Current Green Chemistry (v.2, #2)

Pharmaceutical Green Chemistry Applied to the Chemical Synthesis of Steroid Compounds by Jorge A.R. Salvador, Rui M.A. Pinto, Samuel M. Silvestre (97-134).
Steroid compounds are widely distributed in nature and are challenging substrates for the synthesis of a wide variety of important biologically active molecules. These include the sex hormones, corticosteroids and mineralocorticoids hormones, bile acids, vitamin D derivatives, cardiotonic steroids, among others, that have shown great therapeutic value for a broad array of medical conditions. Due to their biological and synthetic relevance, several chemical processes for the preparation and/or functionalization of the steroid nucleus have been developed. In some of those processes, green chemistry principles have been incorporated, allowing significant advances in synthetic chemistry applied to steroid compounds. In this review, a selection of the most relevant applications of pharmaceutical green chemistry in steroid synthesis, using chemical methods will be presented. Special emphasis will be given to catalytic processes, specially involving the use of heterogeneous catalysts, microwave technologies and ionic liquids as solvents. This review is organized according to the reaction type that include oxidation and reduction reactions, protection/deprotection transformations, glycosylations, rearrangements, olefin methathesis, preparations of heterocycles using different strategies and miscellaneous reactions. Biocatalytic methods applied to steroid synthesis are out of the scope of this review.

Ionic liquids (ILs) are organic salts that are liquid at ambient temperature. They consist of organic cations (imidazolium, pyridinium, sulfonium, phosphonium, etc.) and organic or inorganic anions. Ionic liquids have emerged as a new class of solvents as an alternative to environmentally volatile organic solvents for practical applications due to their unique combination of low volatility, low melting point, chemical stability, high conductivity, wide electrochemical window, ability to dissolve organic and inorganic solutes and gases and catalytic properties. High cost of ionic liquids is compensated by recycling. Some ionic liquids such as Imidazolium and pyridinium salts with long alkyl substituents are toxic. However toxicity of ionic liquids is monitored by understanding ionic liquid structure activity relationships and it is possible to design task specfic, non toxic, environmentally benign ionic liquids. Here we report an overview of synthesis of ionic liquids and their applications in organic chemistry e.g. carbon-cabon bond forming reactions, carbonnitrogen bond forming reactions, condensation reactions and synthesis of various heterocyclic compounds from the point of view of development and practical utility.

An efficient and environmentally benign domino protocol has been presented for the synthesis of structurally diverse 2,3-dihydroquinazolin-4(1H)-one derivatives involving three-component reaction of isatoic anhydride, substituted anilines and aromatic aldehydes in the presence of gluconic acid aqueous solution (50%, wt %). The present protocol offers several advantages such as operational simplicity with easy workup, shorter reaction times, and excellent yields with superior atom economy and environmentally benign reaction conditions with the use of cost-effective, recyclable, non-toxic and bio-degradable GAAS (50%, wt %) as catalyst/solvent.

Montmorillonite K-10 Clay Mediated Green Synthesis of 2-Amino-4-aryl thiazole Derivatives from α-Brominated Aralkyl Ketones in Water by Pinjari Md. K. Mohinuddin, Reddy B. Mohan, Sangita D. Kumar, Nallagondu C.G. Reddy (163-169).
Montmorillonite K-10 clay has been identified as an efficient and green catalyst for the synthesis of 2-amino-4-aryl thiazole derivatives (3) in good to excellent isolated yields (80-96%) from α-brominated aralkyl ketones (1) and thiourea (2) in aqueous medium at 25-30A°C. The present procedure offers advantages of short reaction time, simple work-up, high yields of products and the catalyst is environmentally benign and exhibits remarkable reusable activity by four times.

The strategy of employing directing groups in C-H activation appears to be a big success in today's chemical synthesis due to its high efficiency and environmental friendly process. As various methodologies have been developed by using copper, this widespread and cheap catalyst has also played an important role in C-H activation. This review focuses on copper involved C-H activation assisted by directing groups developed in recent years, with the aim of providing a complementary to existing reviews.

Why Leave a Job Half Done? Recent Progress in Enzymatic Deracemizations by Alba Diaz-Rodriguez, Ivan Lavandera, Vicente Gotor (192-211).
In recent years the usefulness of enzymatic systems to obtain a single stereoisomerically pure compound starting from a racemate has been expanded. Moreover, current advances in protein engineering, molecular biology and modeling tools are the basis to improve the catalytic properties of enzymes to face novel synthetic challenges. Also, medium engineering and novel immobilization methods of (bio)catalysts are enhancing the productivity of biocatalytic processes making them suitable for being scalable. The development of multienzymatic protocols and the combination of enzymes with other catalysts are providing a wide range of synthetic possibilities that are expanding the scope of these transformations even at industrial scale. Herein we will describe an overview of recent (chemo)enzymatic deracemizations, focusing on the strategy employed (dynamic kinetic resolution, stereoinversion, cyclic deracemization or enantioconvergent process), the type of substrate (e.g., alcohols, amines, carboxylic acid derivatives or carbonylic compounds), and the biocatalyst(s) used.