Frogs, alkaloids from neotropical poison

Frogs have alkaloid-containing defensive chemicals in their skin. More than 800 alkaloids are found from the amphibian skin, and 500 alkaloids from neotropical poison frogs. Experimental data found that bufonid poison frogs sequester alkaloid-based defenses from dietary sources, especially from consumption of mites and ants. In the case of Cuban endemic miniaturized frogs (Eleutherodactylus limbatus), the capacity to sequester alkaloids is evolutionary determined and followed by evolution of miniaturization. Moreover, it is in direct relation to the increase of brightness of color and possible aposematic function in this species. ... 

Daly, J. W., G. B. Brown, M. Mensah-Dwumah, and C. W. Myers Classification of skin alkaloids from neotropical poison-dart frogs (Dendrobatidae). Toxicon 16,163— 188 (1978). 

Winkler has extensively examined stereoselective (2 + 2]-cycloaddition reactions in the context of complex molecule synthesis (see also Scheme 18.36). In a fundamental study, intramolecular cycloadditions of tethered enones proved to be highly diastereoselective. Thus, the presence of the stereogenic center on the tether connecting the enone to the dioxinone in 198 controls the stereochemical outcome of the reaction as shown in Scheme 18.31 [133,134). Cycloadduct 200 was isolated in 95 % yield and with superb levels of asymmetric induction (dr= 95 5). A subsequent fragmentation sequence transformed 200 into the neurotoxin perhydrohistrionitoxin (201), an alkaloid isolated from neotropical poison frogs [134]. 

The neotropical dart poison frogs contain a remarkable diversity of alkaloids, and the 2,5-disubstituted decahydroquinolines represent, a major class of these amphibian alkaloids[21]. Isolation of these alkaloids from some ants strengthens a dietary hypothesis for the origin of the above alkaloids that have been detected in extracts of frog skin[22]. In addition, these alkaloids containing both cis and trans ring fusion have been identified as well as diastereomers at the C-2 and C-5 position. 

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