3.

(a) Restricted rotation of C–N bond (highlighted in red) and the corresponding atropisomers formed during Friedel–Crafts monoacylation. 11-α and 12-α stay as an inseparable mixture, which is distinct from the other inseparable mixture consisting of their atropisomers 11-β and 12-β. (b) Single-crystal X-ray structures of 7, 9, and 13. The thermal ellipsoids are scaled to the 50% probability level. Solvent molecules and hydrogen atoms are omitted for clarity. The non-coplanarity is determined by the root-mean-square displacement (δ) of backbone carbon atoms to their respective regression planes.