Round trip radical reactions from acyclic precursors to tricyclo[5.3.1.0(2,6)]undecanes. A new cascade radical cyclization approach to (+/-)-isogymnomitrene and (+/-)-gymnomitrene

The synthesis and "round trip radical cyclization" of 11-iodo-2,7,11-trimet hyldodec-6-en-5-one are described. The round trip cyclization is a sequence of 5-exo, 6-endo, and 5-exo cyclizations in which the last radical cycliza tion occurs at the same carbon atom as the initial radical generation. The key second (6-endo) cyclization produces two stereoisomers, one of which cy clizes efficiently to isogymnomitrene ketone, while the other cyclizes inef ficiently to gymnomitrene ketone. Efforts to influence the kinetic or therm odynamic outcome of the second cyclization were not successful, and the res ults are contrasted with a related cyclization of Jung and Rayle where ther modynamic control was readily established.