Redox modulation of benzene triimides and diimides via noncovalent interactions

[reaction: see text] Mellitic triimides undergo three sequential one-electron reduction processes whose potentials are significantly lowered in the presence of alkyl thioureas. The two sequential reductions of benzene diimides are similarly stabilized. Calculation of the relative free energy change between the different electronic states of the imide acceptors and their corresponding alkyl thiourea complexes indicates dramatic increases in hydrogen bond strength with increasing acceptor charge density.     

Synthesis and characterization of mannosyl mimetic derivatives based on a beta-cyclodextrin core

The synthesis of branched beta-cyclodextrins substituted with mannosyl mimetic derivatives at one primary hydroxy group is described. It was shown that the self-inclusion phenomenon observed for the target compounds in water did not preclude the inclusion properties of the cyclodextrin moiety.     

Electron ionization mass spectra and their reproducibility for trialkylsilylated derivatives of organic acids, sugars and alcohols

Mass spectra of trialkylsilyl derivatives of fatty acids, dicarboxylic acids, hydroxyacids, oxoacids, sugars, amino acids and alcohols were obtained. Amino acids were analyzed as tert-butyldimethylsilyl derivatives; all other model compounds were analyzed as trimethylsilyl derivatives. Reproducibility of the electron ionization (EI) mass spectra for the derivatives obtained was discussed. It was shown that, for many investigated derivatives, composition of the respective mass spectra depended greatly on ion source contamination. The trimethylsilylated alpha-tocopherol mass spectrum composition was most significantly influenced by ion source contamination. This compound can be used to test ion source contamination.     

Validation of a method for arsenic speciation in food by ion chromatography-inductively coupled plasma/mass spectrometry after ultrasonic-assisted enzymatic extraction

A fully validated and rapid quantitative method is presented for determination of inorganic arsenic [arsenite, As(III) and arsenate, As(V)] and organic arsenic species (methylarsonic acid, dimethylarsinic acid, and arsenobetaine) by ion chromatography paired with inductively coupled plasma/MS after ultrasonic-assisted enzymatic extraction (UAEE) in rice- and seafood-based raw materials and finished products. This method gives toxicological meaning to arsenic analysis, since the sum of the toxic chemical forms As(III) and As(V) can be determined. In contrast to classical water-methanol extraction, UAEE enables drastic acceleration of sample extraction (5 min instead of several hours), while total arsenic extraction efficiency is improved without species conversion. Validation was performed to evaluate the method for selectivity, linearity, LOD/LOQ (0.007-0.020 mg/kg), trueness, precision (HorRat values, 0.2-0.6), recovery (93-122%), and uncertainty. The method was also satisfactorily tested using two proficiency tests. Performance characteristics are reported for four certified reference materials, standard reference material (SRM) 1568a (rice flour), Institute for Reference Materials and Measurements 804 (rice flour), SRM 2976 (mussel tissue), certified reference material-627 (tuna fish), and several commercial food samples populating five AOAC triangle food sectors. The results indicated that this speciation method is cost-efficient, time-saving, and accurate, as well as fit-for-purpose, according to International Organization for Standardization/International Electrotechnical Commission 17025:2005 standard, and could be used for routine analysis.     

A short access to the macrocyclic core of cycloviracin and glucolipsin

The macrocyclic core of cycloviracin and glucolipsin has been synthesised in ten steps from levoglucosan and (S)-(−)-dimethyl malate. The limited number of steps to obtain this macrolide makes it a valuable procedure for the synthesis of analogues of cycloviracin and glucolipsin.     

Poly(butyl methacrylate-g-methoxypoly(ethylene glycol)) and poly(methyl methacrylate-g-methoxypoly(ethylene glycol)) graft copolymers: preparation and aqueous solution properties

A series of water-soluble, amphiphilic graft copolymers has been prepared by free-radical copolymerization of methoxypoly(ethylene glycol) macromonomers, with either methyl methacrylate or butyl methacrylate as the comonomers, in water/ethanol solvent mixtures. Lower molecular weight copolymers were obtained by increasing the concentration of the initiator, azobisisobutyronitrile (AIBN), used in the polymerization reaction. However, the route used also led to the formation of significant quantities of tetramethylsuccinodinitrile, a toxic byproduct resulting from the cage reaction of AIBN. Static fluorescence measurements using pyrene as a probe, along with 1H NMR experiments, showed that the graft copolymers form aggregates in water at very low concentrations (approximately 0.01 g l(-1)) with the pendant hydrophilic graft chains forming a stabilizing shell around the hydrophobic backbone. An increase in the hydrophile-lipophile balance of the graft copolymers was found to lead to smaller aggregates with lower aggregation numbers and highly swollen hydrophilic shells, as revealed by small angle neutron scattering (SANS).     

Dearomatization of 3-Nitroindoles with Highly γ-Functionalized Allenoates in Formal (3+2) Cycloadditions

3-Nitroindoles are easily reacted with highly substituted γ-allenoates in the presence of a commercially available phosphine catalyst. For instance, allenoates derived from biomolecules such as amino and deoxycholic acids are combined for the first time with 3-nitroindole. The corresponding dearomatized (3+2) tricyclic cycloadducts are obtained as α-regioisomers exclusively. DFT computations shed light on this multi-step reaction mechanism and on the selectivities observed in the sequence.     

Synthesis and crystallization behavior of rigid copolyesters with biphenyl‐4,4′‐dicarboxylate and 2,6‐naphthalenedicarboxylate in the main chain

Structurally rigid copolyester thermoplastics were synthesized from 1,4‐cyclohexanedimethanol and the diesters dimethyl biphenyl‐4,4′‐dicarboxylate and dimethyl 2,6‐naphthalenedicarboxylate (DMN) via conventional melt transesterification. Conventional differential scanning calorimetry (CDSC) showed all compositions to exhibit multiple endotherms upon heating. Wide‐angle X‐ray diffraction analysis showed copolyester compositions to exhibit the crystalline structure of either the homopolyester Poly(1,4‐cyclohexylenedimethylene 2,6‐naphthalate) (PCN) or the homopolyester Poly(1,4‐cyclohexylenedimethylene 4,4′‐bibenzoate) (PCB), but not both simultaneously. Further thermal analysis using CDSC and fast DSC investigated the origin of the multiple endotherm behavior. While three endotherms are observed for low heating rates, the upper two endotherms appear to merge at heating rates about 1–5 °C s^?1 and a single endotherm remains above heating rates about 10–50 °C s^?1. While the behavior of the upper two endotherms is undeniably consistent with the mechanism of melting–recrystallization–remelting (MRR), we suggest that the low endotherm is likely associated with the melting of constrained secondary crystals, although MRR effects cannot be ruled out. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019 , 57, 973–980     

Alkyne Versus Ynamide Reactivity: Regioselective Radical Cyclization of Yne‐Ynamides

Ynamides are typically more reactive than simple alkynes and olefins. However, a serendipitous observation revealed a rare case where the reactivity of simple alkynes exceeds that of ynamides. This led to the development of a unique sulfur‐radical‐triggered cyclization of yne‐tethered ynamides, which involves attack of the alkyne by a thiyl radical followed by cyclization with the ynamide. A wide range of novel 4‐thioaryl pyrroles that could tolerate common functional moieties and N‐protecting groups were expediently constructed by this strategy. The current method contrasts with the typical cyclization of yne‐ynamides, which involves the attack of the alkyne moiety by the ynamide core. Control experiments and DFT calculations supported the participation of the sulfur radical in the reaction and the regioselective cyclization. The synthetic potential of the substituted pyrroles is also discussed.