RELATIVE REACTIVITIES OF THE EXCITED STATES OF FUROCOUMARINS FOR [2+2] PHOTOCYCLOADDITION REACTION WITH TETRAMETHYLETHYLENE

Abstract— The Stern-Volmer constants for fluorescence quenching by tetramethylethylene decrease in the order DMC ≫ DHP > F-2 > 8-MOP. The same order was observed for the quantum yields of [2+2] cycloaddition reaction with tetramethylethylene on direct irradiation. In [2+2] photocycloaddition of F-2 with tetramethylethylene in ethanol, the ratio of quantum yields deduced from singlet and triplet states of F-2; φ₃⁰/φ₁⁰, is about 5. The excited triplet state is the reactive state for the [2+2] photocycloaddition of F-2 with tetramethylethylene in solution but the excited singlet state of F-2 becomes very important in biological conditions.     

Determination of Glimepiride in Human Plasma by LC-MS-MS and Comparison of Sample Preparation Methods for Glimepiride

A sensitive and selective method for quantitation of glimepiride in human plasma was established using liquid chromatography-electrospray ionization tandem mass spectrometry. Three different methods for the sample preparation of glimepiride and an internal standard were investigated (liquid-liquid extraction, solid-phase extraction and protein precipitation). Glipizide was used as an internal standard. Compounds were separated on a C₁₈ column with 80% acetonitrile and 20% deionized water (adjusted to pH 3.5 with acetic acid), as mobile phase at a flow rate of 200 L min^–1. By use of multiple reaction monitoring mode in MS-MS with liquid-liquid extraction and solid-phase extraction, glimepiride and glipizide were detected without severe interference from the human plasma matrix. Glimepiride produced a protonated precursor ion ([M+H]⁺) at m/z 491 and a corresponding product ion at m/z 352, and the internal standard produced a protonated precursor ion ([M+H]⁺) at m/z 446 and a corresponding product ion at m/z 321. The limit of quantitation was 0.1 ng mL^–1, 0.5 ng mL^–1 and 1.0 ng mL^–1 when using liquid-liquid extraction, solid-phase extraction and protein precipitation, respectively. The validation, reproducibility, stability, and recovery of the different sample preparation methods were comparable and all the methods gave reliable results. The method has been successfully applied to pharmacokinetic study of glimepiride in human plasma.     

Ruthenium‐catalyzed formal alkyl group transfer: Synthesis of quinolines from nitroarenes and alkylammonium halides

Nitroarenes are reductively cyclized with an array of tetraalkylammonium halides and trialkylarnmonium chlorides in the presence of a catalytic amount of a ruthenium catalyst along with tin(II) chloride dihydrate at 180° to afford the corresponding quinolines in moderate to good yields. The addition of tin(II) chloride dihydrate is necessary for the effective formation of quinolines and toluene is the solvent of choice. A reaction pathway involving initial reduction of nitroarenes to anilines and conversion of alkylammonium halides to alkylamines, alkyl group transfer from alkylamines to anilines to form an imine, dimerization of imine, and heteroannulation is proposed for this catalytic process.     

Two organometallic fragments inclusioned in a 1,3-alternate calix[4]arene tetraphosphane: evidence for transition metal-arene interaction through the cavity

The preparation of an 1,3-alternate calix[4]arene tetraphosphane ligand, 25,26,27,28-tetrakis{2-(diphenylphosphino)ethoxy}calix[4]arene (4), is described. Ligand 4 is obtained in four steps in 17% overall yield. Reaction of 4 with AgBF4 produced the encapsulated two silver complex [Ag2{(P,P,P,P)-tetraphencalix[4]arene}](BF4)2. The solid-state structure shows that the encapsulated silver undergoes a substantial pi-interaction by two opposite arene rings. Rhodation of 4 employing [Rh(cot)2]BF4 yielded the encapsulated complex with a bent coordination mode. Two organometallic fragments inclusioned inside a 1,3-alternate calix[4]arene tetraphosphane was also achieved by the reaction of 4 with [PtH(PPh3)2 (thf)]+. Full characterization includes X-ray structural studies of compounds 4-6.     

Electronic structure and field emission of multiwalled carbon nanotubes depending on growth temperature

The electronic structure of multiwalled carbon nanotubes (CNTs) has been investigated, depending on the growth temperature, using synchrotron X-ray photoelectron spectroscopy (XPS) and field emission measurements. The vertically aligned CNTs are grown via pyrolysis of ferrocene and acetylene in a broad temperature range 600-1000 degrees C. The CNTs have a cylindrical structure with a uniform diameter of 20 nm. As growth temperature increases, due to an improved crystallinity of the graphitic sheets, the width of the XPS C 1s peak becomes narrower and the intensity of the valence band increases. Field emission from the as-grown CNTs exhibits a large enhancement of current density with growth temperature, strongly correlated with the electronic structure revealed by XPS.     

