A Raney-cobalt-mediated tandem reductive cyclization route to the 1,5-methanoazocino[4,3-b]indole framework of the uleine and strychnos alkaloids

Abstract

The readily accessible enones 8, 17, and 18 undergo 2-fold reductive cyclization reactions upon exposure to hydrogen in the presence of Raney-cobalt and thereby afford compounds 11 (72%), 19 (47%), and 20 (84%), respectively. These products embody the ABCD-ring system associated with the title alkaloids, and compound 11 can be converted, over four steps and in 33% yield, into congener 24 incorporating the ABCDE-ring system of the Strychnos alkaloids.