Current Organic Chemistry - Volume 15, Issue 22, 2011

The synthesis of unnatural α-amino acids is an area of research that has gained a lot of attention in recent years. The availability of synthetic methods for novel unnatural α-amino acids derivatives and related compounds will be critical in the de novo design of molecules that mimic the conformation of natural, active peptides. These molecules - peptidomimetics - are designed to display high receptor affinity and selectivity, in addition to enhanced bioavailability and metabolic stability. This review presents and discusses selected synthetic methodologies leading to unnatural α-amino acids, including chiral catalysts that enable enantioselective synthesis and microwave-assisted synthesis. Solid phase synthesis and construction of organometallic α-amino acids reveal the scope of influence that this field has in organic chemistry. Additionally, the article is aimed to provide a brief insight into biosynthetic approaches to the synthesis of α-amino acid derivatives, thus providing the reader with an overall appreciation of the field. For the purpose of this issue our previous review on the unnatural α-amino acids synthesis [Perdih A. and Sollner Dolenc M. Curr Org. Chem. 2007, 11, 801-832] has been revised and updated and discusses also selected contributions covering the period from 2007-2010.

In this brief review, we provide an updated summary of the significant advances in organic synthesis using zirconium compounds such as zirconium tetrachloride, zirconium oxide chloride, zirconium sulfate, zirconium hydrogen sulfate, zirconium hydrogen phosphate, zirconyltriflate, zirconium tetrakis(dodecylsulfate), zirconyldodecysulfate, and zirconium complexes, mainly from papers published in the last three years (2008-2010). Application of these compounds allows a reaction to be carried out in a facile and mild manner and affords the corresponding products in high yields with high regio-, chemo- and stereo-selectivity.

The search for suitable ionic liquids that are non-volatile, non-toxic, dissolve a variety of reactants and are recyclable is a real challenge for chemists to be able to replace the traditional solvents that are volatile, flammable and toxic. This segment of chemistry has, these days, become a firm part of green chemistry attracting much attention. The present review summarizes and critically discusses the data accumulated in recent years on the synthesis and utilization of phosphorus containing ionic liquids, such as imidazolium hexafluorophosphates and phosphonium salts. As regards the preparation of ionic liquids, the key steps are quaternization and the introduction of the corresponding anion. Physical properties of the ionic liquids can be fine-tuned by selecting suitable cation-anion pairs. The reactions studied in ionic liquids embraces catalytic hydrogenations, transfer hydrogenations, oxidations, hydroformylations, carbonylations, alkylations, acylations, nucleophilic substitutions, halogenations, condensations, Diels-Alder reactions, Michael additions, coupling reactions and transformations in organophosphorus chemistry. Ionic liquids are especially suitable to accommodate catalytic reactions where one of the components is a gas. Stress was laid to highlight the chemo-, regio-, diastereo- and, especially, the enantioselectivity of the reactions performed in ionic liquids. The data on the ecotoxicity of imidazolium hexafluorophosphates and phosphonium salts are briefly discussed. The updated version of this review covers the corresponding literature up to the first half of 2010.

A brief review on the development of dye-sensitized solar cells (DSSCs) is presented, with the main focus on ruthenium complexes, as photosensitizers of nanocrystalline TiO2 films. In a DSSC, the charge separation is initiated at the adsorbed dye, which is bound at the interface of a TiO2 film. Careful modification of the dyes can tune the spectroscopic, electrochemical properties and photovoltaic performance of the solar cells. It is necessary to optimize the properties of the dye in conjunction with other factors in order to best exploit and be fully compatible with other cell modifications in the development of cheap and efficient photovoltaic systems. In this review, current strategies for the molecular design of the ruthenium complexes including improving the light harvesting in the visible and near-IR region, increasing high optical extinction coefficients and increasing the charge separation and stability of the dyes are discussed.

Diverse organic reactions have been explored using the basic surface of KF/alumina. This article has reviewed on assorted applications of KF/alumina in the synthesis of acyclic, carbocyclic and heterocyclic molecules with special focus on recent applications during 2005 to mid 2010. Examples of reactions under solvent-free dry conditions highlighting the greener context are also discussed.