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.