Alkaloids from Amphibian Skins

Passive accumulation of alkaloids in non-toxic frogs challenges paradigms of the origins of acquired chemical defenses

Understanding the origins of novel, complex phenotypes is a major goal in evolutionary biology. Poison frogs of the family Dendrobatidae have evolved the novel ability to acquire alkaloids from their diet for chemical defense at least three times. However, taxon sampling for alkaloids has been biased towards colorful species, without similar attention paid to inconspicuous ones that are often assumed to be undefended. As a result, our understanding of how chemical defense evolved in this group is incomplete. Here we provide new data showing that, in contrast to previous studies, species from each undefended poison frog clade have measurable yet low amounts of alkaloids. We confirm that undefended dendrobatids regularly consume mites and ants, which are known sources of alkaloids. Further, we confirm the presence of alkaloids in two putatively non-toxic frogs from other families. Our data suggest the existence of a phenotypic intermediate between toxin consumption and sequestration-passive accumulation-that differs from active sequestration in that it involves no derived forms of transport and storage mechanisms yet results in low levels of toxin accumulation. We discuss the concept of passive accumulation and its potential role in the origin of chemical defenses in poison frogs and other toxin-sequestering organisms.

Pyrazole-promoted synthesis of pyrrolo[3,4-c] quinoline-1,3-diones in a novel diketene-based reaction

We describe the first classic example of green synthesis of pyrrolo[3,4-c]quinolones scaffolds by catalyst-free unusual reaction of diketene, isatin, and primary amines in ethanol in the presence of pyrazole as a promoter for 4 h. The whole structure of the new product was confirmed by X-ray analysis. The overall transformation involves the cleavage and generation of multiple carbon-nitrogen and carbon-carbon bonds. This report represents a simple and straightforward approach for the synthesis of pyrrolo[3,4-c]quinoline-1,3-diones, which has significant advantages like readily available precursors, non-use of toxic solvent, operational simplicity, mild conditions, good atom economy, and excellent yields; therefore it provides a green and sustainable strategy for access to a range of interesting N-containing heterocyclic compounds in medicinal and organic chemistry.

Synthesis of substituted pyridines with diverse functional groups via the remodeling of (Aza)indole/Benzofuran skeletons

Substituted pyridines with diverse functional groups are important structural motifs found in numerous bioactive molecules. Several methodologies for the introduction of various bio-relevant functional groups to pyridine have been reported, but there is still a need for a single robust method allowing the selective introduction of multiple functional groups. This study reports a ring cleavage methodology reaction for the synthesis of 2-alkyl/aryl 3-electron-withdrawing groups (esters, sulfones, and phosphonates) 5-aminoaryl/phenol pyridines via the remodeling of 3-formyl (aza)indoles/benzofurans. Totally ninety-three 5-aminoaryl pyridines and thirty-three 5-phenol pyridines were synthesized showing the robustness of the developed methodology. The application of this methodology further provided a privileged pyridine scaffold containing biologically relevant molecules and direct drug/natural product conjugation with ethyl 2-methyl nicotinate.

Inverse hydride shuttle catalysis enables the stereoselective one-step synthesis of complex frameworks

The rapid assembly of complex scaffolds in a single step from simple precursors identifies as an ideal reaction in terms of efficiency and sustainability. Indeed, the direct single-step synthesis of complex alkaloid frameworks remains an unresolved problem at the heart of organic chemistry in spite of the tremendous progress of the discipline. Herein, we present a broad strategy in which dynamically assembled ternary complexes are converted into valuable azabicyclic scaffolds based on the concept of inverse hydride shuttle catalysis. The ternary complexes are readily constructed in situ from three simple precursors and enable a highly modular installation of various substitution patterns. Upon subjection to a unique dual-catalytic system, the transient intermediates undergo an unusual hydride shuttle process that is initiated by a hydride donation event. Furthermore, we show that, in combination with asymmetric organocatalysis, the product alkaloid frameworks are obtained in excellent optical purity.

