Microwave assisted synthesis of 2-aminooxazolo [4,5-b] pyridine derivatives via intramolecular C–O bond formation in aqueous medium

A highly, efficient synthetic protocol for the synthesis of 2-aminooxazolo[4,5-b]pyridine derivatives is established via intramolecular C–O bond coupling using copper iodide as a catalyst and water as solvent. A variety of functionalized substrates were found to react under this reaction conditions to provide products in good to excellent yields.     

An efficient regioselective reaction of indole with 2H-chromenes for the synthesis of new 4H-chromenes in environmentally benign media

An efficient regioselective, catalyst free, and green protocol has been developed for the synthesis of a novel class of 4H-chromene scaffolds at room temperature in ionic liquid. The method is general and applicable for a series of 2H-chromenes as well as indole derivatives.     

Regioselective synthesis of polysubstituted 4H-thiopyrans from β-oxodithioesters

A simple and efficient method for the synthesis of polyfunctionalized 4H-thiopyrans by highly regioselective cyclocondensation of β-oxodithioesters with 1,1,3-trialkyl or aryl substituted prop-2-yn-1-ols using BF₃·Et₂O as the catalyst is described.     

Durability enhancement and degradation of oxygen evolution anodes in seawater electrolysis for hydrogen production

For the anode composed of electrocatalyst oxide, intermediate layer and titanium substrate, the substitution of a certain amount of iridium with tin in the IrO₂ intermediate layer was remarkably effective in elongating the life of the anode in preventing oxidation of the substrate titanium during oxygen evolution. The longest life was realized by preparation of intermediate layer with uniform thickness by brush-coating of H₂IrCl₆-SnCl₄ butanol solution and subsequent calcination. The anode with the intermediate layer prepared from 0.04 M H₂IrCl₆-0.06 M SnCl₄ butanol solution showed the best performance, that is, the oxygen evolution efficiency higher than 99.8% for more than 4300 h in the electrolysis of 0.5 M NaCl solution of pH 1 at the current density of 1000 Am⁻². An increase in SnCl₄ concentration decreased the viscosity of the coating solution with a consequent enhancement of uniformity of the intermediate layer but decreased the thickness of the intermediate layer acting as a barrier to prevent oxidation of titanium. Thus, the best performance was attained at an intermediate SnCl₄ concentration. The growth of an oxide layer on titanium during electrolysis occurred and was found by the potential increase.     

Kinetics and mechanism of oxidation of chloramphenicol by 1-chlorobenzotriazole in acidic medium

Chloramphenicol (CAP) is an antibiotic drug having a wide spectrum of activity. The kinetics of oxidation of chloramphenicol by 1-chlorobenzotriazole (CBT) in HClO₄ medium over the temperature range 293–323 K has been investigated. The reaction exhibits first-order kinetics with respect to [CBT]_o and zero-order with respect to [CAP]_o. The fractional-order dependence of rate on [H⁺] suggests complex formation between CBT and H⁺. It fails to induce polymerization of acrylonitrile under the experimental conditions employed. Activation parameters are evaluated. The observed solvent isotope effect indicates the absence of hydride transfer during oxidation. Effects of dielectric constant and ionic strength of the medium on the reaction rate have been studied. Oxidation products are identified. A suitable reaction scheme is proposed and an appropriate rate law is deduced to account for the observed kinetic data.     

Mechanistic Investigations of Oxidation of Some Dipeptides by Sodium N‐chloro‐p‐toluenesulfonamide in Alkaline Medium:A Kinetic Study

The kinetics of oxidation of five dipeptides (DPP) viz., glycylglycine (Gly-Gly), L-alanyl-L-alanine (Ala-Ala), L-valyl-L-valine (Val-Val), L-leucyl-L-leucine (Leu-Leu) and phenylglycyl-phenylglycine (Phg-Phg) by sodium N-chloro-p-toluenesulfonamide or chloramine-T (CAT) in NaOH medium was studied at 308 K. The reactions follow identical kinetics for all the dipeptides, being first-order dependence each on [CAT]_o, [DPP]_o and fractional-order on [OH^－]. Addition of p-toluenesulfonamide or halide ions (Cl^－ or Br^－) has no significant effect on the rate of reaction. The reaction rate was found to increase with increase in ionic strength of the medium. The solvent isotope effect was studied using D₂O. The activation parameters for the reaction were computed from Arrhenius plots. Equilibrium and decomposition constants were evaluated. The oxidation products of the dipeptides were identified as their corresponding aldehydes. An isokinetic relationship was observed with β＝352 K, indicating that enthalpy factors control the reaction rate. CH₃C₆H₄SO₂NCl^－ of the oxidant has been postulated as the reactive oxidizing species. Under comparable experimental conditions, the rate of oxidation of the dipeptides increases in the order: Phg-Phg＞Ala-Ala＞Val-Val＞Leu-Leu＞Gly-Gly. The kinetics of oxidation of the dipeptides have also been compared with those of their corresponding monomer amino acids. The observed results have been explained by a plausible mechanism and the related rate law has been deduced.     

