Reactivity of homoallylic substituted adamantylideneadamantanes with bromine. Substituent effects on the stability of the ionic and nonionic intermediates

Sterically congested adamantylideneadamantanes (1b-g) (X = Br, Cl, F, OH, OEt, OCOCH(3)), homoallylically substituted with equatorial groups (X), react with bromine in 1,2-dichloroethane to give a stable bromonium ion intermediate or a substitution product depending on the nature of the substituent and on the bromine concentration. The nature of the substituent markedly affects the formation constant of the 1:1 pi-complexes, as well as of the formation constant and reactivity of bromonium ion intermediates. The different reactivity of the ionic intermediates, which depends on the nature of substituents, is attributed to bromonium or bromocarbenium character of the intermediate, with the support of theoretical investigations. Ab initio calculations on 1:1 adamatylideneadamantane-Br(2) complexes (2a-f) show that the substituent affects the stability of these species through electrostatic and dispersion effects. Solvent effects may also contribute to modulate the relative stability of these species.     

Trapping of wide range mass-to-charge ions and dependence on matrix amount in internal source MALDI-FTMS

Internal ionization source MALDI-FTMS shows clear variation of number average molecular weight (M(n)) for an equimolar mixture of four PEG polymers (PEG 2000, PEG 4000, PEG 6000, PEG 8000) when the 2,5-dihydroxy benzoic acid (DHB) matrix to PEG ratio is varied or the laser power is changed. As the matrix to analyte ratio is increased, the analyte signal of higher molecular mass ions increases and the signal of lower mass ions decreases. Laser power dependence studies show a similar trend. Possible explanations for these observations are discussed.     

Synthesis of (-)-centrolobine by Prins cyclizations that avoid racemization

[formula: see text] The segment-coupling Prins cyclization avoids two of the problems common to other Prins cyclization protocols: side-chain exchange and partial racemization by reversible 2-oxonia Cope rearrangement. Model studies demonstrate the stereochemical fidelity of Prins cyclizations using alpha-acetoxy ethers compared with direct aldehyde-alcohol Prins reactions. Furthermore, we propose a mechanism for the racemization observed in some intermolecular Prins cyclizations. Two straightforward syntheses of optically pure (-)-centrolobine highlight the utility of Prins cyclizations.     

Biomimetic complexes-graphene composites for redox processes

Biomimetic complexes such as porphyrins, phthalocyanines, and hemins show promising catalytic activities in redox processes. Donor-acceptor hybrids of these biomimetic macrocyclic complexes with graphene as one of the most critical allotropes of carbon are currently an outstanding scientific frontier. Notably, the composites that consist of these efficient and bio-inspiring complexes anchored on the surface of the graphene and their widespread applications have gained ever-increasing interest in the last decade, which expresses their efficiency in practice. Accordingly, this review covers the current status of biomimetic complexes-graphene composites with particular emphasis on significant signs of their synthesis and characterization also highlighted applications of these composites in academia and industrial redox processes such as oxidation of organic compounds, epoxidation, oxygen reduction reactions, CO₂ reduction, and hydrogen evolution reactions.     

Thermal Behavior of d‐Ribose Adsorbed on Silica: Effect of Inorganic Salt Coadsorption and Significance for Prebiotic Chemistry

Understanding ribose reactivity is a crucial step in the “RNA world” scenario because this molecule is a component of all extant nucleotides that make up RNA. In solution, ribose is unstable and susceptible to thermal destruction. We examined how ribose behaves upon thermal activation when adsorbed on silica, either alone or with the coadsorption of inorganic salts (MgCl₂, CaCl₂, SrCl₂, CuCl₂, FeCl₂, FeCl₃, ZnCl₂). A combination of ¹³C NMR, in situ IR, and TGA analyses revealed a variety of phenomena. When adsorbed alone, ribose remains stable up to 150 °C, at which point ring opening is observed, together with minor oxidation to a lactone. All the metal salts studied showed specific interactions with ribose after dehydration, resulting in the formation of polydentate metal ion complexes. Anomeric equilibria were affected, generally favoring ribofuranoses. Zn²⁺ stabilized ribose up to higher temperatures than bare silica (180 to 200 °C). Most other cations had an adverse effect on ribose stability, with ring opening already upon drying at 70 °C. In addition, alkaline earth cations catalyzed the dehydration of ribose to furfural and, to variable degrees, its further decarbonylation to furan. Transition‐metal ions with open d‐shells took part in redox reactions with ribose, either as reagents or as catalysts. These results allow the likelihood of prebiotic chemistry scenarios to be evaluated, and may also be of interest for the valorization of biomass‐derived carbohydrates by heterogeneous catalysis.     

Lead optimization mapper: automating free energy calculations for lead optimization

Alchemical free energy calculations hold increasing promise as an aid to drug discovery efforts. However, applications of these techniques in discovery projects have been relatively few, partly because of the difficulty of planning and setting up calculations. Here, we introduce lead optimization mapper, LOMAP, an automated algorithm to plan efficient relative free energy calculations between potential ligands within a substantial library of perhaps hundreds of compounds. In this approach, ligands are first grouped by structural similarity primarily based on the size of a (loosely defined) maximal common substructure, and then calculations are planned within and between sets of structurally related compounds. An emphasis is placed on ensuring that relative free energies can be obtained between any pair of compounds without combining the results of too many different relative free energy calculations (to avoid accumulation of error) and by providing some redundancy to allow for the possibility of error and consistency checking and provide some insight into when results can be expected to be unreliable. The algorithm is discussed in detail and a Python implementation, based on both Schrödinger’s and OpenEye’s APIs, has been made available freely under the BSD license.     

Imperfect production process with learning and forgetting effects

Computational Management Science - Wright’s learning curve (WLC) assumes every unit of production has an acceptable level of quality, which is not the case in many production environments....     

The impact of consanguinity worldwide

Consanguineous marriages have been practiced for hundreds of years in many parts of the world. The rate of congenital malformations among the offspring is 2.5 times higher than that among the offspring of unrelated parents, mainly due to the expression of autosomal-recessive disorders, and hospitalization for these reasons causes a major financial burden. An increase in sterility and in the rates of abortion, stillbirths, perinatal losses and neonatal deaths has been reported by some authors but not by others. It is generally accepted that the advantages of consanguinity outweigh the disadvantages; however, strategies for reducing the burden require a unique approach as discussed in this article.