Total Synthesis of Crenulatan Diterpenes: Strategy and stereocontrolled construction of a bicyclic keto-lactone building block

The bicyclic keto lactone 26 was synthesized for the purpose of developing a viable route to marine diterpenes of the crenulatan type. Following the efficient conversion of (S)-citronellol ( 5 ) to the allylated alcohol 9a (Scheme 2), the αβ-unsaturated lactone 12 was efficiently accessed in preparation for stereocontrolled conjugate addition. The hydroxymethyl equivalent most suited to this task was (i-PrO)Me₂SiCH₂MgCl, which gave 13 predominantly in the presence of CuI and Me₃SiCl. Once the OH group was deprotected (→ 14 ), it proved an easy matter to implement acid-catalyzed isomerization to lactone 15 , oxidation of which gave the pivotal aldehyde 16 . Condensation of 16 with PhSeCH₂Li led via 21 to 22 (Scheme 3). Once the OH group was protected (→ 22b ), it proved possible to effect aldolization with crotonaldehyde (→ 23 ). Exposure of 23 to acid gave the sub-target compound 25 . Its subsequent oxidation and thermal activation resulted in sequential selenoxide elimination with Claisen rearrangement (→ 26 ). The structural features of 26 require that a chair-like transition state be adopted during the [3.3]sigmatropic event. With the clarification of these issues, a highly serviceable and more advanced assault on the crenulatans should prove capable of being mounted.     

The Kashiwara conjugation and wave-front sets of regular holonomic distributions on a complex manifold

Erratum

The Kashiwara conjugation and wave-front sets of regular holonomic distributions on a complex manifold     

Impact of the Cooking Process on Metabolite Profiling of Acanthocereus tetragonus,a Plant Traditionally Consumed in Mexico

Acanthocereus tetragonus (L.) Hummelinck is used as an alternative food source in some Mexican communities. It has been shown that the young stems of A. tetragonus provide crude protein, fiber, and essential minerals for humans. In this work, we analyzed the phytochemical profile, the total phenolic content (TPC), and the antioxidant activity of cooked and crude samples of A. tetragonus to assess its functional metabolite contribution to humans. The phytochemical profile was analyzed using Ultra-High-Performance Liquid Chromatography coupled to High-Resolution Mass Spectrometry (UHPLC-PDA-HESI-Orbitrap-MS/MS). Under the proposed conditions, 35 metabolites were separated and tentatively identified. Of the separated metabolites, 16 occurred exclusively in cooked samples, 6 in crude samples, and 9 in both crude and cooked samples. Among the detected compounds, carboxylic acids, such as threonic, citric, and malic acids, phenolic acids, and glycosylated flavonoids (luteolin-O-rutinoside) were detected. The TPC and antioxidant activity were analyzed using the Folin–Ciocalteu method and the 2,2-diphenyl-1-picrylhydrazyl (DPPH) free radical inhibition method, respectively. The TPC and antioxidant activity were significantly reduced in the cooked samples. We found that some metabolites remained intact after the cooking process, suggesting that A. tetragonus represents a source of functional metabolites for people who consume this plant species.     

Surface Modification Strategy for Enhanced NO2 Capture in Metal–Organic Frameworks

The interaction strength of nitrogen dioxide (NO₂) with a set of 43 functionalized benzene molecules was investigated by performing density functional theory (DFT) calculations. The functional groups under study were strategically selected as potential modifications of the organic linker of existing metal–organic frameworks (MOFs) in order to enhance their uptake of NO₂ molecules. Among the functional groups considered, the highest interaction energy with NO₂ (5.4 kcal/mol) was found for phenyl hydrogen sulfate (-OSO₃H) at the RI-DSD-BLYP/def2-TZVPP level of theory—an interaction almost three times larger than the corresponding binding energy for non-functionalized benzene (2.0 kcal/mol). The groups with the strongest NO₂ interactions (-OSO₃H, -PO₃H₂, -OPO₃H₂) were selected for functionalizing the linker of IRMOF-8 and investigating the trend in their NO₂ uptake capacities with grand canonical Monte Carlo (GCMC) simulations at ambient temperature for a wide pressure range. The predicted isotherms show a profound enhancement of the NO₂ uptake with the introduction of the strongly-binding functional groups in the framework, rendering them promising modification candidates for improving the NO₂ uptake performance not only in MOFs but also in various other porous materials.     

Generation and detection of acoustic solitons in crystalline slabs by laser ultrasonics

Acoustic solitons have been recently observed in different systems (Si, Sapphire, MgO, alpha-quartz). Such acoustic waves could lead to sub-picosecond acoustic pulses. In this paper, we report on the formation of acoustic solitons in a GaAs crystalline slab. A short picosecond acoustic pulse is generated by absorption of a femtosecond laser pulse in an aluminum thin film deposited on one side of the slab. This strain pulse travels through the sample up to the opposite side where it is detected by a time delayed laser pulse reflected by an aluminum transducer. We use interferometric detection to measure independently the real and imaginary parts of the relative change in optical reflectivity induced by the acoustic pulse. We find that, at low temperature and with a laser pump pulse energy of 10 nJ, an acoustic soliton clearly separates from the acoustic pulse in GaAs slab. The soliton shape is compared with numerical simulations for different excitation conditions. From the very unique properties of solitons, we infer a soliton pulse duration of about 2.3 ps which corresponds to a spatial extent of only 12 nm.     

