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Showing posts with label STEREOCHEMISTRY. Show all posts
Showing posts with label STEREOCHEMISTRY. Show all posts

Thursday, 19 September 2013

Weygand/Hilgetag Preparative Organic Chemistry

Weygand/Hilgetag Preparative Organic Chemistry

Friday, 2 August 2013


A chemist in the 1950s using column chromatography. The Erlenmeyer receptacles are on the floor.
Column chromatography in chemistry is a method used to purify individual chemical compounds from mixtures of compounds. It is often used for preparative applications on scales from micrograms up to kilograms. The main advantage of column chromatography is the relatively low cost and disposability of the stationary phase used in the process. The latter prevents cross-contamination and stationary phase degradation due to recycling.
The classical preparative chromatography column, is a glass tube with a diameter from 5 mm to 50 mm and a height of 5 cm to 1 m with a tap and some kind of a filter (a glass frit or glass wool plug – to prevent the loss of the stationary phase) at the bottom. Two methods are generally used to prepare a column: the dry method, and the wet method.
  • For the dry method, the column is first filled with dry stationary phase powder, followed by the addition of mobile phase, which is flushed through the column until it is completely wet, and from this point is never allowed to run dry.
  • For the wet method, a slurry is prepared of the eluent with the stationary phase powder and then carefully poured into the column. Care must be taken to avoid air bubbles. A solution of the organic material is pipetted on top of the stationary phase. This layer is usually topped with a small layer of sand or with cotton or glass wool to protect the shape of the organic layer from the velocity of newly added eluent. Eluent is slowly passed through the column to advance the organic material. Often a spherical eluent reservoir or an eluent-filled and stoppered separating funnel is put on top of the column.
The individual components are retained by the stationary phase differently and separate from each other while they are running at different speeds through the column with the eluent. At the end of the column they elute one at a time. During the entire chromatography process the eluent is collected in a series of fractions. Fractions can be collected automatically by means of fraction collectors. The productivity of chromatography can be increased by running several columns at a time. In this case multi stream collectors are used. The composition of the eluent flow can be monitored and each fraction is analyzed for dissolved compounds, e.g. by analytical chromatography, UV absorption, or fluorescence. Colored compounds (or fluorescent compounds with the aid of an UV lamp) can be seen through the glass wall as moving bands.


    Stationary phase

    The stationary phase or adsorbent in column chromatography is a solid. The most common stationary phase for column chromatography is silica gel, followed by aluminaCellulosepowder has often been used in the past. Also possible are ion exchange chromatographyreversed-phase chromatography(RP), affinity chromatography or expanded bed adsorption(EBA). The stationary phases are usually finely ground powders or gels and/or are microporous for an increased surface, though in EBA a fluidized bed is used. There is an important ratio between the stationary phase weight and the dry weight of the analyte mixture that can be applied onto the column. For silica column chromatography, this ratio lies within 20:1 to 100:1, depending on how close to each other the analyte components are being eluted.

    Mobile phase (eluent)

    The mobile phase or eluent is either a pure solvent or a mixture of different solvents. It is chosen so that the retention factor value of the compound of interest is roughly around 0.2 - 0.3 in order to minimize the time and the amount of eluent to run the chromatography. The eluent has also been chosen so that the different compounds can be separated effectively. The eluent is optimized in small scale pretests, often using thin layer chromatography (TLC) with the same stationary phase.
    There is an optimum flow rate for each particular separation. A faster flow rate of the eluent minimizes the time required to run a column and thereby minimizes diffusion, resulting in a better separation. However, the maximum flow rate is limited because a finite time is required for analyte to equilibrate between stationary phase and mobile phase, see Van Deemter's equation. A simple laboratory column runs by gravity flow. The flow rate of such a column can be increased by extending the fresh eluent filled column above the top of the stationary phase or decreased by the tap controls. Faster flow rates can be achieved by using a pump or by using compressed gas (e.g. air,nitrogen, or argon) to push the solvent through the column (flash column chromatography).
    The particle size of the stationary phase is generally finer in flash column chromatography than in gravity column chromatography. For example, one of the most widely used silica gel grades in the former technique is mesh 230 – 400 (40 – 63 µm), while the latter technique typically requires mesh 70 – 230 (63 – 200 µm) silica gel.

    A spreadsheet that assists in the successful development of flash columns has been developed. The spreadsheet estimates the retention volume and band volume of analytes, the fraction numbers expected to contain each analyte, and the resolution between adjacent peaks. This information allows users to select optimal parameters for preparative-scale separations before the flash column itself is attempted.

    An automated ion chromatography system.

