Electrophilic Cyclization of Doubly Activated Imines with Lewis Acids and Trialkylsilyl Triflates Synthesis of Pyrrole and Azepine Derivatives

Abstract

AbstractThe cyclization of the (pentenylimino)malonate 8a gives either the pyrrolidines 11a and 12a or the hexahydro‐1H‐furo[3,4‐b]‐pyrrole 13a as main products depending on the Lewis acid used. The isopropyl ester 8b cyclizes under all reaction conditions preferentially to give the lactone ester 13b. Cyclization of the heptenylimine 9 yields exclusively the epimeric octahydro‐1H‐furo‐[3,4‐b]azepines 17 and 18. The thermodynamically more stable 17 could be obtained in large excess. Saponification and decarboxylation of 17 gives the cyclic nonproteinogenic α‐amino acid derivative 20. The octenylimine 10 could not be cyclized under several conditions tested.