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Narasaka–Prasad Reduction
The Narasaka–Prasad reduction, sometimes simply called Narasaka reduction, is a diastereoselective reduction of β-hydroxy ketones to the corresponding ''syn''- dialcohols. The reaction employs a boron chelating agent, such as BBu2OMe, and a reducing agent, commonly sodium borohydride. This protocol was first discovered by Narasaka in 1984. The reaction proceeds through the 6-membered transition state shown below. Chelation by the boron agent favors hydride delivery from the top face because it leads directly to the more stable chair-like conformation of the product ( Fürst-Plattner Rule). The intermolecular hydride delivery from NaBH4 therefore proceeds via an axial attack from the opposite face with respect to the existing alcohol. This reaction can be contrasted with the similar Evans–Saksena reduction that employs a different boron reagent in order to achieve intramolecular hydride delivery from the same face of the alcohol, thus producing the ''anti''-diol. The Na ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Diastereoselective
In stereochemistry, diastereomers (sometimes called diastereoisomers) are a type of stereoisomer. Diastereomers are defined as non-mirror image, non-identical stereoisomers. Hence, they occur when two or more stereoisomers of a compound have different configurations at one or more (but not all) of the equivalent (related) stereocenters and are not mirror images of each other. When two diastereoisomers differ from each other at only one stereocenter, they are epimers. Each stereocenter gives rise to two different configurations and thus typically increases the number of stereoisomers by a factor of two. Diastereomers differ from enantiomers in that the latter are pairs of stereoisomers that differ in all stereocenters and are therefore mirror images of one another. Enantiomers of a compound with more than one stereocenter are also diastereomers of the other stereoisomers of that compound that are not their mirror image (that is, excluding the opposing enantiomer). Diastereomers h ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
