Current Organic Chemistry - Volume 9, Issue 10, 2005

Isoxazoles are a class of heterocyclic compounds having a remarkable number of applications and have been demonstrated to be very versatile building blocks in organic synthesis. The wide range of biological activities includes pharmacological properties such as hypoglycemic, analgesic, antiinflammatory, anti-bacterial and HIV-inhibitory activity. Some isoxazole derivatives display agrochemical properties namely herbicidal and soil fungicidal activity and have applications as pesticides and insecticides. Isoxazoles have also been used as dyes, electric insulating oils, high temperature lubricants and polyisoxazoles have applications as semicondutors. The key feature of these heterocycles is that they possess the typical properties of an aromatic system but contain a weak nitrogen-oxygen bond which under certain reaction conditions, particularly in reducing or basic conditions, is a potential site of ring cleavage. Thus, isoxazoles are very useful intermediates since the ring system stability allows the manipulation of substituents to give functionally complex derivatives, yet it is easily cleaved when necessary. The ring opening provides difunctionalized compounds, namely 1,3-dicarbonyl, enaminoketone, γ-amino alcohol, α,β- unsaturated oxime, β-hydroxy nitrile or β-hydroxy ketone compounds, so that isoxazoles can be considered masked forms of these synthetic units. Consequently, isoxazoles have become an important synthetic tool. The construction of the isoxazole ring can be achieved by several synthetic approaches. However, the two major routes to isoxazoles are the 1,3-dipolar cycloadditon of alkenes and alkynes with nitrile oxides and the reaction of hydroxylamine with a three-carbon atom component, such as 1,3-diketone or an α,β-unsaturated ketone. This review aims to provide coverage of the recent developments on the synthesis and reactivity of isoxazoles.

Room temperature ionic liquids (RTILs) are a rapidly growing area of chemistry. Although much of the interest has been focussed on the applications of these materials, there has also been significant effort directed toward extending the range of types of these materials. Unfortunately, information about the physical properties of these compounds is often poorly reported and difficult to find. The purpose of this review is partially to collect this information (to aid those considering using RTILs) and partially to encourage those working in this area to be more thorough in their reporting of the physical properties of these unique materials.

One popular model suggests that life on Earth originates from a prebiotic soup, which is the idea that a large amount of different elements spontaneously came together in a pool to form basic organic moleculars. These basic organic moleculars are believed to have then condensed and reacted to form more complex organic moleculars and then eventually formed the original life. In the process of chemical evolution, the conversion of inorganic compounds to small organic moleculars is the first and indispensable step for the origin of life. Scientists have designed many experiments that could provide evidence in support of the theory of the prebiotic soup. In these experiments, small moleculars of CO2, CO, CH4 or CH3SH have ever been taken as the building blocks for the synthesis of organic molecules. When introducing H2S, N2, NH3 et al., amino acids, nucleotides and other possible monomers, nonenzymatically condensing to form oligomeric products, could be synthesized. Meanwhile, pyrite, coprecipitated (Ni,Fe)S, Fe3O4 as well as some other minerals might take part in the processes of selection, concentration and energy supplement, especially of the emergence of chirality preference in the extant amino acids and sugars in living organisms. Herein recent progress made in the research into the prebiotic soup theory of the origin of life is reviewed. Finally, after discussion of the possible existence of carbon nanotubes in primitive earth, their morphology similarity, affinity and selectivity with biology moleculars, it is proposed that carbon nanotubes might play a significant role in the origin of life.

The 5'-terminus of eukaryotic messenger RNA (mRNA) molecules contains an unique structure, viz. a dinucleoside 5',5'-triphosphate moiety where the terminal nucleoside is 7-methylguanosine. This 5'-cap structure serves as a site of recognition for numerous enzymes involved in splicing, transport and translation of mRNA, and protects mRNA against intracellular exonucleases. In addition, viral RNA polymerases use capped mRNA sequences of the host cell as primers for viral RNA synthesis. Understanding of molecular mechanism of all these processes requires detailed information on the chemical properties of the cap structure, including capability to prepare conjugates and structural analogs of the cap for research tools. This review tends to summarize the present knowledge on various aspects of the chemistry of the cap structure, including chemical stability and mechanisms of breakdown, protolytic and complexing equilibria, stacking interactions and synthetic methodology.