Stereoselective conjugate addition reactions to phenylglycinol-derived, unsaturated oxazolopiperidone lactams

Phenylglycinol-derived, unsaturated oxazolopiperidone lactams are extremely useful building blocks that undergo stereoselective conjugate addition reactions with organocuprates, enolates, and sulfur-stabilized anions, allowing the stereocontrolled introduction of substituents at the piperidine 4-position. The factors governing the exo- or endo-facial selectivity are discussed. The methodology can be used for the preparation of a variety of enantiopure piperidines, including pharmaceuticals, alkaloids precursors, piperidine-fused derivatives, as well as complex piperidine-containing natural products.     

Third-order iterative methods with applications to Hammerstein equations: A unified approach

The geometrical interpretation of a family of higher order iterative methods for solving nonlinear scalar equations was presented in [S. Amat, S. Busquier, J.M. Gutiérrez, Geometric constructions of iterative functions to solve nonlinear equations. J. Comput. Appl. Math. 157(1) (2003) 197-205]. This family includes, as particular cases, some of the most famous third-order iterative methods: Chebyshev methods, Halley methods, super-Halley methods, C-methods and Newton-type two-step methods. The aim of the present paper is to analyze the convergence of this family for equations defined between two Banach spaces by using a technique developed in [J.A. Ezquerro, M.A. Hernández, Halley’s method for operators with unbounded second derivative. Appl. Numer. Math. 57(3) (2007) 354-360]. This technique allows us to obtain a general semilocal convergence result for these methods, where the usual conditions on the second derivative are relaxed. On the other hand, the main practical difficulty related to the classical third-order iterative methods is the evaluation of bilinear operators, typically second-order Fréchet derivatives. However, in some cases, the second derivative is easy to evaluate. A clear example is provided by the approximation of Hammerstein equations, where it is diagonal by blocks. We finish the paper by applying our methods to some nonlinear integral equations of this type.     

Improving the compression rate versus L 1 error ratio in cell-average error control algorithms

Multiresolution representations of data are powerful tools in data compression. A common framework for applications is the cell-average setting. For a proper adaptation to singularities, it is interesting to develop nonlinear methods. Thus, one needs to control the stability of these representations. We introduce a generalization, depending on a parameter λ, of the classical cell-average error-control algorithms and we study the choice of the parameter to get the best relation quality vs. ratio of compression. It turns out that λ can be chosen nonlinearly and that for the L ¹ norm we can get a significant improvement over the classical error-control algorithms.     

One pot synthesis of imidazo[2,1-b]thiazoles and benzo[d]thiazolo[3,2-a]imidazoles

A series of imidazo[2,1-b]thiazole and benzo[d]thiazolo[3,2-a]imidazole analogues were synthesized by stirring an equimolar mixture of dibenzoylacetylene with imidazole/thiazole derivatives in toluene or acetonitrile at room temperature. The products were generated in good yields and characterized by standard analytical techniques such as IR, ¹H NMR, ¹³C NMR and mass spectrometry. The structure of products 19, 20, 22, 24 and 25 were also unambiguously confirmed by single crystal X-ray analysis.     

Structure-directed reversion in the pi-facial stereoselective alkylation of chiral bicyclic lactams

The reversal of the pi-facial diastereoselectivity observed in the benzyl bromide alkylation of the enolate of phenylglycinol-derived oxazolopiperidone is examined by means of theoretical calculations and experimental assays. When the angular carbon adopts an R configuration, the endo addition is intrinsically favored due to the lower torsional strain induced in the TS, but for the reaction with benzyl bromide, which affords the exo product. This reversal is attributed to the formation of a C-H...pi hydrogen bond between the C-H unit of the C8a angular position and the benzene ring of the alkylating reagent. A similar secondary interaction explains the stereochemical reversion observed in the benzyl alkylation of the enolate of oxazolopyrrolidinone. These findings point out that the delicate balance between factors that dictate the diastereoselectivity in the alkylation of chiral byciclic lactams can be unexpectedly altered by weak secondary interactions. The results presented here provide valuable guidelines to tune the selective preparation of enantiopure bioorganic and pharmaceutical compounds.     

Kinetics and mechanism of large rate enhancement in the alkaline hydrolysis of N'-morpholino-N-(2'-methoxyphenyl)phthalamide

The apparent second-order rate constant (k OH) for hydroxide-ion-catalyzed conversion of 1 to N-(2'-methoxyphenyl)phthalamate (4) is approximately 10(3)-fold larger than k OH for alkaline hydrolysis of N-morpholinobenzamide (2). These results are explained in terms of the reaction scheme 1 --> k(1obs) 3 --> k(2obs) 4 where 3 represents N-(2'-methoxyphenyl)phthalimide and the values of k(2obs)/k(1obs) vary from 6.0 x 10(2) to 17 x 10(2) within [NaOH] range of 5.0 x 10(-3) to 2.0 M. Pseudo-first-order rate constants (k(obs)) for alkaline hydrolysis of 1 decrease from 21.7 x 10(-3) to 15.6 x 10(-3) s(-1) with an increase in ionic strength (by NaCl) from 0.5 to 2.5 M at 0.5 M NaOH and 35 degrees C. The values of k obs, obtained for alkaline hydrolysis of 2 within [NaOH] range 1.0 x 10(-2) to 2.0 M at 35 degrees C, follow the relationship k(obs) = kOH[HO(-)] + kOH'[HO (-)] (2) with least-squares calculated values of kOH and kOH' as (6.38 +/- 0.15) x 10(-5) and (4.59 +/- 0.09) x 10(-5) M (-2) s(-1), respectively. A few kinetic runs for aqueous cleavage of 1, N'-morpholino-N-(2'-methoxyphenyl)-5-nitrophthalamide (5) and N'-morpholino-N-(2'-methoxyphenyl)-4-nitrophthalamide (6) at 35 degrees C and 0.05 M NaOH as well as 0.05 M NaOD reveal the solvent deuterium kinetic isotope effect (= k(obs) (H 2) (O)/ k(obs) (D 2 ) (O)) as 1.6 for 1, 1.9 for 5, and 1.8 for 6. Product characterization study on the cleavage of 5, 6, and N-(2'-methoxyphenyl)-4-nitrophthalimide (7) at 0.5 M NaOD in D2O solvent shows the imide-intermediate mechanism as the exclusive mechanism.     

