Application of vinylogous carbamates and vinylogous aminonitriles to the regiospecific synthesis of uniquely functionalized pyrroles and quinolones

Pyrroles and quinolones represent core structures, which are routinely found in both natural and synthetic bioactive substances. Consequently, the development of efficient and regiospecific methods for the preparation of such heterocycles with unique functionality is of some importance. We describe herein the regiospecific synthesis of 1,2,3,4-tetrasubstituted pyrroles containing polar substituents and such products are prepared from vinylogous carbamates and vinylogous aminonitriles. We also describe the regiospecific synthesis of 3-aryl containing 1,3,6-trisubstituted quinolones from vinylogous carbamates. The use of an amine exchange reaction to prepare precursors for the pyrrole and quinolone forming cyclizations represents a key factor in the strategy.     

Using electrical noise to enhance the ability of humans to detect subthreshold mechanical cutaneous stimuli

Stochastic resonance (SR) is a phenomenon wherein the response of a nonlinear system to a weak input signal is optimized by the presence of a particular, nonzero level of noise. Our objective was to demonstrate cross-modality SR in human sensory perception. Specifically, we were interested in testing the hypothesis that the ability of an individual to detect a subthreshold mechanical cutaneous stimulus can be significantly enhanced by introducing a particular level of electrical noise. Psychophysical experiments were performed on 11 healthy subjects. The protocol consisted of the presentation of: (a) a subthreshold mechanical stimulus plus electrical noise, or (b) no mechanical stimulus plus electrical noise. The intensity of the electrical noise was varied between trials. Each subject's ability to identify correctly the presence of the mechanical stimulus was determined as a function of the noise intensity. In 9 of the 11 subjects, the introduction of a particular level of electrical noise significantly enhanced the subject's ability to detect the subthreshold mechanical cutaneous stimulus. In 2 of the 11 subjects, the introduction of electrical noise did not significantly change the subject's ability to detect the mechanical stimulus. These findings indicate that input electrical noise can serve as a negative masker for subthreshold mechanical tactile stimuli, i.e., electrical noise can increase the detectability of weak mechanical signals. Thus, for SR-type effects to be observed in human sensory perception, the noise and stimulus need not be of the same modality. From a bioengineering and clinical standpoint, this work suggests that an electrical noise-based technique could be used to improve tactile sensation in humans when the mechanical stimulus is around or below threshold. (c) 1998 American Institute of Physics.     

Correlation in redundancy systems

Queueing Systems - We study the tight upper bound of the transient mean waiting time in the classical GI/GI/1 queue over candidate interarrival-time distributions with finite support, given the...     

A stable hydroxide-conducting polymer

A stable hydroxide-conducting membrane based on benzimidazolium hydroxide and its analogous anion-exchange polymer is reported for the first time. The molecular and polymeric analogues possess unprecedented hydroxide stability in neutral and KOH solutions as the soluble benzimidazolium salt, made possible by steric crowding around the benzimidazolium C2 position, which is usually susceptible to nucleophilic attack by OH(-). The polymers were cast and insolubilized for the purpose of forming membranes by blending with a poly(benzimidazole) followed by hydroxide-activated electrostatic interactions. The resulting membranes possess ionic (OH(-)) conductivities of up to 13.2 mS cm(-1) and represent a new class of anion-exchange polymers and membranes.     

C-H bond strengths and acidities in aromatic systems: effects of nitrogen incorporation in mono-, di-, and triazines

The negative ion chemistry of five azine molecules has been investigated using the combined experimental techniques of negative ion photoelectron spectroscopy to obtain electron affinities (EA) and tandem flowing afterglow-selected ion tube (FA-SIFT) mass spectrometry to obtain deprotonation enthalpies (Δ(acid)H(298)). The measured Δ(acid)H(298) for the most acidic site of each azine species is combined with the EA of the corresponding radical in a thermochemical cycle to determine the corresponding C-H bond dissociation energy (BDE). The site-specific C-H BDE values of pyridine, 1,2-diazine, 1,3-diazine, 1,4-diazine, and 1,3,5-triazine are 110.4 ± 2.0, 111.3 ± 0.7, 113.4 ± 0.7, 107.5 ± 0.4, and 107.8 ± 0.7 kcal mol(-1), respectively. The application of complementary experimental methods, along with quantum chemical calculations, to a series of nitrogen-substituted azines sheds light on the influence of nitrogen atom substitution on the strength of C-H bonds in six-membered rings.     

Spectroscopic investigation of the adsorption mechanisms of polyacrylamide polymers onto iron oxide particles

The mechanisms of high-molecular-weight polyacrylamide nonionic homopolymer and 25 mol% anionic acrylate-substituted copolymer adsorption onto iron oxide particles were investigated via DRIFT and UV-vis spectroscopies at three pH values (6, 8.5, and 11). While electrostatic interactions play an important role in charged polymer adsorption, this information is not spectroscopically available. At pH values above and below pH 8.5 (the isoelectric point for the anionic polymer), bidentate chelation and hydrogen bonding were the main adsorption mechanisms. At the isoelectric point, monodentate chelation was observed to be the main mode of adsorption, along with hydrogen bonding. For the nonionic polymer, in all cases, hydrogen bonding through the carbonyl group was the main mode of adsorption. The adsorption of both polymers conformed well to the Freundlich model, suggesting that the adsorbed polymer amount increases with increasing polymer concentration up to 7500 g/t solid, rather than approaching monolayer coverage. Spectroscopic evidence was found to suggest that hydrolysis of nonionic polyacrylamide occurs at high pH.     

Substituents on quinone methides strongly modulate formation and stability of their nucleophilic adducts

Electronic perturbation of quinone methides (QM) greatly influences their stability and in turn alters the kinetics and product profile of QM reaction with deoxynucleosides. Consistent with the electron-deficient nature of this reactive intermediate, electron-donating substituents are stabilizing and electron-withdrawing substituents are destabilizing. For example, a dC N3-QM adduct is made stable over the course of observation (7 days) by the presence of an electron-withdrawing ester group that inhibits QM regeneration. Conversely, a related adduct with an electron-donating methyl group is very labile and regenerates its QM with a half-life of approximately 5 h. The generality of these effects is demonstrated with a series of alternative quinone methide precursors (QMP) containing a variety of substituents attached at different positions with respect to the exocyclic methylene. The rates of nucleophilic addition to substituted QMs measured by laser flash photolysis similarly span 5 orders of magnitude with electron-rich species reacting most slowly and electron-deficient species reacting most quickly. The reversibility of QM reaction can now be predictably adjusted for any desired application.     

Synthesis and structures of acyclic monoanionic tetradentate aza beta-diketiminate complexes of magnesium, zinc, and cadmium

An acyclic monoanionic tetradentate nitrogen ligand was prepared through the condensation of 2-(4-tolyl)-malondialdehyde and 8-aminoquinoline to give (BDI(QQ))H where (BDI(QQ))H = (8-quinolyl)-NCHC(4-tolyl)CHNH-(8-quinolyl). Metal complexes, (BDI(QQ))MX, were prepared where MX = MgBr 2, ZnCl 3, and CdOAc 4. The spectroscopic and crystallographic properties of compounds 2, 3, and 4 were explored. Structures of complexes 2, 3, 4, and the tridentate ligand, (BDI(Q))OH, 5, are reported.