Palladium-catalyzed olefin migration and 7-endo-trig cyclization of dehydroalanines

1,4- or 1,5-migration of a double bond of dehydroalanines under palladium catalysis is reported. The process occurred with several highly-substituted dehydroalanines under mild typical Heck reaction conditions. Although this was one of the first reports of a palladium-catalyzed 1,5-olefin migration, the outcome of the reaction was susceptible to the starting material. Mechanistically, the process involved a five- (1,4-migration) or six-membered ring (1,5-migration) organopalladium intermediate, which cleaved a C–N bond in an β-amino elimination process.     

Synthesis and crystal structures of two novel 3,4,5- trimethoxyphenyl derivatives from (Z)-1-[(2′,3′-dinitro-4′-methoxy)-phenyl]-2-[(3″,4″,5″-trimethoxy)-phenyl] ethene

The title compounds, (Z)-1-[(2′,4′-dimethoxy-3′-nitro)-phenyl]-2-[(3″,4″,5″-trimethoxy)-phenyl]ethene, C₁₉H₂₁O₇N (I), and 2-amino-4-methyl-6-(3,4,5-trimethoxy-phenyl)-pyrimidine, C₁₄H₁₇O₃N₃ (II), were obtained unexpectedly from a reaction involving (Z)-1-[(2′,3′-dinitro-4′-methoxy)-phenyl]-2-[(3″,4″,5″-trimethoxy)-phenyl]ethene (IV). The molecular structures of these compounds were obtained by single-crystal X-ray diffraction. Crystallization of I occurs in the centrosymmetric monoclinic space group P2₁/c (No. 14) with a=7.7311(5), b=19.9239(13), c=11.5725(8); and β=92.193(3) and Z=4. Crystallization of II occurs in the centrosymmetric monoclinic space group C2/c (No. 15) with a=21.296(2), b=6.8963(7), c=20.001(2); and β=114.121(6) and Z=8. Details of the synthesis and the structural characterization of the title compounds are presented and discussed.     

Cycloreversion of azetidines via oxidative electron transfer. steady-state and time-resolved studies

Cycloreversion of cis- and trans-1,2,3-triphenylazetidine (c-2 and t-2) is achieved by electron transfer to (tris(4-bromophenyl)aminium radical cation (5 (*+)). Stepwise C-N and C-C bond cleavage of azetidine radical cations leads to cis- and trans-stilbene, together with N-benzylideneaniline, as final products. Mechanistic evidence is provided by quenching studies, using laser flash photolysis to generate 5 (*+) from its neutral precursor.     

Super-Strong Hydrogel Composites Reinforced with PBO Nanofibers for Cartilage Replacement

Cartilage replacement materials exhibiting a set of demanding properties such as high water content, high mechanical stiffness, low friction, and excellent biocompatibility are quite difficult to achieve. Here, poly(p-phenylene-2,6-benzobisoxazole) (PBO) nanofibers are combined with polyvinyl alcohol (PVA) to form a super-strong structure with a performance that surpasses the vast majority of previously existing hydrogels. PVA–PBO composites with water contents in the 59–76% range exhibit tensile and compressive moduli reaching 20.3 and 4.5 MPa, respectively, and a coefficient of friction below 0.08. Further, they are biocompatible and support the viability of chondrocytes for 1 week, with significant improvements in cell adhesion, proliferation, and differentiation compared to PVA. The new composites can be safely sterilized by steam heat or gamma radiation without compromising their integrity and overall performance. In addition, they show potential to be used as local delivery platforms for anti-inflammatory drugs. These attractive features make PVA–PBO composites highly competitive engineered materials with remarkable potential for use in the design of load-bearing tissues. Complementary work has also revealed that these composites will be interesting alternatives in other industrial fields where high thermal and mechanical resistance are essential requirements, or which can take advantage of the pH responsiveness functionality.     

Theoretical characterization of the chemical bonds of some three-membered ring compounds through QTAIM theory

Structural Chemistry - In this paper, we investigate the nature of the carbonyl and the intraring C–C, C–N, C–O, N–N, O–O and N–O bonds of cyclopropanone and the...     

Solvent Mixture Optimization in the Extraction of Bioactive Compounds and Antioxidant Activities from Garlic (Allium sativum L.)

Garlic is a health promoter that has important bioactive compounds. The bioactive extraction is an important step in the analysis of constituents present in plant preparations. The purpose of this study is to optimize the extraction with the best proportion of solvents to obtain total phenolic compounds (TPC) and thiosulfinates (TS) from dried garlic powder, and evaluate the antioxidant activities of the optimized extracts. A statistical mixture simplex axial design was used to evaluate the effect of solvents (water, ethanol, and acetone), as well as mixtures of these solvents, after two ultrasound extraction cycles of 15 min. Results showed that solvent mixtures with a high portion of water and pure water were efficient for TPC and TS recovery through this extraction procedure. According to the regression model computed, the most significant solvent mixtures to obtain high TPC and TS recovery from dried garlic powder are, respectively, the binary mixture with 75% water and 25% acetone and pure water. These optimized extracts presented oxygen radical absorbance capacity. Pure water was better for total antioxidant capacity, and the binary mixture of water–acetone (75:25) was better for DPPH scavenging activity. These optimized extracts can be used for industrial and research applications.     

Crosslinking and degradation of PVP hydrogels as a function of dose and PVP concentration

Swelling was performed on dried membranes, normal ones and dried gel to unravel the role of crosslinking and degradation on polymeric structure during drying and hydration process. The comparison of the swelling results suggested that the network were formed only by PVP molecules. The complex mixture of macromolecules showed a irreversible behavior upon drying and hydration, probably as a function of PEG and/or agar entangling in effective physical crosslink. The best network regularity and useful properties was found at 20–30 kGy.     

Synthesis of polystyrene-polysiloxane side-chain liquid crystalline block copolymers

The synthesis of a new liquid crystalline block copolymer consisting of a polystyrene block and a side-chain liquid crystalline siloxane block is reported. The synthetic approach described is based on the anionic polymerization of styrene and cyclic trimethyltrivinyltrisiloxane monomers, followed by functionalization of the siloxane block with side chain mesogens. The siloxane block has a T_g well below 25°C and is designed to exhibit a chiral smectic C^* phase at room temperature. These block copolymers are the first side-chain liquid crystalline block copolymers which contain both a high T_g glassy block and a low T_g liquid crystalline block.