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    Typical set up for manual column chromatography





    Sunday, 21 July 2013

    The first total synthesis of fuscain

    First total synthesis of fuscain

    First total synthesis of fuscain

    Fuscain is a new furanolactam isolated from the sponge Phacellis fusca from the South China Sea. Furan analogues isolated from marine organisms have valuable medicinal properties. The first total synthesis of fuscain is reported in Journal of Chemical Research December issue. The key step in the synthesis is the formation of seven-membered lactam by acylation of a furan ring using the mild Lewis acid CuSO4•5H2O.
    Fuscain, a new furanolactam which was originally isolated from the sponge Phacellis fusca collected in South China Sea, showed a moderate cytotoxicity toward P388 and L1210 cell lines. The same sponge yielded three pyrrololactam alkaloids: saldisin, 2-bromoaldisin and debromohymenialdisin.2 Recently, furan analogues isolated from marine organisms have shown anticancer,3–5 antibacterial,6 anticoagulant, antifungal, antimalarial,  antiplatelet, antituberculosis and antiviral activities11. Aldisin-based derivatives can be easily synthesised. However, it is still a challenge to synthesise fuscain. Hence the biological effects of fuscain and its derivatives on cell cycle progression and antitumour activities have rarely been reported. The synthetic route to fuscain is shown below.
    The key step is an intramolecular Friedel–Crafts cyclisation to form the seven-membered ring. Various Lewis acids (polyphosphoric acid, POCl3, polyphosphoric acid–acetic acid, POCl3–P2O5, TFA or MSA) have been reported for Friedel– Crafts cyclisation.13,14. Initially, we selected PPA and P2O5 as catalysts but no product was obtained. Because of the structural difference between Alidisin and fuscain, the aromaticity of furan ring is less than a pyrrole ring, and a furan ring usually polymerised under acidic conditions, we selected a relatively mild Lewis acid CuSO4•5H2O to complete the intramolecular cyclisation to form fuscain.

    Source: Journal of Chemical Research, Volume 36, Number 12, December 2012 , pp. 736-737(2)
    doi: 10.3184/174751912X13528167435099
    Yuan-wei Liang, Xiao-jian Liao, Chang-jun Wang, Jin-zhi Guo, Shuo Li and Shi-hai Xu*
    Department of Chemistry, Jinan University, Guangzhou 510632, P. R. China


    Friday, 12 July 2013

    Turning carbon dioxide into something useful -Carbon dioxide reduced to formate by iridium pincer catalyst


    New research shows that a water-soluble catalyst developed by scientists in the US can electrocatalytically transform carbon dioxide into a useful chemical feedstock.
    The global demand for fuel is rising, as are carbon dioxide levels in the atmosphere. Recent studies have attempted to address the global carbon imbalance by exploring ways to recycle carbon dioxide into liquid fuels. Formate, the anion of formic acid, is an intermediate of carbon dioxide reduction and can be used as a fuel in formic acid fuel cells. However, the selective production of formate, without using organic solvents, is challenging. Water, being inexpensive and environmentally-friendly, is obviously preferred over organic solvents as a reaction medium. On the other hand, the reduction of carbon dioxide in water is complicated by the reduction of water to hydrogen being a more kinetically favourable process.

    Thursday, 4 July 2013

    Catalyst duo exerts powerful stereocontrol

    The dual catalyst enables selective access to the required stereoisomer © Science/AAAS

    Chemists from the Swiss Federal Institute of Technology, ETH Zurich, have teamed chiral catalysts in pairs to selectively drive a reaction towards desired stereoisomeric products with high selectivity. Each catalyst activates one reagent and controls its substituent arrangement as it bonds to the other to form two neighbouring chiral centres. ‘We have shown that it is possible to develop fully stereodivergent reaction processes,’ says Erick Carreira, who led the work. ‘We expect that additional reactions displaying full stereodivergency will be identified.’

    read all at


    S Krautwald et alScience, 2013, DOI: 10.1126/science.1237068

    Thursday, 27 June 2013

    A molecular database for developing organic solar cells


    Harvard researchers have released a massive database of more than 2 million molecules that might be useful in the construction of solar cells that rely on organic compounds for construction of organic solar cells for the production of renewable energy. Developed as part of the Materials Genome Initiative launched by the White House’s Office of … more…


    Tuesday, 11 June 2013

    Bruce Roth Awarded 2013 Perkin Medal

    Bruce Roth
    Credit: Genentech

    Bruce Roth Awarded 2013 Perkin Medal

    Honors: Chemist was the first to synthesize the cholesterol-lowering drug atorvastatin, also known as Lipitor
    The Society of Chemical Industry (SCI) has selected Bruce D. Roth, vice president of discovery chemistry at Genentech, as the winner of the 2013 Perkin Medal. The annual award is recognized as the highest honor given for outstanding work in applied chemistry in the U.S.

    A Molecule Of Many Colors-With rigid wings and a flexible core, a new compound can switch between two shapes and glow one of three colors.

    Structure of a flexible molecule in its flat and bent shapes
    Flexible And Fluorescent
    A molecule combining rigid anthraceneimide wings and a flexible cyclooctatetraene core switches between a flat and a bent V shape. The R groups are either hydrogens or n-butyl groups.
    Credit: J. Am. Chem. Soc.

    A Molecule Of Many Colors

    Organic Chemistry: With rigid wings and a flexible core, a new compound can switch between two shapes and glow one of three colors.

    A new, flexible, multi-ring organic compound fluoresces red, green, or blue depending on its environment (J. Am. Chem. Soc. 2013, DOI: 10.1021/ja404198h). The molecule’s combination of rigid wings and a flexible center could serve as a general design strategy for molecular sensors, the researchers say.
    The molecule, developed by a team of researchers, including Shohei Saito, Stephan Irle, and Shigehiro Yamaguchi of Nagoya University in Japan, has two rigid anthraceneimide wings on opposite sides of a floppy cyclooctatetraene core

    read all at

    Monday, 10 June 2013