Electroanalytical Determination of N‐Nitrosamines in Aqueous Solution Using a Boron‐Doped Diamond Electrode

The electrochemical methods cyclic and square‐wave voltammetry were applied to develop an electroanalytical procedure for the determination of N‐nitrosamines (N‐nitrosopyrrolidine, N‐nitrosopiperidine and N‐nitrosodiethylamine) in aqueous solutions. Cyclic voltammetry was used to evaluate the electrochemical behaviors of N‐nitrosamines on boron‐doped diamond electrodes. It was observed an irreversible electrooxidation peak located in approximately 1.8 V (vs. Ag/AgCl) for both N‐nitrosamines. The optimal electrochemical response was obtained using the following square‐wave voltammetry parameters: f=250 Hz, E_sw=50 mV and E_s=2 mV using a Britton–Robinson buffer solution as electrolyte (pH 2). The detection and quantification limits determined for total N‐nitrosamines were 6.0×10^?8 and 2.0×10^?7 mol L^?1, respectively.     

Base-Catalyzed Synthesis of Spiropyrazoloaminonitriles with Arylidene Pyrazolone and Vinylogous Malononitriles through a Cascade Michael Addition and Cyclization

Synthesis of title compounds were accomplished by a reaction of vinyl malononitriles and arylidene pyrazolones catalyzed by base. This protocol proceeds via Michael addition followed by intramolecular cyclization leading to the formation of two new C−C bonds. Further the same reaction was also conducted with α,α-dicyano olefins and vinyl malononitriles to furnish 1,6-dihydro biphenyl compounds. Simple reaction conditions, high yields and compatibility are the advantages of this protocol.     

Mechanochemistry-Amended Barbier Reaction as an Expedient Alternative to Grignard Synthesis**

Organomagnesium halides (Grignard reagents) are essential carbanionic building blocks widely used in carbon-carbon and carbon-heteroatom bond-forming reactions with various electrophiles. In the Barbier variant of the Grignard synthesis, the generation of air- and moisture-sensitive Grignard reagents occurs concurrently with their reaction with an electrophile. Although operationally simpler, the classic Barbier approach suffers from low yields due to multiple side reactions, thereby limiting the scope of its application. Here, we report a mechanochemical adaptation of the Mg-mediated Barbier reaction, which overcomes these limitations and facilitates the coupling of versatile organic halides (e.g., allylic, vinylic, aromatic, aliphatic) with a diverse range of electrophilic substrates (e.g., aromatic aldehydes, ketones, esters, amides, O-benzoyl hydroxylamine, chlorosilane, borate ester) to assemble C−C, C−N, C−Si, and C−B bonds. The mechanochemical approach has the advantage of being essentially solvent-free, operationally simple, immune to air, and surprisingly tolerant to water and some weak Brønsted acids. Notably, solid ammonium chloride was found to improve yields in the reactions of ketones. Mechanistic studies have clarified the role of mechanochemistry in the process, indicating the generation of transient organometallics facilitated by improved mass transfer and activation of the surface of magnesium metal.     

Synthesis and Conformational Studies of a Hybrid Cyclic Peptide Based on cis‐β‐Furanoid Sugar Amino Acid (FSAA) and Ornithine

The conformational control of a 14‐helix nucleating template, cis‐β‐furanoid sugar amino acid (FSAA), over a flexible δ‐amino acid, ornithine is studied in a FSAA‐ornithine cyclic tetrapeptide. Extensive NMR and MD studies reveal that the cyclic peptide adopts a three‐dimentional bowl‐shape cavity, which promotes six‐ and seven‐membered intra‐ and inter‐residue H‐bonding, in polar and non‐polar solvents, respectively.