Current Organic Chemistry - Volume 21, Issue 7, 2017

The deoxygenation of tertiary phosphine oxides is an important reaction, as it provides useful intermediates and reagents, makes the available P-ligands useful in transition metal complexes, and regenerates wastes produced by the Wittig-, Mitsunobu and Appel reactions. Methods for the reduction of the P=O moiety comprising different metal and other hydrides, silanes and other species are surveyed, especially from the point of view of environmentally-friendly (green) chemistry, and practical applicability to be able to select the most suitable reagent. Oxalyl chloride converting the phosphine oxide to the corresponding chlorophosphonium salt may also be used, but this step should be followed by a reduction. Obviously, silanes are the reagents of choice. The most widespread, but corrosive trichlorosilane may be substituted by more userfriendly alternatives, such as phenylsilane, tetramethyldisiloxane (TMDS) and polymethylhydrosiloxane (PMHS). The latter two species are cheap, but of lower reactivity, however, they may be useful in microwave-assisted and solvent-free deoxygenations. Stereochemical and mechanistic aspects of the most often applied silane reductions are also discussed. In most cases, the P-configuration is preserved.

Ultrafast photoreactions play a key-role in numerous biological systems. They provide vision, phototaxis, UV-protection as well as energy storage and energy conversion with an efficiency which, so far, has not been reached under laboratory conditions. Detailed mechanistic studies can help to elucidate the underlying principles, but the systems of interest are often too large to be treated solely with accurate quantum methods. This paper reviews current strategies for studying ultrafast biological photodynamics by use of combined QM/MM schemes with surface hopping. In these methods, the photoreactive component is described with a quantum mechanical approach, while surrounding effects provided by solvent molecules or a protein environment are considered through computationally less expensive molecular mechanics techniques. To obtain meaningful results in the simulation of photochemical reactions, the combination of these methods must be carefully balanced. The choice of QM method, the definition of starting conditions for photodynamics simulations, as well as the description of the non-adiabatic event and statistical analysis of ensemble averaged data are critically discussed, highlighting the strengths of the currently applied approaches but also noting common pitfalls.

Coumarins are naturally occurring phytochemicals with heterocyclic structures which display a wide range of biological activities against neurological diseases such as Alzheimer´s disease (AD). This study reviews recent research into the design, synthesis and pharmacological profile of several series of synthetic coumarin ligands with clearcholinesterase, and assesses the monoamino oxidases (MAO-A and MAO-B) inhibitory activity, Aβ anti-aggregation potency and antioxidant properties reported for these novel derivatives. Our attention is focused on a comparison of the neuroprotective effects of these coumarin derivatives in terms of their potential as mono-, homo- and heterodimers as agents in the treatment of AD. The monocoumarin derivatives 13a & 13b with benzyl pyridinium group showed outstanding levels of acetylcholinesterase inhibitory activity (IC50 = 0.11, 0.16 nM). Bis-coumarin ligands showed high levels of inhibitory activity and selectivity for MAO-A. Tacrinecoumarin heterodimer 21b was the most active inhibitor of hBChE (IC50 = 38 pM).

Salts of various oniums such as ammonium, phosphonium, tellurium, arsonium, bismuthenium have been used as phase transfer catalyst in many oxidation reactions. These ions are also in use as carriers of anionic oxidants such as permanganate, chromate, dichromate, etc. Among these oniums, alkylammonium ions have been extensively studied. Alkylammonium ions are charged molecules, susceptible for acquiring hydrophobic characteristics through carboneous groups present in the molecule. Variation of these groups can tune the hydrophobicity of the oniums; thereby these molecules can acquire amphipathic characteristics. In different solutions, these ions aggregate to form different organized assemblies providing different localization sites for the oxidants. These oxidants exist as tight ion pairs with the oniums and follow different reaction mechanism during the oxidation reactions of various irganic substrates. The X-ray crystal strudies as well as reaction kinetics support the existence of tight ion pairs in both solid state and in solutions. The variation of substituent in the substrate, and the polarity of the solvent are found to have significant effect on the oxidation kinetics and reaction mechanism. Herein, we focus the review on the alkyl ammonium ions as carriers of oxidants and described the kinetics and reaction mechanism of the oxidation processes.

Background: Since introduction of chalcones in 1899, chalcones have been considered as precursors of flavones in the biosynthesis of flavonoids. However, because the chalcones include multi-functional groups on aromatic rings, developments of efficient synthetic methodologies as well as synthetic strategies involving protection/ deprotection should be considered. Objective: The primary objective of this study was to describe chalcone synthesis under mild condition and its mechanism study. Method: The chalcones synthesis is aldol condensation of acetophenones with aldehydes in the presence of catalytic mount of copper(II) bromide at 70 °C. Result: Proposed transformation of the chalcone synthesis is to proceed via catalytically generated abromocarbonyl intermediate by copper(II) bromide, aldol condensation of a-bromocarbonyl intermediate is promoted by HBr, while its byproducts, HOBr and HBr reconstitute the catalyst. Using this method, the natural product echinatin (1) was conveniently synthesized in a single step with yield of 82%. Conclusion: Cu(II) mediated aldol condensation via a-bromocarbonyl species involving Cu(II) catalyst reconstitution were noted in this study. It was thought that it has the potential for application as a simple and quick synthesis method for pharmaceutical ingredients.