The basicity of pyridine and its tendency towards cationic homoconjugation in non‐aqueous media

Acid dissociation constant, pK_a, of protonated pyridine, determined in the polar protophobic aprotic solvent, acetone, has been compared with pK_a values of the pyridinium ion in a variety of other polar solvents including aprotic protophobic ones, acetonitrile, benzonitrile, nitrobenzene, nitromethane, and propylene carbonate, in the protophilic aprotic dimethyl sulfoxide and N,N‐dimethylformamide, as well as in the amphiprotic methanol. On the basis of the set of these pK_a values, the effect of the medium on the basicity of pyridine is discussed. Further, based on the cationic homoconjugation constants of pyridine conjugated with the pyridinium cation in the non‐aqueous solvents, the tendency of pyridine towards homoconjugation reactions has also been discussed. Finally, both the basicity of pyridine and its tendency towards cationic homoconjugation has been compared with analogous properties of pyridine N‐oxide.     

Divergent Solid‐Phase Synthesis of Natural Product‐Inspired Bipartite Cyclodepsipeptides: Total Synthesis of Seragamide A

Macrocyclic natural products (NPs) and analogues thereof often show high affinity, selectivity, and metabolic stability, and methods for the synthesis of NP‐like macrocycle collections are of major current interest. We report an efficient solid‐phase/cyclorelease method for the synthesis of a collection of macrocyclic depsipeptides with bipartite peptide/polyketide structure inspired by the very potent F‐actin stabilizing depsipeptides of the jasplakinolide/geodiamolide class. The method includes the assembly of an acyclic precursor chain on a polymeric carrier, terminated by olefins that constitute complementary fragments of the polyketide section and cyclization by means of a relay‐ring‐closing metathesis (RRCM). The method was validated in the first total synthesis of the actin‐stabilizing cyclodepsipeptide seragamide A and the synthesis of a collection of structurally diverse bipartite depsipeptides.     

Structure formation in monolayers composed of hard bent-core molecules

ABSTRACT

We investigate the role of excluded volume interactions instabilising different structures in monolayers filled with bent-shaped molecules using the Onsager type of density functional theory supplemented by constant-pressure Monte-Carlo simulations. We study influence of molecular features, like the apex angle, thickness of the arm and the type of the arm edges on the stability of layered structures. For simple molecular shapes taken the observed phases are dominated by the lamellar antiferroelectric type as observed experimentally, but a considerable sensitivity of the ordering to details of the molecular shape is found for order parameters and wave vectors of the structures. Interestingly, for large opening angles and not too thick molecules, a window of stable nematic splay-bend phase is shown to exist.     

Microsynthesis of Dianhydrohexitols

Abstract

Conditions have been established for the microsyntheses of dianhydro-hexitols by reaction of 1, 4-monoanhydro-D, L-galactitols and 1, 5-monoanhydro-D-galactitol as well as of 1, 4- and 1, 5-monoanhydro-D-glucitols and -D-mannitols with tosyl chloride in pyridine followed by cyclization of the resulting 6-O-tosyl derivatives in methanolic solution of sodium methoxide. The dianhydrohexitols were formed by intramolecular nucleophilic substitution of the C-6 O-tosyl group with a properly stereochemically oriented hydroxyl group.

Components of the mixtures were separated by capillary gas chromato-graphy using columns coated with SP-2340 and identified by GC-MS. The identities of the synthesized dianhydrohexitols were confirmed by comparison with the GC retention times and mass spectra of authentic samples.     

P-Chiral Thioxaphosphoranesulphenyl Chlorides RR1 P(S)SC1. A New Tool in Stereochemistry of Organophosphorus-Sulphur Compounds

Abstract

Phosphoranesulphenyl halides of the general formula RR'P(Y)SX and their selenium analogues RR'P(Y)SeX (X=Cl,Br; Y=O,S) have been shown to be useful intermediates for access to many new classes of compounds con-taining phosphorus, sulphur, or selenium centers.'     

The Effect of Filtration on Physical and Chemical Properties of Osmo-Dehydrated Material

Osmotic dehydration (OD) performed in concentrated fruit juices used as osmotic solution (OS) comes with some limitations resulting from the material cell structure and is not entirely recognized at the moment. Filtration of the juice could provide some insight into the phenomena occurring throughout the OD. Therefore, the main aim of the study was to recognize the mechanism of selective penetration during OD and evaluate the effect of filtration on physical and chemical properties of osmo-dehydrated material. For this purpose, OD of pumpkin in non-filtrated and filtrated (filters 0.2, 0.45, 0.8, 1.2, 3, 5 and 8 μm) concentrated chokeberry juice was carried out in the study. Moreover, scanning electron microscope (SEM) images were provided. Total phenolic content (TPC) and antioxidant capacity measured by Ferric Reducing Antioxidant Potential (FRAP) and Trolox Equivalent Antioxidant Capacity (TEAC ABTS) of OS and the material were determined. It was found that even though filtration of osmotic solution had a moderate influence on the mass transfer, it greatly affected the chemical composition of dehydrated material. The best option, considering both chemical and physical properties of the dehydrated material, is the use of non-filtrated solution. However, when shorter time of OD is considered, much better results are obtained for filtrated solutions.     

External cavity diode lasers with E-beam written silicon diffraction gratings

Performance of external cavity diode lasers with silicon gratings produced by E-beam writing and subsequent reactive ion etching is described. Optical amplifiers used in the experiments were based on single quantum well heterostructures grown by molecular beam epitaxy. The lasers were set up in the Littrow configuration and have been designed to allow for wavelength tuning in the range centred at 960 nm.     

Interpolation of weakly compact operators

Archiv der Mathematik -     

14N and 13C NMR of tautomeric systems of mercapto- and amino-pyridines

Nitrogen chemical shifts are shown to provide a means of estimating equilibrium compositions of tautomeric systems of mercapto- and amino-derivatives of pyridine. Carbon chemical shifts can afford only qualitative information about such equilibria.     

Conformational studies by nuclear magnetic resonance—V: 13C spectra and structural problems of enamino carbonyl compounds

A ¹³C magnetic resonance investigation of 22 compounds of the general formula O?C¹(R¹)C²(R²)?C³(R³) NR⁴R^4′ has been carried out in order to confirm the conformational assignments estimated previously by PMR and IR spectroscopy. The effects of nonplanarity and hindered rotation have been discussed based on chemical shifts and shapes of the C¹, C² and C³ signals.