What is Felkin–Anh model and chelation control?

The Felkin–Anh model predicts the major diastereomer of a nucleophilic addition to a carbonyl group next to a stereocentre: the largest group on that centre lies perpendicular to the carbonyl plane, and the nucleophile approaches anti to it, past the smallest group, along the Bürgi–Dunitz trajectory. Under chelation control, a metal ion bound to both the carbonyl oxygen and a donor group on the stereocentre locks the conformation, and the nucleophile adds to the less hindered face of the chelate ring.