Evidence for a stepwise mechanism in formal hetero-Diels-Alder reactions of N-arylimines

Article abstract of DOI:10.1016/0040-4020(95)00947-7

In order to investigate the mechanism of Lewis acid-catalyzed cyclizations of ω-unsaturated N-arylimines, various prolinal-derived N-arylimines 17a-f were synthesized and treated with Lewis acids. Whereas imine 17a bearing a C-2 tether and imines 17d, f with terminal alkene or alkyne moieties could not be cyclized under these conditions, imine 17b with a C-3 tether gave three diastereomeric pyrrolo-[1',2':1,2]azepino[3,4-b]quinolines 18a-c and imine 17c gave two diasteromeric 7,7-diphenyl-indolizino-[3,4-b]quinolines 19a, b. High cis/trans-ratios were observed in both cases. Imine 17e bearing an internal alkyne underwent a cyclization/dehydrogenation to give the indolizino[3,4-b]quinoline 21. From these results a stepwise mechanism was concluded. The configuration of 19a was established by an X-ray crystal structure analysis.