Current Organic Chemistry - Volume 16, Issue 17, 2012

The Editorial Board wishes to dedicate this issue of “Current Organic Chemistry” to the Editor-in-Chief of the journal, Prof. Dr. Atta-ur- Rahman, FRS, on his 70th birthday. The Editorial Board acknowledges the tremendous achievements of Prof. Rahman in the fields of Organic Chemistry in general and on bioactive natural products in particular. Prof. Rahman's honors include: the first scientist from the Muslim world to have won the prestigious UNESCO Science Prize (1999) in the 35 year old history of the Prize; elected as Fellow of Royal Society (London) in July 2006; conferred honorary doctorate degrees from many prestigious universities, including the University of Cambridge. On a national level his services are acknowledged in the form of four prestigious civil awards, including these highest national awards: Nishan-e-Imtiaz (2002), Hilal-e-Imtiaz (1998), Sitara-e-Imtiaz (1991) and Tamgha-e-Imtiaz (1983). At an academic level, Prof. Rahman has 856 publications in several fields of organic chemistry including 658 research publications, 21 patents, 113 books and 65 chapters in books published largely by major U.S. and European presses. He is currently the Editor-in-Chief of 12 scientific journals in fields ranging from Medicinal Chemistry to Pharmaceutical Drug Design. Seventy six students have completed their Ph.D. degrees under his supervision. He chairs the Network of Academies of Sciences of Islamic Countries (NASIC) and is the Vice-President (Central & South Asia) of the Academy of Sciences for the Developing World (TWAS) Council, and Foreign Fellow of Korean Academy of Sciences. Prof. Atta-ur-Rahman was the President of the Pakistan Academy of Sciences (2003-2006) and was again elected as the President of the Academy from 1st of January 2011. In particular, Prof. Rahman has made outstanding contribution to Current Organic Chemistry. The contributions of Prof. Rahman to the uplifting of science in Pakistan in his capacity as the Federal Minister for Science & Technology and later as Chairman Higher Education Commission were acknowledged by a high Civil Award of the government of Austria and TWAS prize for institution building as well as by four editorials in Nature. Professor Atta-ur-Rahman has proved his outstanding talents in organic and analytical chemistry. His scientific achievements are indeed amazing. The Editorial board wishes health and prosperity to Prof. Atta-ur-Rahman in the years to come.

Aim of the present communication is to show experimental results, and related conclusions, on the electrochemical oxidation (EO) of tartaric acid (TA), which has been oxidized at Ti/PbO2, Ti/Pt, Pt and HBDD electrodes at different current densities in acidic media. TA complete mineralization has been achieved only at HBDD and Ti/PbO2, being higher the faradaic efficiency at the latter electrode. At Pt electrode, the electroxidation was found to be extremely slow, in acidic conditions. The experimental evidence has shown that the main factor is the interaction of the organic substrate and hydroxyl radicals with the electrode surface, during TA oxidation. In the case of the EO of oxalic acid (OA) in acidic media that was previously studied, better results were obtained at the Pt electrode, supporting the idea that the interaction of organic substrate with the electrode surface, was the main determining parameter and based on the results here reported, this idea was confirmed for TA, more complex compound than OA.

Comninellis and his research group have elucidated a theoretical model that permits us to predict the chemical oxygen demand (COD) and instantaneous current efficiency (ICE), during the electrochemical oxidation of organic pollutants on a synthetic boron-doped diamond thin film electrodes (BDD) in a batch recirculation system under galvanostatic conditions. Several studies highlight the good correlation between theoretical predictions and empirical data, but it was noted that some data, from some experiments, were not in agreement with the theoretical model, since it achieved efficiencies above 100%. Recently, few studies have reported phenomena that occur in electrocatalytic systems which were not deducted in the first models. Thus, we emphasize that the mineralization of organic compounds on BDD electrodes involves not only hydroxyl radicals but also the molecular oxygen present in air, in saturated aqueous solutions or strong oxidants generated from simultaneous reactions. Therefore, this highlights summarizes the results reported by other electrochemists until nowadays to understand the electrochemical oxidation mechanisms.

