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Enantioselective synthesis, also called asymmetric synthesis, [1] is a form of chemical synthesis.It is defined by IUPAC as "a chemical reaction (or reaction sequence) in which one or more new elements of chirality are formed in a substrate molecule and which produces the stereoisomeric (enantiomeric or diastereomeric) products in unequal amounts."
An enantioselective reaction is one in which one enantiomer is formed in preference to the other, in a reaction that creates an optically active product from an achiral starting material, using either a chiral catalyst, an enzyme or a chiral reagent. The degree of selectivity is measured by the enantiomeric excess.
This term has become very popular and commonly used in practice. But the appropriate expression is "enantioselective chromatography". [34] Chiral chromatography has advanced to turn into the most preferred technique for the determination of enantiomeric purity as well as separation of pure enantiomers both on analytical and preparative scale.
Asymmetric counteranion directed catalysis (ACDC) [1] or chiral anion catalysis [2] in enantioselective synthesis is the "induction of enantioselectivity in a reaction proceeding through a cationic intermediate by means of ion pairing with a chiral, enantiomerically pure anion provided by the catalyst". [1]
The Cram's rule of asymmetric induction named after Donald J. Cram states In certain non-catalytic reactions that diastereomer will predominate, which could be formed by the approach of the entering group from the least hindered side when the rotational conformation of the C-C bond is such that the double bond is flanked by the two least bulky groups attached to the adjacent asymmetric center. [3]
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Obeying the same kinetic rate law, each of the three catalytic complexes catalyze the desired reaction to form product. [7] As enantiomers of each other, the homochiral complexes catalyze the reaction at the same rate, although opposite absolute configuration of the product is induced (i.e. r RR =r SS ).
Asymmetric versions of the above reaction have taken advantage of a number of strategies for achieving asymmetric induction. The highest yielding and most enantioselective methods include: Use of stoichiometric chiral oxidant [5] Use of stoichiometric metal peroxides substituted with chiral ligands [6] Use of stoichiometric chiral base [7]