Challenges in the simulation of dye-sensitized ZnO solar cells: quantum confinement, alignment of energy levels and excited state nature at the dye/semiconductor interface

We report a first principles density functional theory/time-dependent density functional theory (DFT/TDDFT) computational investigation on a prototypical perylene dye anchored to realistic ZnO nanostructures, approaching the size of the ZnO nanowires used in dye-sensitized solar cells devices. DFT calculations were performed on (ZnO)(n) clusters of increasing size, with n up to 222, of 1.3 × 1.5 × 3.4 nm dimensions, and for the related dye-sensitized models. We show that quantum confinement in the ZnO nanostructures substantially affects the dye/semiconductor alignment of energy levels, with smaller ZnO models providing unfavourable electron injection. An increasing broadening of the dye LUMO is found moving to larger substrates, substantially contributing to the interfacial electronic coupling. TDDFT excited state calculations for the investigated dye@(ZnO)(222) system are fully consistent with experimental data, quantitatively reproducing the red-shift and broadening of the visible absorption spectrum observed for the ZnO-anchored dye compared to the dye in solution. TDDFT calculations on the fully interacting system also introduce a contribution to the dye/semiconductor admixture, due to configurational excited state mixing. Our results highlight the importance of quantum confinement in dye-sensitized ZnO interfaces, and provide the fundamental insight lying at the heart of the associated DSC devices.     

A practical synthetic route to enantiopure 6-substituted cis-decahydroquinolines

Starting from 4-substituted cyclohexanones, a practical synthetic route to enantiopure 6-substituted cis-decahydroquinolines has been developed, the key steps being a stereoselective cyclocondensation of an unsaturated δ-keto ester derivative with (R)-phenylglycinol and the stereoselective hydrogenation of the resulting tricyclic oxazoloquinolone lactams.     

On the origin of the stereoselectivity in the alkylation of oxazolopiperidone enolates

The origin of the diastereoselective alkylation of enolates of oxazolopiperidones is studied by means of theoretical calculations and experimental assays. For the unsubstituted oxazolopiperidone, the alkylation with methyl chloride is predicted to afford mainly the exo product, a finding further corroborated from the analysis of the experimental outcome obtained in the reaction of the racemic oxazolopiperidone. However, such a preference can be drastically altered by the presence of substituents attached to the fused ring. In particular, when the angular carbon adopts an R configuration in a phenylglycinol-derived oxazolopiperidone, the presence of a phenyl ring at position 3 forces the pseudo-planarity of the bicyclic lactam, and the diastereoselectivity is dictated by the internal torsional strain induced in the enolate. However, when the angular carbon adopts an S configuration, the preference for the exo alkylation stems from the intermolecular steric hindrance between the enolate and the alkylating reagent. Interestingly, the intramolecular hydrogen bond formed between the phenyl ring and the carbonyl oxygen in the enolate largely reduces the difference in stability of the two TSs compared to the unsubstituted oxazolopiperidone.     

Mechanistic study of hydrolytic erosion and drug release behaviour of PLGA nanoparticles: Influence of chitosan

The hydrolytic erosion of poly(d,l-lactide-co-glycolide) (PLGA) nanoparticles (PLGA-NPs) was investigated in vitro. The changes in physical properties of the nanoparticles with time were evaluated by ultra high-pressure liquid chromatographic (UHPLC) analysis, particle size analysis and scanning electron microscopy (SEM). Mass reduction data demonstrated a triphasic erosion pattern for PLGA-NPs with nearly no mass loss (3.0%) up to a week, followed by a rapid mass loss (weeks 1-3, 61.4%), and further followed by slow mass loss (weeks 3-5, 19.8%). SEM revealed microcavitation on the surface of nanoparticles, which tended to increase with the erosion time and eventually particle fragmentation was evident at 5 weeks. A significant increase in particle size was observed at 4 weeks which can be attributed to particle aggregation, however, at about 5 weeks, the particle size decreased significantly owing to particle fragmentation. The hydrolytic erosion of PLGA-NPs was found to be specifically proton catalyzed. The release profile of the model drug, moxifloxacin, from PLGA-NPs was closely related to nanoparticle erosion except for the initial burst release which was based on diffusion. The presence of chitosan in the PLGA-NPs accelerated the rate of erosion of the nanoparticles and reduced the burst release of the drug. An understanding of the erosion mechanism and alteration in erosion by chitosan could give desirable and more uniform drug release kinetics from PLGA-NPs.