The kinetics of the electrooxidation of m-cresol in aqueous solution was investigated in a one-compartment flow electrochemical cell with a boron-doped diamond electrode (BDD). Cyclic voltammograms recorded on BDD revealed that cresol oxidation takes place at a potential very close to the discharge of water. Under potentiostatic conditions, at a working potential lower than water discharge, a passive layer was rapidly formed on the electrode surface due to cresol polymerization. The anode fouling was not observed during electrolysis performed with the flow electrochemical cell operating under galvanostatic conditions. In this case, the decay of mcresol concentration followed a pseudo-first-order kinetics. The abatement of chemical oxygen demand (COD) showed that the kinetics of m-cresol oxidation was limited by mass transfer and that a full mineralization was achieved. A good agreement between predicted and experimental COD and instantaneous current efficiency values was obtained, although some deviations were observed at high current since the experimental data decreased faster than those predicted ones. These deviations can be explained by the occurrence of oxygen evolution which increases the mass transfer coefficient.

The electrochemical degradation of nitrobenzene in undivided and divided cell using Ti/SnO2-Sb2O5 anode was studied. The electrochemical degradation of nitrobenzene is of the results together by anode and cathode. Two general possible paths of the electrochemical degradation of nitrobenzene are proposed: (1) Nitrobenzene was first oxidized to p-nitrophenol. Then p-nitrophenol was degraded to hydroquinone, benzoquinone, maleic acid and oxalic acid, etc. Finally, all of the intermediates were oxidized into CO2 and H2O. (2) Nitrobenzene was mainly reduced to aniline at the cathode，aniline was degraded to 4-aminopheol, hydroquinone, benzoquinone and other intermediates at the anode, then all of the intermediates are oxidized till complete degraded and mineralized at last.

Electrochemical oxidation of 3-chloropyridine (3-ClPYR), as a model pollutant, on a modified β-PbO2 electrode was examined by bulk electrolysis. Parameters affecting the removal and reaction rate of 3-ClPYR, such as current density, pH and initial concentration, were investigated. The results showed that 3-ClPYR could be readily decomposed under the attack of hydroxyl radicals (·OH) electrochemically generated on the modified β-PbO2 anode, the removal of 3-ClPYR was confirmed satisfactorily to fit pseudo-first order kinetics, the rate constant was mainly governed by current density and pH of 3-ClPYR solution. The evolution of intermediates was examined with 200 mg·L-1 of 3-ClPYR at pH 6.2, 3.0 g·L-1 of Na2SO4 and 160 mA·cm-2 of current density. Pyridine was firstly formed via starting with the scission of C-Cl bond. When the initial concentration of 3-ClPYR was above 600 mg·L-1, 2, 3-dichloropyridine and 3, 5- dichloropyridine were also detected in a very small quantities owing to the addition of chlorine radicals (·Cl). Then, pyridine and chlorinated pyridine were further cleaved to organic acids (oxalic acid, acetic acid, formic acid and propionic acid) and inorganic ions (Cl-, NO2 -, NO3 -, ClO-) via opening the nitrogen containing heterocyclic ring, indicating the promising use of electrochemical technique for clean and safe removal of nitrogen containing heterocyclic compounds from industrial effluents.

Anodic oxidation is a promising process for degrading toxic and biologically refractory organic pollutants present in wastewater. Proper selection of electrodes is the key to reach effective and economic treatment. In this study, the electrochemical oxidation of Alizarin Red has been studied in neutral media using lead dioxide (PbO2), boron-doped diamond (BDD) and platinum (Pt) anodes in bulk electrolysis experiments under same conditions. Obtained results have clearly shown that the electrode material is an important parameter for the optimization of such process. Kinetics analysis showed that Alizarin Red is readily oxidized on PbO2 and BDD anodes, Pt anode having a moderate ability to oxidize it. Different current efficiencies were obtained for PbO2 and BDD, depending on the applied current density in the range from 33 to 150 mA cm-2. The effect of the initial Alizarin Red concentration on the performance of PbO2, BDD and Pt anodes, as well as, their comparative oxidation ability was studied. Faster Alizarin Red elimination and TOC removal were obtained using BDD anode with current density of 33 mA cm-2. Bulk electrolysis showed that the complete TOC and color removal were achieved using PbO2 and BDD anodes while Pt anode performed only a partial oxidation of Alizarin Red.

