Selective aldol condensation or cyclotrimerization reactions catalyzed by FeCl₃

The inexpensive and commercially available FeCl₃ catalyzes the selective homo-aldol condensation of aldehydes into the corresponding α-β unsaturated aldehydes, presumably through an unobserved aldol intermediate. Surprisingly the reaction course can be diverted to produce 1,3,5-trioxanes via aldehyde cyclotrimerization. Selectivity for trioxanes is achieved at lower temperatures and is enhanced by the presence of water, contrary to prior reports. A broad range of aliphatic and aromatic aldehydes are capable of undergoing these transformations; short reaction times, high yields, scalability, and easy purification processes are also described.