Acid-catalysed epimerization of indolo[2,3-a]quinolizidine derivatives: Role of the nitrogen lone pairs in the mechanism

摘要

The role of the nitrogen lone pairs in the mechanism of the acid-catalysed epimerization of indolo[2,3-a]quinolizidines is investigated using lactams as model compounds. Deethyleburnamonine (3) did not epimerize with trifluoroacetic acid (TFA), whereas deethyldihydroebumamenine (7) underwent epimerization smoothly. Under treatment with TFA, lactams 12 and 13 both epimerized with ease to a mixture of lactams 12 and 13. An analogous equilibrium was achieved when the experiment was repeated with lactams 18 and 19. Intermediate 20 was trapped (Zn reduction) in the acid-catalysed epimerization of lactam 12, allowing conclusions about the mechanism. (C) 1998 Elsevier Science Ltd. All rights reserved. [References: 21]