Steviol glycosides, a collection of natural sweeteners extracted from the leaves of the Paraguayan shrub Stevia rebaudiana, have been drawing an increased amount of attention in the last decades. Very recently, the steviol glycosides were approved as a novel food additive in the European Union, which resulted in a growing interest by the general public. The structure of the steviol glycosides consists out of a diterpene ent-kaurene core, steviol, linked to one or more glucose units. Various benign medicinal properties have been assigned to the structure of stevioside, including antihypertensive and antihyperglycemic effects. These effects have encouraged many organic chemists to investigate the synthesis of possible analogues of the structure of both stevioside and its aglycon steviol. In previous work we discussed the known modifications of the structure of the ent-beyerane isosteviol, which is obtained via a Wagner-Meerwein rearrangement of steviol. In this review, we offer an organized overview of the available modifications of both stevioside and steviol. These modifications cover various fields in research, including medicinal chemistry, combinatorial chemistry or the synthesis of sweeteners with improved taste. For stevioside, the enzymatic transglycosylations are discussed, followed by all reported chemical transformations, which are not widespread in the literature. The possible microbiological transformations of stevioside and steviol are also discussed. These transformations have been carried out with various microorganisms or enzymes, resulting mainly in gibberellin-like structures or hydroxylated steviol analogues. An overview of all possible hydroxylations and their corresponding yields is provided. Compared to stevioside, the chemical modifications of steviol are more numerous. An overview, where each analogue is grouped according to the reactive site of steviol used, is provided. If possible, a comparison between the resulting yields of similar reactions is made.

C5-position in pyrimidine nucleosides and nucleotides is an invaluable anchor point for tagging diverse kind of labels like haptens, dyes and signaling molecules. Oligonucleotides with such C5-modified pyrimidine nucleosides and nucleotides are being used in molecular genetics and next generation nucleic acid sequencing as indispensable diagnostic tools. In view of their requirement in a wide range of primers and probes, chemistry synthesis and manufacture of C5-modified substituted pyrimidines had elicited continued interest over the last several decades. These diverse classes of molecules also have pharmaceuticals significance. The objective of this review is to capture the recent strategies available to synthesize C5-modified pyrimidine nucleosides and nucleotides that include organometallic, “Click” and photochemistry.

The organic electroluminescent (EL) technology, which has been the research focus in the field of electro-optical information, holds a wide range of potential application in domains of communication, information, display, illumination and so on. Organic EL materials have many advantages when compared to their inorganic counterparts. Carbazole, as a kind of rigid plane biphenyl compound with wide band gap, high luminescent efficiency and high flexibility to modify the molecule skeleton, has been a key chromophore in organic EL materials. In this regard, carbazole-based chromophores are widely used in the synthesis of highly efficient blue light-emitting materials, charge-transporting materials and host materials. In this review paper, different chemical modifying methods to improve the performance of carbazole-based EL material are presented in detail, and these methods include different linkage methods, synthesis of molecules with non-conjugation linkage conformation, bipolar characteristic, heterocycle replacements, multifunctional integration strategies and so on. Finally, some issues to be addressed and hotspots to be further investigated are also discussed.

This review presents the organic compounds and reactions in which boronic acids are featured together with chalcogens. Structurally varied sulfur-, selenium- and tellurium-containing boronic acids are described. Survey of their preparation methods as well as the scope of applications in organic, medicinal, analytical and supramolecular chemistry is provided. Reactions involving both the boronic acids and chalcogen-containing compounds are discussed, with emphasis placed on the role of the chalcogen atom in these transformations.

Natural products have not only yielded new and effective drugs but have also provided insight into new mechanisms of action. Paclitaxel (Taxol) as the first promoters of tubulin polymerization plays important roles in the war against cancer and in the exploring new microtubule-binding drugs. The present review summarized the development of paclitaxel and its analogues which can be used as the precurce of synthesis of Taxol and its analogues.