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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."
Several years later in 1987, E. J. Corey and coworkers developed the reaction between chiral amino alcohols and borane (BH 3), generating oxazaborolidine products which were shown to rapidly catalyze the enantioselective reduction of achiral ketones in the presence of BH 3 •THF. [2] [3]
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.
The reaction has a wide scope, as shown in Figure 16, owing to the high Lewis acidity of the catalysts, which can activate even weakly nucleophilic olefins, such as 1-hexene and cyclohexene. Figure 15. C2-symmetric Cu(II) catalysts developed for the enantioselective carbonyl-ene reactions of olefins and ethyl glyoxylate Figure 16. Scope of the ...
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]
As one of the few highly enantioselective reactions during its time, many manipulations of the 2,3-epoxyalcohols have been developed. [13] The Sharpless epoxidation has been used for the total synthesis of various saccharides, terpenes, leukotrienes, pheromones, and antibiotics. [6]
Jacobsen's catalysts R = Alkyl, O-alkyl, O-trialkyl Best Jacobsen catalyst: R = t Bu Katsuki's catalysts R 1 = Aryl, substituted aryl R 2 = Aryl, Alkyl. The Jacobsen epoxidation, sometimes also referred to as Jacobsen-Katsuki epoxidation is a chemical reaction which allows enantioselective epoxidation of unfunctionalized alkyl- and aryl- substituted alkenes.
Enantioselective ketone reductions convert prochiral ketones into chiral, non-racemic alcohols and are used heavily for the synthesis of stereodefined alcohols. [ 1 ] Carbonyl reduction, the net addition of H 2 across a carbon-oxygen double bond, is an important way to prepare alcohols.