Self-assembled metallocycles with two interactive binding domains

Five metallocycles 1 a-e have been self-assembled from S-shaped bispyridyl ligands 2 a-e and a palladium complex, [Pd(dppp)(OTf)(2)] (dppp=1,3-bis(diphenylphosphanyl)propane), and have been characterized by elemental analysis and various spectroscopic methods including (1)H NMR spectroscopy and electrospray ionization (ESI) mass spectrometry. These metallocycles all are monocyclic compounds, but can fold to generate two binding domains bearing hydrogen-bonding sites based on pyridine-2,6-dicarboxamide units. The binding properties of the metallocycles with N,N,N',N'-tetramethylterephthalamide (G) have been probed by means of ESI mass spectrometry and (1)H NMR spectroscopy. The results both in the gas phase and in solution are consistent with the fact that the metallocycles accommodate two molecules of the guest G. Thus, the ESI mass spectra clearly show fragments corresponding to the 1:2 complexes in all cases. (1)H NMR studies on 1 a and G support the formation of a 1:2 complex in solution; the titration curves are nicely fitted to a 1:2 binding isotherm, but not to a 1:1 binding isotherm. In addition, a Job plot also suggests a 1:2 binding mode between 1 a and G, showing maximum complexation at approximately 0.33 mol fraction of the metallocycle 1 a in CDCl(3). The binding constants K(1) and K(2) are calculated to be 1600 and 1400 M(-1) (+/-10 %), respectively, at 25 degrees C in CDCl(3), indicative of positively cooperative binding. This positive cooperativity was confirmed by the Hill equation, affording a Hill coefficient of n = 1.6. Owing to insufficient solubility in CDCl(3), for comparison purposes the binding properties of the metallocycles 1 b-e were investigated in a more polar medium, 3 % CD(3)CN/CDCl(3). (1)H NMR titrations revealed that the metallocycles all bind two molecules of the guest G with Hill coefficients ranging from 1.4 to 1.8. This positive cooperativity may be attributed to a structural reorganization of the second binding cavity when the first guest binds to either one of the subcavities present in the metallocycles.     

NBS-Promoted reactions of symmetrically hindered methylphenols via p-benzoquinone methide

Symmetrically hindered methylphenols 1 react smoothly with NBS to form transient intermediates, p-benzoquinone methides (BM), which can be further processed to give hydroxybenzaldehydes in the presence of DMSO. This reaction is initiated by the formation of the phenoxy radical, followed by disproportionation to afford BM. None of the side-chain-brominated product is observed. The existence of BM is supported by the following observations: the formation of BM in solution can be monitored by GC and GC-MS; the electrophilic methine part participates in electrophilic aromatic substitution with anisoles to give hydroxybenzylated products 15; and the double bond character of the exocyclic methine plays a role in [4 + 2] cycloaddition with diene to afford Diels-Alder adducts. In contrast, unsymmetrically hindered or simple methylphenol (p-cresol) with NBS gives the nuclear brominated products, as usual. The energies of symmetrically hindered BMs, unsymmetrically hindered BM, and simple BM were calculated using density functional theories. Relative stabilization energies calculated at the B3LYP/6-31G//B3LYP/6-31G level by an isodesmic equation are enhanced 3-6 kcal/mol for symmetrically hindered BMs.     

Stacking sequence effect of aramid–UHMPE hybrid composites by flexural test method: MATERIAL PROPERTIES

Aramid fibre–ultra-high modulus polyethylene (UHMPE) fibre interply hybrid composites were fabricated with changes in the stacking sequence. The flexural strength and modulus of hybrid composites were measured in order to investigate the effect of stacking sequence. Scanning electron microscopy (SEM) was used to examine the fracture surface of interply hybrid composites. When aramid fibre was at the compressive side and dispersion extent of fibers was small, the higher flexural strength and positive hybrid effect were observed. In addition, the different stacking sequence resulted in a change in flexural failure mechanism which had an effect on the flexural strength. As the dispersion extent of fibers decreased, the introduction of cohesive failure in aramid–aramid interface and PE–PE interface improved the flexural strength of hybrid composites.     

Chelators for radioimmunotherapy: I. NMR and ab initio calculation studies on 1,4,7,10-tetra(carboxyethyl)-1,4,7,10-tetraazacyclododecane (DO4Pr) and 1,4,7-tris(carboxymethyl)-10-(carboxyethyl)-1,4,7,10-tetraazacyclododecane (DO3A1Pr)

This work describes the modification of the chelating agent 1,4,7,10-tetraazacyclododecane-N,N',N' ',N' "-tetraacetic acid (DOTA) to improve the rate of metal loading for radioimmunotherapy applications. Previous ab initio calculations predicted that the compounds 1,4,7,10-tetra(carboxyethyl)-1,4,7,10-tetraazacyclododecane (DO4Pr) and 1,4,7-tris(carboxymethyl)-10-(carboxyethyl)-1,4,7,10-tetraazacyclododecane (DO3A1Pr) have a ca. 2000-fold improvement in yttrium metal loading rates compared to those of DOTA (Jang, Y. H.; Blanco, M.; Dasgupta, S.; Keire, D. A.; Shively, J. E.; Goddard, W. A., III. J. Am. Chem. Soc. 1999, 121, 6142-6151). In this study, we report the synthesis, purification, (1)H-NMR chemical shift assignments, pK(a) values, metal loading rate measurements, and additional ab initio calculations of these two compounds. The yttrium loading rates of DO3A1Pr are approximately twice those of DOTA, at pH 4.6 and 37 degrees C. The NMR data indicates that the DO4Pr analogue forms a stable type I complex but does not form a type II complex. The new ab initio calculations performed on DO4Pr and DO3A1Pr indicate that the rate-determining step is the deprotonation of the first macrocycle amine proton, not the second proton as assumed in the previous calculations. The new calculations predict an improvement in the rate of metal loading that more closely matches the experimentally observed change in the rate.