- Evidence for a stepwise mechanism in formal hetero-Diels-Alder reactions of N-arylimines
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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.
- Linkert, Frank,Laschat, Sabine,Kotila, Sirpa,Fox, Thomas
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p. 955 - 970
(2007/10/03)
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