Intramolecular Electrophilic Cyclization of Doubly Activated Imines Induced by Lewis Acids and Trialkylsilyl Triflates. An Efficient Route to Substituted Piperidines and Annulated Piperidine Lactones

Intramolecular electrophilic cyclization of imines 11a – c with two electron‐withdrawing groups at the CN bond using Lewis and Brønsted acids as well as trialkylsilyl trifluoromethanesulfonates gives the piperidines 12a – c and the annulated piperidine lactones 13a – c and 14 . 11b yields mostly 12b with the trialkylsilyl triflates and 13b with Lewis acids such as FeCl 3 on Al 2 O 3 or GaCl 3 . The reaction of 11a and 11c always results, using different methods, in the formation of the lactones 13a , 14 , and 13c , respectively. Treatment of 13b with aqueous base affords 19 , a derivative of a cyclic nonproteinogenic α‐amino acid.