Tetrabutylammonium Bromide (TBAB) Catalyzed Synthesis of Bioactive Heterocycles.

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.

6-Amino-3-methyl-4-(3,4,5-tri-meth-oxy-phen-yl)-2,4-di-hydro-pyrano[2,3-c]pyrazole-5-carbo-nitrile.

In the title compound, C17H18N4O4, the dihedral angle between the benzene ring and 2,4-di-hydro-pyrano[2,3-c]pyrazole ring system is 89.41 (7)°. The pyran moiety adopts a strongly flattened boat conformation. In the crystal, mol-ecules are linked by N-H⋯N, N-H⋯O, C-H⋯N and C-H⋯O hydrogen bonds into an infinite two-dimensional network parallel to (110). There are π-π inter-actions between the pyrazole rings in neighbouring layers [centroid-centroid distance = 3.621 (1) Å].

Crystal structure of 5,5'-[(4-fluoro-phen-yl)methyl-ene]bis-[6-amino-1,3-di-methyl-pyrimidine-2,4(1H,3H)-dione].

In the title mol-ecule, C19H21FN6O4, the dihedral angles between the benzene ring and essentially planar pyrimidine rings [maximum deviations of 0.036 (2) and 0.056 (2) Å] are 73.32 (7) and 63.81 (8)°. The dihedral angle between the mean planes of the pyrimidine rings is 61.43 (6)°. In the crystal, N-H⋯O hydrogen bonds link mol-ecules, forming a two-dimensional network parallel to (001) and in combination with weak C-H⋯O hydrogen bonds, a three-dimensional network is formed. Weak C-H⋯π inter-actions and π-π inter-actions, with a centroid-centroid distance of 3.599 (2) Šare also observed.

Ethyl 6-amino-5-cyano-4-phenyl-2,4-di-hydro-pyrano[2,3-c]pyrazole-3-carboxyl-ate dimethyl sulfoxide monosolvate.

In the asymmetric unit of the title compound, C16H14N4O3·C2H6OS, there are two independent main mol-ecules (A and B) and two dimethyl sulfoxide solvent mol-ecules. In mol-ecule A, the pyran ring is in a flattened sofa conformation, with the sp (3)-hydridized C atom forming the flap. In mol-ecule B, the pyran ring is in a flattened boat conformation, with the sp (3)-hydridized C atom and the O atom deviating by 0.073 (3) and 0.055 (3) Å, respectively, from the plane of the other four atoms. The mean planes the pyrazole and phenyl rings form dihedral angles of 84.4 (2) and 84.9 (2)°, respectively, for mol-ecules A and B. In the crystal, N-H⋯O and N-H⋯N hydrogen bonds link the components of the structure into chains along [010]. In both solvent mol-ecules, the S atoms are disordered over two sites, with occupancy ratios of 0.679 (4):0.321 (4) and 0.546 (6):0.454 (6).

Synthesis of Isatin-derived Heterocycles with Promising Anticancer Activities.

Isatin or 1H-indole-2,3-dione skeleton has been playing a significant role in drug de-sign and development. Isatin itself and many of its derivatives are widely distributed in naturally occurring bioactive compounds. Various synthetic isatin derivatives were found to possess a broad range of significant pharmacological efficacies especially anti-cancer activity against a wide variety of cancer cell lines. Interestingly, on a few occasions, some isatin-derived scaffolds were reported as more potent than the tested reputed drug molecules. As a result, isatin-derived compounds have been gaining significant attention in cancer-based drug developments. In this re-view, we have summarized literature reported during the last two decades related to the synthesis of structurally diverse isatin-derived scaffolds with promising anti-cancer activities.

Trimeth-yl(triphenyl-meth-oxy)silane.

In the title mol-ecule, C(22)H(24)OSi, the Si-O-C angle is 139.79 (11)°, the O-C-C angles of the triphenyl-meth-oxy group are in the range 106.13 (13)-109.22 (14)° and the O-Si-C angles of the trimethyl-sil-yloxy group are in the range 103.08 (10)-113.53 (10)°. In the crystal, face-to-face π-π interactions are observed between the phenyl rings [centroid separation = 4.194 (1) Å, interplanar spacing = 3.474 Šand centroid shift = 2.35 Å]. The three phenyl groups of the triphenyl-methyl substituent are mutually nearly perpendicular, with dihedral angles in the range 80.49 (8)-81.53 (8)°. There are only weak inter-molecular van der Waals inter-actions in the crystal.

2-Amino-7,7-dimethyl-5-oxo-4-(p-tol-yl)-5,6,7,8-tetra-hydro-4H-chromene-3-carbonitrile.

In the title mol-ecule, C(19)H(20)N(2)O(2), the cyclo-hexene ring adopts a sofa conformation, while the pyran ring adopts a flattened boat conformation. In the crystal, mol-ecules are linked by N-H⋯N and N-H⋯O hydrogen bonds, forming a two-dimensional network parallel to (010).

Recent developments on ultrasound-assisted one-pot multicomponent synthesis of biologically relevant heterocycles.

