A Biomimetic Enantioselective Approach to the Decahydroquinoline Class of Dendrobatid Alkaloids

Abstract

Frogs of the neotropical family Dendrobatidae produce a remarkably diverse array of biologically active alkaloids. One of the major classes of these amphibian alkaloids[1] are the decahydroquinolines, which have been isolated not only from skin extracts of dendrobatid and mantelline frogs,[2] but also from bufonid toads,[3] tunicates,[4] marine flatworms,[4b] and myrmicine ants.[5] They possess either a cis or trans decahydroquinoline ring fusion, with a side-chain substituent at both the C2 and C5 positions and, in the lepadin series,[4] an acylated hydroxy group at the C3 position. The most representative decahydroquinoline alkaloid is cis-195A (formerly called pumiliotoxin C), first isolated in 1969 from a Panamanian population of Dendrobates pumilio. [6] The source of amphibian alkaloids remains an unresolved and challenging question,[1] in particular after the discovery that some of these alkaloids also occur in ants, thus strengthening a dietary hypothesis for their origin in frogs.[5] Although there are no conclusive studies concerning the biosynthesis of these toxins and, consequently, little is known about the biosynthetic pathways, there has been speculation as to possible derivation from the polyketide route by aminocyclization of polycarbonyl intermediates (A), leading to either 2,5-disubstituted decahydroquinolines (C) or spiropiperidines (histrionicotoxins).[1a,b, 7] In accordance with this hypothesis, a plausible biosynthetic pathway to the decahydroquinoline class of dendrobatid alkaloids is depicted in