Abstract
AbstractLewis acid‐catalyzed cyclization of proline‐derived N‐arylimines 8 gives the tetracyclic decahydro‐7,7,11‐trimethyl‐in‐dolizino[3,4‐b]quinolines 17a, b in a ratio, which is highly dependent on the coordination number of the acidic catalyst. Whereas monodentate Brønsted and Lewis acids as well as octahedral bidentate Lewis acids preferably gave the all‐trans product 17b, the all‐cis product 17a was obtained by tetrahedral aluminium Lewis acids. In contrast to this chelation‐controlled reversal of diastereoselectivity, mixtures of cis/trans undecahydro‐6,6,10‐trimethyl‐11‐azacyclopenta[a]‐anthracenes 18b‐e, h were isolated from the cyclization of the corresponding cyclopentane‐derived N‐arylimines 16 irrespective of the Lewis acid. Force field calculations support a stepwise iminium ion cyclization under thermodynamic control rather than a concerted hetero‐Diels‐Alder mechanism.
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