Pyridinethiones. Part 11. Diastereoselective Aldol Condensations with 2(1H)‐Pyridones, 2(1H)‐Pyridinethiones, and the Crystal Structure of 3‐(1‐Hydroxy‐2‐methyl‐3‐oxo‐3‐(4‐chlorophenyl)propyl)‐1‐isopropyl‐2(1H)‐pyridinethione.