Mini-Reviews in Organic Chemistry - Volume 9, Issue 3, 2012

The Editorial Board wishes to dedicate this issue of “Mini-Reviews in 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, 114 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 Mini-Reviews in 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 level award of the government of Austria and TWAS prize for institution building as well as by four articles/ editorials in Nature. The Editorial board wishes health and prosperity to Prof. Atta-ur-Rahman in the years to come.

Pressure is a fundamental parameter which can accelerate reaction rates. The use of ultra high pressure (0.2-2 GPa) in organic chemistry can be particularly efficient, because it can fix problems of reactivity or regio- and stereoselectivity which could appear at ambient pressure, while being considered as an activation tool. The present review points out the last progresses in the area of glycochemistry with this technique. Selective protection or deprotection of the hydroxyl group of saccharides are described, as well as the most studied cycloaddition reactions, leading to the sugar-base structure dihydropyran or employing a sugar to enhance the stereoselectivity of the reaction. The Wittig reaction, homoallylation with allylstannane and aza-Michael reaction are also presented.

Due to their particular properties the use of ionic liquids is growing. Because of their chiral discriminated properties chiral ionic liquids synthesized from mono, polysaccharides or cyclodextrins are used as catalysts in asymmetric synthesis, chiral selectors in NMR studies or as stationary phases in chiral chromatographic analysis. Chiral carbohydrate ionic liquids are a recent topic and only few examples in the literature describe applications of ionic liquids derived from monosaccharides. Isomannide and isosorbide are the most explored and demonstrate important features as chiral selectors and as catalysts in asymmetric synthesis. Carbohydrate imidazolium salts have been explored as soluble supports for oligosaccharide synthesis and polymers resulting from carbohydrate ionic liquids polymerization have been reported. Cyclodextrin ionic liquids have been described and the ability to produce inclusion complexes was explored.

The biosynthesis of glycoconjugates in bacteria and eukaryotic cells involves the formation of lipid-linked sugar precursors. The lipid component is a polyisoprenyl-phosphate (undecaprenyl phosphate in bacteria, and dolichyl-P in eukaryotes and Archaea). All cell types employ phosphoisoprenol-linked saccharides in the early stages of protein glycosylation, as well as for the synthesis of bacterial cell wall and surface polysaccharides. Nucleotide sugars, which donate carbohydrates for the synthesis of the saccharide moiety, are available as soluble molecules in cytosolic compartments, while polyisoprenyl-phosphates are embedded in the lipid membrane bilayer. Once assembled, phosphoisoprenol-linked saccharide molecules must cross the lipid bilayer for further processing. Thus, transmembrane movement of phosphoisoprenol-linked saccharides is an obligatory, conserved step of significant biologic importance in all cells, which requires the assistance of membrane proteins known as flippases or translocases. The bacterial lipopolysaccharide consists of a lipid A-core oligosaccharide (OS) capped in many bacteria by an O-antigen polysaccharide. The O antigen is synthesized and assembled as a phosphoisoprenyl-linked saccharide. To date, the membrane flippases or translocases involved in O-antigen assembly can be broadly separated into two groups. One group utilizes ATP hydrolysis for the membrane translocation step, while the other operates in an ATP hydrolysis-independent fashion. Based on this, up to four different assembly pathways for the O-antigen polysaccharides are currently recognized. Additional proteins to those involved in the membrane translocation step are also required in each system to properly catalyze the assembly of the O-antigen polysaccharide, which ultimately becomes the substrate for the ligation reaction with the lipid A-core OS. Here, we provide a review of all of the different types of membrane flippases involved in the export of LPS O antigen components.

Imidazoles comprise a major class of five-membered heterocyclic compounds and have been found to be an integral part of many significant pharmacological active compounds in the field of medicinal chemistry. Imidazoles are also of general synthetic utility, since they permit functional group and structural modifications to synthesize a number of substituted heterocycles. In literature, various conventional methods are reported for the synthesis of imidazoles. Microwave assisted synthesis has become a useful technique to medicinal chemists for rapid organic synthesis to avoid formation of side product. This review concludes various existing synthetic methods of imidazoles particularly focusing on microwave assisted approach developed up to 2011.

The pharmacological importance of substituted 1,4-and 1,5-benzodiazepines has been well established and some derivatives are known as anxiolytic, anticonvulsant, hypnotic and anticancer agents. In recent years, much effort has been done in the benzodiazepine area to develop new members of this family. This review focuses on the synthesis and the biological activities of benzodiazepam derivatives.

The genus of Forsythia (Oleaceae), composing ca. 10˜15 species, is mostly native to eastern Asia and used in oriental traditional medicines. Phytochemical investigations on Forsythia species have shown that triterpenoids, lignans, flavonoids, phenylethanoid glycosides and C6-C2 natural alcohols, accumulated mainly in the fruits, are the major components. Many studies have elucidated that phenolic compounds, including lignans, phenylethanoid glycosides and flavonoids are responsible for the various biological activities of the herbal medicine. This review lists 141 chemical constituents as well as their biosynthetic pathway, bioactivities and quality control as reported until September, 2011 (104 references).

This review describes recent development in the aerobic oxidation of amines. A variety of ruthenium catalysts have shown excellent activity for the aerobic oxidation of amines. Several gold and copper catalysts are also effective for this reaction. Other metalcontaining complexes are revealed to be active catalysts and an enzyme is also investigated for this oxidation. Meanwhile, various supports have been explored for this reaction and the availability of various supports with differing physical properties allows chemists to design and create many catalytic systems.