Narasaka–Prasad reduction

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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.[1]

Scheme for the Narasaka–Prasad reduction

The reaction proceeds through the 6-membered transition state shown below. Chelation by the boron agent blocks the bottom face by placing R2 in an axial position. The intermolecular hydride delivery from NaBH4 therefore proceeds via an axial attack from the opposite face with respect to the existing alcohol.[1]

Transition state for the Narasaka–Prasad reduction showing the reasons for the observed diastereoselectivity

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,[2] such as discodermolide[3]

See also[edit]

References[edit]

  1. ^ a b Jaemoon Yang (2008). Six-Membered Transition States in Organic Synthesis. John Wiley & Sons. pp. 151–155. ISBN 9780470199046. 
  2. ^ Priepke, Henning; Weigand, Stefan; Brückner, Reinhard (2006). "A Butyrolactone → 1,3-Diol Strategy for the Obtention of Tolypothrix Polyethers – Stereoselective Synthesis of a Key Lactone Precursor". Liebigs Annalen (Wiley Online Library) 1997 (8): 1635–1644. doi:10.1002/jlac.199719970804. Retrieved 31 January 2013. 
  3. ^ Mickel, Stuart; Niederer, Daniel; Daeffler, Robert; Adman, Osmani; Kuesters, Ernst; Schmid, Emil; Schaer, Karl; Gamboni, Remo (2004). "Large-Scale Synthesis of the Anti-Cancer Marine Natural Product (+)-Discodermolide. Part 5:  Linkage of Fragments C1-6 and C7-24 and Finale". Org. Proc. Res. Dev. (American Chemical Society) 8 (1): 122–130. doi:10.1021/op034134j. Retrieved 31 January 2013.