwitterionic solvent in many organic transformations under molten conditions. In this review, we have summarized the recent developments on TBAB-catalyzed protocols for the efficient synthesis of various biologically promising heterocyclic scaffolds.

The classical Hantzsch reaction is one of the simplest and most economical methods for the synthesis of biologically important and pharmacologically useful 1,4-dihydropyridine derivatives. Bismuth nitrate pentahydrate under microwave irradiation is proven to act as a very efficient catalyst for a one-pot, three-component synthesis of 1,4-dihydropyridines in excellent yields from diverse amines/ammonium acetate, aldehydes and 1,3-dicarbonyl compounds within 1–3 min under solvent-free conditions. The present environmentally benign procedure for the synthesis of 1,4-dihydropyridines is suitable for library synthesis and it will find application in the synthesis of potent biologically active molecules. The excellent yield and extreme rapidity of the method is due to a concurrent effect of the catalyst and microwave irradiation.