W(CO)₅-Catalyzed Synthesis of 2- and 3-Azabicyclo[3.3.0]octanes

Significance

Reported is a tungsten-catalyzed regioselective synthesis of 3-azobicyclo[3.3.0]-octane and 2-azobicyclo[3.3.0]octane derivatives C and D from π-acetylenic dienol silyl ether 1. The reaction proceeds via different pathways a and b as a function of base. Thus, the initial vinylidene complex A undergoes double cyclizations and nitrogen facilitated 1,2-alkyl migration to lead to product C. This mechanism is substantiated by ¹³C- as indicated and D-labeled experiments. In the absence of base, the tungsten-catalyzed process is envisaged to proceed via the zwitterionic intermediates B to lead to products D. None of the intermediates were isolated.