Narasaka–Prasad reduction

[1] 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.

[1] 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]