Structure-reactivity correlation in the reactions of pyrrolidine with O-ethyl S-aryl dithiocarbonates in aqueous ethanol

The reactions of pyrrolidine with O-ethyl S-(X-phenyl) dithiocarbonates (X = 4-methyl, 4-methoxy, H, 4-chloro, 4-nitro, 2,4-dinitro, and 2,4,6-trinitro) are subjected to a kinetic study in 44 wt% aqueous ethanol, 25.0°C, and ionic strength 0.2 M (maintained with KCl). Pseudo-first-order kinetics are found under amine excess. Linear plots of the pseudo-first-order rate coefficient against concentration of free-base pyrrolidine are obtained for all the reactions, the nucleophilic rate coefficient (k_N) being the slope of such plots. The Bronsted-type plot (log k_N vs. pK_a for the leaving group) is linear with slope β_lg = − 0.2, which is consistent with a mechanism through a tetrahedral intermediate (T^±) where its formation is rate determining. The β_lg value is very similar to that found in the same reactions in water. There is a great difference in the mechanism of the reactions of O-ethyl S-phenyl dithiocarbonate with pyrrolidine (order one in amine) and piperidine (complex order in amine) in aqueous ethanol, and this is attributed to a greater nucleofugality from T^± of piperidine rather than pyrrolidine. © 1997 John Wiley & Sons, Inc.     

Two intermediates in the synthesis of deca­hydro­iso­quinolines with NMDA and AMPA receptor antagonist activity

In 6‐methyl‐N‐(4‐nitro­benzoyl)‐5,6‐di­hydropyridin‐2(1H)‐one, C₁₃H₁₂N₂O₄, (I), the piperidone ring is in a distorted half‐chair conformation. In 8‐methoxy‐3‐methyl‐N‐(4‐nitro­benzoyl)‐1,2,3,4,5,6,7,8‐octa­hydro­iso­quinoline‐1,6‐dione, C₁₈H₂₀N₂O₆, (II), the heterocyclic ring is in a slightly distorted half‐boat conformation, while the other six‐membered ring is in a distorted chair conformation. Compound (II) presents a strong intramolecular C—H?O hydrogen bond. In both (I) and (II), the mol­ecules interact through C—H?O interactions.     

Quintessential inflation from a variable cosmological constant in a 5D vacuum

We explore an effective 4D cosmological model for the universe where the variable cosmological constant governs its evolution and the pressure remains negative along all the expansion. This model is introduced from a 5D vacuum state where the (space-like) extra coordinate is considered as noncompact. The expansion is produced by the inflaton field, which is considered as nonminimally coupled to gravity. We conclude from experimental data that the coupling of the inflaton with gravity should be weak, but variable in different epochs of the evolution of the universe.     

Computing Approximate Solutions of the Maximum Covering Problem with GRASP

We consider the maximum covering problem, a combinatorial optimization problem that arises in many facility location problems. In this problem, a potential facility site covers a set of demand points. With each demand point, we associate a nonnegative weight. The task is to select a subset of p > 0 sites from the set of potential facility sites, such that the sum of weights of the covered demand points is maximized. We describe a greedy randomized adaptive search procedure (GRASP) for the maximum covering problem that finds good, though not necessarily optimum, placement configurations. We describe a well-known upper bound on the maximum coverage which can be computed by solving a linear program and show that on large instances, the GRASP can produce facility placements that are nearly optimal.     

Multiresolution schemes for strongly degenerate parabolic equations in one space dimension

An adaptive finite volume method for one‐dimensional strongly degenerate parabolic equations is presented. Using an explicit conservative numerical scheme with a third‐order Runge‐Kutta method for the time discretization, a third‐order ENO interpolation for the convective term, and adding a conservative discretization for the diffusive term, we apply the multiresolution method combining two fundamental concepts: the switch between central interpolation or exact computing of numerical flux and a thresholded wavelet transform applied to cell averages of the solution to control the switch. Applications to mathematical models of sedimentation‐consolidation processes and traffic flow with driver reaction, which involve different types of boundary conditions, illustrate the computational efficiency of the new method. © 2007 Wiley Periodicals, Inc. Numer Methods Partial Differential Eq 2007     

Minimal free resolutions that are not supported by a CW-complex

We provide the first examples of minimal free resolutions of monomial ideals which cannot be supported by any CW-complex. Their existence answers negatively the fundamental question of whether every monomial ideal admits a minimal cellular resolution.