Advances on asymmetric organocatalytic 1,4-conjugate addition reactions in aqueous and semi-aqueous media

Asymmetric Michael addition reactions or 1,4-conjugate addition reactions are considered to be the fundamental C–C bond-forming reaction for the construction of chiral β-nitro, β-carbonyl and several other important structural motifs in organic synthesis. After the development of many chiral metal complexes as catalysts, a tremendous growth in the design and applications of variety of new chiral organocatalysts for asymmetric 1,4-conjugate addition reactions has been witnessed over the last two decades. Initially, asymmetric organocatalysis has been performed in organic solvents, but gradually performing the same catalysis in aqueous and semi-aqueous media became a necessity due to environmental aspects as well as to achieve better reactivity and selectivity. The structural and functional diversity of the chiral organocatalysts derived from natural and synthetic sources utilized differently the water either as the sole solvent, co-solvent or additive for optimising their best performances. In the present review, we discuss a detailed and comprehensive report on the advancement in the field of asymmetric organocatalytic 1,4-conjugate addition reactions in aqueous and semi-aqueous media.     

Investigations in the transition metal catalyzed aziridination of olefins, amination, and other insertion reactions with Bromamine-T as the source of nitrene.

Investigations into the transition metal catalyzed aziridination of olefins with Bromamine-T as a new source of nitrene is presented in this account. Comparison of Chloramine-T and Bromamine-T in this reaction indicates that the latter is superior as the source of nitrene. Systematic study with several transition metal based catalysts suggests that Cu-halides are the best catalysts. A first report of aziridination under microwave and ultrasound irradiation conditions is also presented. Copper-catalyzed aziridination of methyl cinnamate with Bromamine-T did not proceed at ambient temperature but was effected smoothly under ultrasound irradiation to furnish trans-aziridine selectively, while under microwave irradiation, a mixture of cis and trans isomers, was obtained. It has been demonstrated that aziridination of olefins proceeds smoothly with inexpensive bleaching powder. Preliminary results of Rh-catalyzed benzylic insertion reactions with Bromamine-T are included in this account.     

Study of ion–solvent interactions of some tetraalkylammonium halides in THF + CCl4 mixtures by conductance measurements

Conductivities of some tetraalkylammonium halides, viz. tetrapentylammonium chloride (Pen₄NCl), tetrahexylammonium chloride (Hex₄NCl), tetraheptylammonium chloride (Hep₄NCl), and tetraoctylammonium chloride (Oct₄NCl) were measured at 298.15 K in THF + CCl₄ mixtures with 40, 60 and 80 mass% of THF. A minimum in the conductometric curves (molar conductance, Λ vs. square root of concentration, √c) was observed at concentrations which is dependent both on the salt and the solvent. The observed molar conductivities were explained by the formation of ion-pairs (M⁺ + X⁻ ↔ MX, K_P) and triple-ions (2M⁺ + X⁻ ↔ M₂X⁺; M⁺ + 2X⁻ ↔ MX₂⁻, K_T). A linear relationship between the triple-ion formation constants [log(K_T/K_P)] and the salt concentrations at the minimum conductivity (log C_min) was given for all salts in THF + CCl₄ mixtures. The formation of triple-ions might be attributed to the ion sizes in solutions in which coulombic interactions and covalent bonding forces act as the main forces between the ions (R₄N⁺X⁻).     

LC–QTOF–MS-based metabolite profiling and evaluation of α-glucosidase inhibitory kinetics of Coccinia grandis fruit