Attenuation in trabecular bone: A comparison between numerical simulation and experimental results in human femur

Numerical simulations (finite-difference time domain) are compared to experimental results of ultrasound wave propagation through human trabecular bones. Three-dimensional high-resolution microcomputed tomography reconstructions served as input geometry for the simulation. The numerical simulation took into account scattering, but not absorption. Simulated and experimental values of the attenuation coefficients (alpha, dB/cm) and the normalized broadband ultrasound attenuation (nBUA, dB/cm/MHz) were measured and compared on a set of 28 samples. While experimental and simulated nBUA values were highly correlated (R(2)=0.83), and showed a similar dependence with bone volume fraction, the simulation correctly predicted experimental nBUA values only for low bone volume fraction (BV/TV). Attenuation coefficients were underestimated by the simulation. The absolute difference between experimental and simulated alpha values increased with both BV/TV and frequency. As a function of frequency, the relative difference between experimental and simulated alpha values decreased from 60% around 400 kHz to 30% around 1.2 MHz. Under the assumption that the observed discrepancy expresses the effect of the absorption, our results suggests that nBUA and its dependence on BV/TV can be mostly explained by scattering, and that the relative contribution of scattering to alpha increases with frequency, becoming predominant (>50 %) over absorption for frequencies above 600 kHz.     

Simple approach for charge renormalization in highly charged macroions

We revisit the notion of renormalized charge, which is a concept of central importance in the field of highly charged colloidal or polyelectrolyte solutions. Working at the level of a linear Debye-Hückel-like theory only, we propose a versatile method to predict the saturated amount of charge renormalization, which is, however, a nonlinear effect arising at strong electrostatic coupling. The results are successfully tested against nonlinear Poisson-Boltzmann theory for polyions of various shapes (planar, cylindrical, and spherical), both in the infinite dilution limit or in confined geometry, with or without added electrolyte. Our approach, accurate for monovalent microions in solvents such as water, is finally confronted against experimental results on charged colloids and B-DNA solutions.     

Adsorption kinetics of 4-chlorophenol onto granular activated carbon in the presence of high frequency ultrasound

This work describes the results of investigations carried out to examine the adsorption kinetics of 4-chlorophenol (4-CP) from aqueous solution containing tert-butyl alcohol (10%, v/v) onto granular activated carbon (GAC) in the presence of ultrasound of different high frequencies (516, 800 and 1660 kHz) and acoustic powers (15.2, 21.5, 31.1 and 38.3 W). The main objective of this study is to describe the mechanism of ultrasound-assisted adsorption rather than the enhancement of adsorption capacity. Sonochemical degradation of 4-CP was studied in the absence and presence of tert-butyl alcohol. The sonolysis of 4-CP is effectively inhibited by the addition of tert-butyl alcohol (10%, v/v) and very little 4-CP degradation occurs, indicating that little or no pyrolysis of the compound occurs. Without addition of tert-butyl alcohol, after 300 min and at 1660 kHz, the removal of 4-CP in the presence of ultrasound for an acoustic power of 38.3 W was nearly total (99%), but in the conventional method only 60% was eliminated. In this case, the removal of 4-CP by GAC in the ultrasound-assisted technique is due to both adsorption and ultrasonic degradation, but the removal by simple stirring is only due to adsorption, which makes a direct comparison unacceptable. In order to distinguish sonochemical degradation and adsorption of 4-CP onto GAC and to make an exact and practical comparison of the adsorption in the absence and presence of ultrasound, kinetic adsorption experiments were conducted using aqueous solution containing 10% (v/v) tert-butyl alcohol. The obtained results show that both adsorption rate and adsorbed amount were significantly enhanced and improved in the presence of ultrasound for all the studied frequencies and powers. The enhancement of adsorption is favored by increasing ultrasonic power. Adsorption kinetic data were modeled using the liquid-film mass transfer equation and intraparticle diffusion model. The values of the intraparticle diffusion coefficient obtained in the presence of ultrasound are greater than that obtained in the absence of ultrasound. In the initial period of adsorption, where external mass transfer is assumed to predominate, liquid-film mass transfer coefficients significantly increased by the assistance of ultrasound. These results indicate that ultrasound enhances the mass transport in the pores as well as across the boundary layer. This effect increased with increasing ultrasonic power for the three studied frequencies. The average order for the studied ultrasonic waves according to the initial adsorption rate, the intraparticle diffusion coefficient and the liquid-film mass transfer coefficient is 516 kHz>800 kHz>1660 kHz.     

Radiative heat transfer at nanoscale: Closed-form expression for silicon at different doping levels

Radiative heat transfer at the nanoscale is becoming an important technological issue with the development of nano electromechanical systems (NEMS). In this article, we derive asymptotic expressions to compute near-field radiative heat transfer between two planes of silicon. We identify two physical mechanisms that give the dominant contribution at small gaps. For intrinsic and low-doped silicon, the main contribution is due to evanescent waves coming from propagating waves undergoing frustrated total internal reflections at the interfaces. For doping levels larger than N_e=10¹⁶ cm^?3 surface mode coupling contributes to the heat transfer. Asymptotic expressions are also given in that case. In all cases, we compare analytical formulas with exact numerical calculations when varying the temperature and the doping concentration. We also give their range of validity.