Heterocycles are the backbone of organic compounds. Specially, N- &O-containing heterocycles represent privileged structural subunits well distributed in naturally occurring compounds with immense biological activities. Multicomponent reactions (MCRs) are becoming valuable tool for synthesizing structurally diverse molecular entities. On the other hand, the last decade has seen a tremendous outburst in modifying chemical processes to make them sustainable for the betterment of our environment. The application of ultrasound in organic synthesis is fulfilling some of the goals of 'green and sustainable chemistry' as it has some advantages over the traditional thermal methods in terms of reaction rates, yields, purity of the products, product selectivity, etc. Therefore the synthesis of biologically relevant heterocycles using one-pot multi-component technique coupled with the application of ultrasound is one of the thrusting areas in the 21st Century among the organic chemists. The present review deals with the "up to date" developments on ultrasound assisted one-pot multi-component synthesis of biologically relevant heterocycles reported so far.

Recent developments on ultrasound assisted catalyst-free organic synthesis.

Mother Nature needs to be protected from ever increasing chemical pollutions associated with synthetic organic processes. The fundamental challenge for today's methodologists is to make their protocols more environmentally benign and sustainable by avoiding the extensive use of hazardous reagents and solvents, harsh reaction conditions, and toxic metal catalysts. However, the people of the twenty-first century are well aware about the side effects of those hazardous substances used and generated by the chemical processes. As a result, the last decade has seen a tremendous outburst in modifying chemical processes to make them 'sustainable' for the betterment of our environment. Catalysts play a crucial role in organic synthesis and thus they find huge applications and uses. Scientists' continuously trying to modify the catalysts to reduce their toxicity level, but the most benign way is to design an organic reaction without catalyst(s), if possible. It is worthy to mention that the involvement of ultrasound in organic synthesis is sometimes fulfilling this goal. In many occasions the applications of ultrasound can avoid the use of catalysts in organic reactions. Such beneficial features as a whole have motivated the organic chemists to apply ultrasonic irradiation in more heights and as a results, in recent past, there were immense applications of ultrasound in organic reactions for the synthesis of diverse organic scaffolds under catalyst-free condition. The present review summarizes the latest developments on ultrasound assisted catalyst-free organic synthesis reported so far.

Sunlight-induced rapid and efficient biogenic synthesis of silver nanoparticles using aqueous leaf extract of Ocimum sanctum Linn. with enhanced antibacterial activity.

BACKGROUND: Nanotechnology is now regarded as a distinct field of research in modern science and technology with multifaceted areas including biomedical applications. Among the various approaches currently available for the generation of metallic nanoparticles, biogenic synthesis is of increasing demand for the purpose of green nanotechnology. Among various natural sources, plant materials are the most readily available template-directing matrix offering cost-effectiveness, eco-friendliness, and easy handling. Moreover, the inherent pharmacological potentials of these medicinal plant extracts offer added biomedical implementations of the synthesized metal nanoparticles. RESULTS: A robust practical method for eco-friendly synthesis of silver nanoparticles using aqueous leaf extract of Ocimum sanctum (Tulsi) as both reducing and capping agent, under the influence of direct sunlight has been developed without applying any other chemical additives. The nanoparticles were characterized with the help of UV-visible spectrophotometer and transmission electron microscopy (TEM). The prepared silver nanoparticles exhibited considerable antibacterial activity. The effects were more pronounced on non-endospore-forming Gram-positive bacteria viz., Staphylococcus aureus, Staphylococcus epidermidis, and Listeria monocytogenes than endospore-forming species Bacillus subtilis. The nanoparticles also showed prominent activity on Gram-negative human pathogenic Salmonella typhimurium, Escherichia coli, Pseudomonas aeruginosa, and plant pathogenic Pantoea ananatis. A bactericidal mode of action was observed for both Gram-positive and Gram-negative bacteria by the nanoparticles. CONCLUSIONS: We have developed a very simple, efficient, and practical method for the synthesis of silver nanoparticles using aqueous leaf extract of O. sanctum under the influence of direct sunlight. The biosynthesis of silver nanoparticles making use of such a traditionally important medicinal plant without applying any other chemical additives, thus offers a cost-effective and environmentally benign route for their large-scale commercial production. The nanoparticles dispersed in the mother solution showed promising antibacterial efficacy. Graphical AbstractSunlight-induced rapid and efficient biogenic synthesis of silver nanoparticles using aqueous leaf extract of Ocimum sanctum Linn. with enhanced antibacterial activity.

Facile synthesis of symmetrical bis(benzhydryl)ethers using p-toluenesulfonyl chloride under solvent-free conditions.

BACKGROUND: The benzhydryl ether moiety is widely distributed in nature and constitutes a key structural motif in numerous molecules of significant biological potential and of prospective clinical uses. Solvent-free and cost-effective facile synthesis of symmetrical bis(benzhydryl)ethers is, thus, much desirable. RESULTS: A simple and efficient method for the facile synthesis of symmetrical bis(benzhydryl)ethers directly from the corresponding benzhydrols has been developed using a catalytic amount of p-toluenesulfonyl chloride (5 mol%) at an oil bath temperature of 110°C under solvent-free conditions. CONCLUSIONS: Operational simplicity, low reagent loading, high product yields, short reaction time, and solvent-free conditions are the notable advantages of the present method.