Efficient proton-templated synthesis of 18- to 38-membered tetraimino(amino)diphenol macrocyclic ligands: structural features and spectroscopic properties

A whole range of Robson-type tetraiminodiphenol macrocyclic ligands have been prepared as their perchlorate salts [H4L](ClO4)2 in high yield (ca. 90%) by a single-step [2 + 2] condensation reaction between 4-methyl(or tert-butyl)-2,6-diformyl(or diacyl)phenols and alpha,omega-diaminoalkanes (C2-C12) in the presence of acetic acid and NaClO4. The reduction of these 18- to 38-membered macrocyclic salts with NaBH4 have afforded corresponding tetraaminodiphenol macrocycles H2L'. The X-ray crystal structures of two of the tetraiminodiphenol macrocycles with the C2 and C4 lateral chains have been determined, and the optimized configurations for all of the macrocycles have been obtained by molecular mechanics calculations. The macrocycles have been characterized by elemental analysis and by IR, absorption, emission, and NMR spectroscopic study. The protonated tetraiminodiphenol macrocycles exhibit strong fluoroscence in methanol, acetonitrile, and nitromethane and undergo quenching when treated with triethylamine. The neutral macrocycles H2L, isolated by treating [H4L](ClO4)2 with excess of triethylamine, lack luminescence, as do the reduced tetraaminodiphenol macrocycles H2L'. The hydrolytic cleavage of [H4L](ClO4)2 has been studied.     

Penicillamine and captopril: mechanistic exploration of defensive actions of thiol drugs against a metal bound-superoxo complex

In acid-media ([H⁺] = 0.01–0.06 M), each of the thiol compounds, D-penicillamine (PEN, L_PH₂) and captopril (CAP, L_CH₂) exist in several proton-dependent forms which can reduce the superoxo complex [(en)(dien)Co^III(O₂)Co^III(en)(dien)]⁵⁺ (1) to the corresponding peroxo [(en)(dien)Co^III(O₂)Co^III(en)(dien)]⁴⁺ (2) or the hydroperoxo complex [(en)(dien)Co^III(OOH)Co^III(en)(dien)]⁵⁺ (3). The observed first-order rate constants, k_o,P and k_o,C for PEN and CAP increase with the increase in [T_PEN] and [T_CAP] (which are the analytical concentrations of the respective thiols) but decrease with the increase in the media-acidity ([H⁺]) and the media ionic strength (I). The protolytic equilibria in aqueous solution allow several potentially reducing forms to coexist for both PEN (L_PH₃⁺, L_PH₂, L_PH^?, and L_P^2?) and CAP (L_CH₂, L_CH^?, L_C^2?) but the kinetic analyses reveal that the order of reactivity for the species are L_PH₃⁺ ~ L_PH₂ <<< L_PH^? and L_CH₂ < L_CH^? <<< L_C^2?, respectively. The predominance and higher reactivities of the anionic species, L_PH^? and L_C^2? are supported by the negative slopes of the plots of k_o,P or k_o,C versus I. Moreover, a large value of k_H/k_D for PEN suggests an inner-sphere electroprotic reaction pathway while the absence of such effect for CAP strongly supports an outer-sphere electron transfer reaction. These propositions are supported by the structural features of L_PH^? and L_C^2?.     

Thermal generation of pentacenes from soluble 6,13-dihydro-6,13-ethenopentacene precursors by a Diels-Alder-retro-Diels-Alder sequence with 3,6-disubstituted tetrazines

3,6-Substituted tetrazines 2 (a: R(2) = 2-pyridyl or b: CO(2)Me) react with 2,3,9,10-(R(1))(4)-dihydro-6,13-ethenopentacene 3 in solution at elevated temperature to the corresponding pentacene 1 (a: R(1) = H, b: OBn, c: F).     

Kinetic insights on the oxidation of acetaminophen and caffeine by a Mn(IV)3 complex

Transition Metal Chemistry - The kinetics of the oxidation reactions of APAP and Cafn by a Mn(IV)-oxidant, [MnIV3(µ-O)4(phen)4(H2O)2]4+ (1) has been studied at room temperature. Under the...     

Photochemically generated elemental selenium forms conjugates with serum proteins that are preferentially cytotoxic to leukemia and selected solid tumor cells

The objective of this study was to determine if and how photoproducts contribute to the antitumor effect of merocyanine-mediated PDT. A panel of barbituric, thiobarbituric and selenobarbituric acid analogues of Merocyanine 540 was photobleached, and the resulting photoproducts were characterized by absorption, fluorescence emission, mass, energy dispersive X-ray, and X-ray photoelectron spectroscopy and tested for cytotoxic activity against tumor cell lines and freshly explanted bone marrow cells. While all dyes were readily photobleached, only photoproducts of selone dyes showed cytotoxic activity. One-hour incubations with micromolar concentrations of selone-derived photoproducts were sufficient to reduce leukemia/lymphoma cells ≥10 000 fold, whereas preserving virtually all normal CD34-positive bone marrow cells. Of six multidrug-resistant tumor cell lines tested, five were as sensitive or more sensitive to photoproducts than the corresponding wild-type lines. Physicochemical characterizations of the cytotoxic activity indicated that it consisted of conjugates of subnano particles of elemental selenium and (lipo)proteins. The discovery of cytotoxic Se-protein conjugates provides a rare example of photoproducts contributing substantially to the antitumor effect of PDT and challenges the long-held view that Se in oxidation state zero is biologically inert. Agents modeled after our Se-protein conjugates may prove useful for the treatment of leukemia.     

The effect of filler surface modification and processing conditions on distribution behaviour of silica nanofillers in polyesters

Dispersion and distribution of nanosized precipitated amorphous silica particles in poly(ethylene terephthalate) PET and poly(butylene terephthalate) PBT matrices have been studied. The effect of silane coupling agent as well as processing conditions has been analysed on the basis of microscopic analysis of sample morphology. N-2-(aminoethyl)-3-aminopropyltrimethoxysilane (A-1120) has been used as a silica surface modifier. The effect of the extrusion process performance with a single and a twin screw extruder has been tested. It has been found that the processing conditions are more important factors determining fine dispersion and homogenous distributions of filler particles in the matrix than the filler surface modification. The results obtained have revealed that single screw extrusion is preferred only for processing the composites comprising the silica modified with aminosilane, while the application of twin screw extrusion leads to homogenous dispersion and fully deagglomeration of filler particles without silane treatment. It has been established that when the concentration of silica filler increased from 3 up to 7% by weight, the secondary process of particle aggregation occurs.