Coccinia grandis is an important food crop of the Cucurbitaceae family, widely used for culinary purposes in India. It is reported to possess hypoglycemic, hypolipidemic and antioxidant activities. The current study was aimed to explore the inhibition kinetics as well as major constituents of the active fraction of C. grandis against α-glucosidase. The kinetic study was performed through spectrophotometric assay, with p-nitrophenyl-α-d -glucopyranoside as a substrate with varying concentrations. An in vitro antioxidant study was performed by DPPH assay. In addition, UPLC–QTOF–MS analysis was carried out for metabolite profiling of the bioactive fraction of C. grandis. The results showed that the difference between the α-glucosidase inhibitory activity of the ethyl acetate fraction of C. grandis (EFCG) (IC₅₀ 2.43 ± 0.27 mg/ml), and standard inhibitor, acarbose (2.08 ± 0.19 mg/ml), was not statistically significant at a P-value of 0.05. The enzyme kinetics confirmed the inhibition mode in a mixed manner. The EFCG also showed the highest antioxidant activity (101.74 ± 1.95 μg/ml) among all of the fractions. A significant correlation between antioxidant and α-glucosidase inhibitory activity of EFCG was observed. The LC–QTOF–MS study of the EFCG putatively identified 35 metabolites, which may be responsible for its antioxidant and α-glucosidase inhibitory properties. Thus, C. grandis fruits can serve as a functional food to address diabetes-related disorders associated with α-glucosidase.     

Photophysical and Duplex‐DNA‐Binding Properties of Distamycin Dimers Based on 4,4′‐ and 2,2′‐Dialkoxyazobenzenes as the Core

Distamycin‐based tetrapeptide ( 1 ) was covalently tethered to both ends of the central dihydroxyazobenzene moiety at either the 2,2′ or 4,4′ positions. This afforded two isomeric, distamycin–azobenzene–distamycin systems, 2 (para) and 3 (ortho), both of them being photoisomerizable. Illumination of these conjugates in solution at approximately 360 nm induced photoisomerization and the time course of the process was followed by UV/Vis and ¹H NMR spectroscopy. The kinetics of the thermal reversion at various temperatures of cis to trans isomers of the conjugates obtained after photoillumination were also examined. This afforded the respective thermal‐activation parameters. Both the molecular architecture and the location of the substituent around the core azobenzene determined the rate and activation‐energy barrier for the cis‐to‐trans back‐isomerization of these conjugates in solution. Duplex–DNA binding of the conjugates and the changes in DNA‐binding efficiency upon photoisomerization was also examined by CD spectroscopy, thermal denaturation studies, and a Hoechst displacement assay. The conjugate 2 showed higher DNA‐binding affinity and a greater change in the DNA‐binding efficiency upon photoisomerization compared with its 2,2′‐disubstituted counterpart. The experimental findings were substantiated by using molecular‐docking studies involving each conjugate with a model duplex d[(GC(AT)₁₀CG)]₂ DNA molecule.     

Molar volumes,viscosity, and isentropic compressibility of some primary monoalkanols in aqueous N,N-dimethylformamide solutions

The densities (ρ) and viscosities (η) for the ternary liquid mixtures of water + N,N-dimethylformamide + monoalkanols, have been measured as a function of the composition at 298.15, 308.15, and 318.15 K. From the experimental measurements excess molar volumes (V ^E), Viscosity deviation (Δη), and synergy index (I _s) have been evaluated. The speeds of sound have been also measured and excess isentropic compressibilities (K _s^E) are calculated al 298.15 K. The results are discussed and interpreted in terms of molecular package and specific interaction predominated by hydrogen bonding, been investigated.     

Metal-complex/DNA conjugates: a versatile building block for DNA nanoarrays

The use of DNA networks as templates for forming nanoarrays of metallic centres shows an exciting potential to generate addressable nanostructures. Inorganic units can be photoactive, electroactive and/or can possess magnetic and catalytic properties and can adopt different spatial arrangements due to their varied coordination nature. All these properties influence both the structure and function of passive DNA scaffolds and provide DNA nanostructures as a new platform for new materials in emerging technologies, such as nanotechnology, biosensing or biocomputing.     

Solvent- or temperature-controlled diastereoselective aldol reaction of methyl phenylacetate

Unlike the enolboration-aldolization of methyl propanoate, the choice of either the solvent or temperature determines the diastereoselectivity during the enolboration-aldolization of methyl phenylacetate. In CH(2)Cl(2), the reaction favors the anti-pathway at -78 °C and the syn-pathway at rt. Conversely, the reaction produces the anti-isomer up to rt and the syn-isomer at refluxing temperatures in nonpolar solvents.