Enantioselective Brønsted Acid-Catalyzed N-Acyliminium Cyclization Cascades

An enantioselective Bronsted acid-catalyzed N-acyliminium cyclization cascade of tryptamines with enol lactones to form architecturally complex heterocycles in high enantiomeric excess has been developed. The reaction is technically simple to perform as well as atom-efficient and may be coupled to a gold(I)-catalyzed cycloisomerization of alkynoic acids whereby the key enol lactone reaction partner is generated in situ. Employing up to 10 mol % bulky chiral phosphoric acid catalysts in boiling toluene allowed the product materials to be generated in good overall yields (63−99%) and high enantioselectivities (72−99% ee). With doubly substituted enol lactones, high diastereo- and enantioselectivities were obtained, thus providing a new example of a dynamic kinetic asymmetric cyclization reaction.