Diastereoselective ketone reduction Felkin-Anh Transition State

Additions to Carbonyl groups can be diastereoselective

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The Felkin-Anh conformation has the largest (and most electronegative in other cases) group perpendicular to the carbonyl. Attack on the Bürgi-Dunitz trajectory leads to the nucleophile, hydride in this case, passing close the substituents on the alpha carbon. The nucleophile, in this case, hydride, attacks from the least hindered approach.

Link to chelation-controlled example

A. Mengel and O. Reiser, Chem. Rev., 1999, 99, 1191–1224.