The IBOGA alkaloids and their role as precursors of anti-neoplastic bisindole catharanthus alkaloids

Abstract

Iboga alkaloids are found in plants of the family Apocynaceae, which falls within the order Gentianales. The iboga alkaloids are examples of the broad class of monoterpenoid indole alkaloids that are considered a chemical marker for the Apocynaceae. This chapter discusses the chemistry of iboga alkaloids and their role as precursors of anti-neoplastic bisindole Catharanthus alkaloids. The iboga alkaloids are comprised of indole and isoquinuclidine rings fused by a seven-membered C-ring. There is a carboxy group at C-16 in the C20 alkaloids, but other iboga alkaloids lack this substituent. Ibogamine and catharanthine are prototypical structures. Studies have shown the catharanthine isolated from natural sources is usually of the opposite absolute configuration from ibogamine. The chapter uses the biosynthetic numbering scheme of Le Men and Taylor. The chapter also discusses the structures in which the C-16-C-2 1 bond is cleaved. The C-16,C-21 fragmented structure occurs in the anti-neoplastic bisindole alkaloids isolated from Catharanthus roseus. The iboga structure also frequently occurs in combination with the vobasan skeleton.