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The Narasaka–Prasad reduction, sometimes simply called Narasaka reduction, is a diastereoselective reduction of β-hydroxy 
The reaction proceeds through the 6-membered 
This reaction can be contrasted with the similar
ketone
In organic chemistry, a ketone is a functional group with the structure R–C(=O)–R', where R and R' can be a variety of carbon-containing substituents. Ketones contain a carbonyl group –C(=O)– (which contains a carbon-oxygen double bon ...
s to the corresponding ''syn''- dialcohols. The reaction employs a boron chelating agent
Chelation is a type of bonding of ions and molecules to metal ions. It involves the formation or presence of two or more separate coordinate bonds between a polydentate (multiple bonded) ligand and a single central metal atom. These ligands are ...
, such as BBu2OMe, and a reducing agent
In chemistry, a reducing agent (also known as a reductant, reducer, or electron donor) is a chemical species that "donates" an electron to an (called the , , , or ).
Examples of substances that are commonly reducing agents include the Earth m ...
, commonly sodium borohydride
Sodium borohydride, also known as sodium tetrahydridoborate and sodium tetrahydroborate, is an inorganic compound with the formula Na BH4. This white solid, usually encountered as an aqueous basic solution, is a reducing agent that finds applic ...
. This protocol was first discovered by Narasaka in 1984.
The reaction proceeds through the 6-membered transition state
In chemistry, the transition state of a chemical reaction is a particular configuration along the reaction coordinate. It is defined as the state corresponding to the highest potential energy along this reaction coordinate. It is often marked ...
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
An intermolecular force (IMF) (or secondary force) is the force that mediates interaction between molecules, including the electromagnetic forces of attraction
or repulsion which act between atoms and other types of neighbouring particles, e.g. a ...
hydride delivery from NaBH4 therefore proceeds via an axial attack from the opposite face with respect to the existing alcohol
Alcohol most commonly refers to:
* Alcohol (chemistry), an organic compound in which a hydroxyl group is bound to a carbon atom
* Alcohol (drug), an intoxicant found in alcoholic drinks
Alcohol may also refer to:
Chemicals
* Ethanol, one of sev ...
.
This reaction can be contrasted with the similar Evans–Saksena reduction The Saksena–Evans reduction is a diastereoselective reduction of β-hydroxy ketones to the corresponding ''anti''-dialcohols, employing the reagent tetramethylammonium triacetoxyborohydride (Me4NHB(OAc)3). The reaction was first described by Anil ...
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 Narasaka–Prasad reduction has been employed in many total syntheses in the literature, such as discodermolide
(+)-Discodermolide is a polyketide natural product found to stabilize microtubules. (+)-discodermolide was isolated by Gunasekera and his co-workers at the Harbor Branch Oceanographic Institute from the deep-sea sponge '' Discodermia dissoluta'' ...
See also
*Evans–Tishchenko reaction The Evans–Tishchenko reaction is the diastereoselective reduction of β-hydroxy ketones to the corresponding 1,3-''anti'' diol monoesters. The reaction employs a Lewis acid, often samarium iodide, and an aldehyde. It was first described in 19 ...
References