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.{{Cite book|first = Jaemoon|last = Yang|chapter-url = https://books.google.com/books?id=A_vUdr6ABGIC&pg=PA151|chapter = Diastereoselective Syn-Reduction of β-Hydroxy Ketones|title = Six-Membered Transition States in Organic Synthesis|year = 2008|publisher = John Wiley & Sons|pages = 151–155|isbn = 9780470199046}}
File:Narasaka–Prasad reduction.png
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.
File:Narasaka–Prasad reduction transition state.png
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 Narasaka–Prasad reduction has been employed in many total syntheses in the literature,{{cite journal |last1= Priepke|first1=Henning |last2=Weigand |first2= Stefan|last3=Brückner|first3=Reinhard|year= 2006|title= A Butyrolactone → 1,3-Diol Strategy for the Obtention of Tolypothrix Polyethers – Stereoselective Synthesis of a Key Lactone Precursor|journal= Liebigs Annalen|volume= 1997|issue= 8|pages=1635–1644 |doi= 10.1002/jlac.199719970804}} such as discodermolide{{cite journal |last1= Mickel|first1= Stuart|last2=Niederer |first2= Daniel|last3=Daeffler|first3=Robert|last4=Adman|first4=Osmani|last5=Kuesters|first5=Ernst|last6=Schmid|first6=Emil|last7=Schaer|first7=Karl|last8=Gamboni|first8=Remo|year=2004 |title= Large-Scale Synthesis of the Anti-Cancer Marine Natural Product (+)-Discodermolide. Part 5: Linkage of Fragments C1-6 and C7-24 and Finale|journal= Org. Process Res. Dev.|volume=8 |issue= 1|pages= 122–130|doi=10.1021/op034134j}}