EVIDENCE FOR A STEPWISE MECHANISM IN FORMAL HETERO-DIELS-ALDER REACTIONS OF N-ARYLIMINES

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