Stereoselective Synthesis of the Decahydroquinoline Alkaloid cis-

The first enantioselective synthesis of two C-5 diastereomers of the proposed structure of the decahydroquinoline alkaloid cis-195J has been achieved. The key step of our strategy is the highly stereoselective vinylogous Mukaiyama-Mannich reaction (VMMR), which gave rise to the first two stereogenic centers at the ring fusion with excellent diastereo- and enantiocontrol. Through alkyne cyclization and enamine reduction the correct cis-configuration between C-2, C-4a, and C-8a in the decahydroquinoline backbone was established. Subsequently, a radical cyclization of a tethered alkyl iodide onto the enoate assembled the bicyclic cis-decahydroquinoline as a mixture of two C-5 diastereomers. Further elaboration of the C-5 side chain eventually provided both diastereomers of cis-195J, which were readily separated, and their constitution and configuration were thus unambiguously proven for the first time.

Applications of Transition-Metal-Catalyzed Asymmetric Allylic Substitution in Total Synthesis of Natural Products: An Update

Metal-catalyzed asymmetric allylic substitution (AAS) reaction is one of the most synthetically useful reactions catalyzed by metal complexes for the formation of carbon-carbon and carbon-heteroatom bonds. It comprises the substitution of allylic substrates with a wide range of nucleophiles or S_N 2'-type allylic substitution, which results in the formation of the above-mentioned bonds with high levels of enantioselective induction. AAS reaction tolerates a broad range of functional groups, thus has been successfully applied in the asymmetric synthesis of a wide range of optically pure compounds. This reaction has been extensively used in the total synthesis of several complex molecules, especially natural products. In this review, we try to highlight the applications of metal (Pd, Ir, Mo, or Cu)-catalyzed AAS reaction in the total synthesis of the biologically active natural products, as a key step, updating the subject from 2003 till date.

Studies on the Synthesis of Phlegmarine-Type Lycopodium Alkaloids: Enantioselective Synthesis of (-)-Cermizine B, (+)-Serratezomine E, and (+)-Luciduline

The synthesis of the Lycopodium alkaloids, (-)-cermizine B, (+)-serratezomine E, and (+)-luciduline using phenylglycinol-derived tricyclic lactams as chiral scaffolds, is reported. The requisite lactams are prepared by a cyclocondensation reaction between ( R)- or ( S)-phenylglycinol and the substituted δ-keto ester 11, easily accessible from ( R)-pulegone. The factors governing the stereoselectivity of these cyclocondensation reactions are discussed. Key steps of the synthesis from the stereochemical standpoint are the stereoselective elaboration of the allyl substituent to the ( S)-2-(piperidyl)methyl moiety and the stereoselective removal of the chiral inductor to give a cis-decahydroquinoline.

Interacting amino acid replacements allow poison frogs to evolve epibatidine resistance

Animals that wield toxins face self-intoxication. Poison frogs have a diverse arsenal of defensive alkaloids that target the nervous system. Among them is epibatidine, a nicotinic acetylcholine receptor (nAChR) agonist that is lethal at microgram doses. Epibatidine shares a highly conserved binding site with acetylcholine, making it difficult to evolve resistance yet maintain nAChR function. Electrophysiological assays of human and frog nAChR revealed that one amino acid replacement, which evolved three times in poison frogs, decreased epibatidine sensitivity but at a cost of acetylcholine sensitivity. However, receptor functionality was rescued by additional amino acid replacements that differed among poison frog lineages. Our results demonstrate how resistance to agonist toxins can evolve and that such genetic changes propel organisms toward an adaptive peak of chemical defense.

Enantioselective Total Synthesis of (+)-Gephyrotoxin 287C

A synthesis of (+)-gephyrotoxin 287C using (S)-phenylglycinol-derived tricyclic lactam 7 as the starting enantiomeric scaffold is reported. From the stereochemical standpoint, the key steps are the generation of the DHQ C-5 stereocenter by hydrogenation of the C-C double bond, removal of the chiral inductor to give a cis-DHQ, introduction of the DHQ C-2 substituent, completion of the (Z)-enyne moiety, and generation of the C-1 stereocenter during closure of the pyrrolidine ring.

Access to Enantiopure 5-, 7-, and 5,7-Substituted cis-Decahydroquinolines: Enantioselective Synthesis of (-)-Cermizine B

Stereoconvergent cyclocondensation reactions of (R)- or (S)-phenylglycinol with appropriately substituted cyclohexanone-based δ-keto esters are the key steps of short synthetic routes to enantiopure 5-, 7-, and 5,7-substituted cis-decahydroquinolines. The factors governing the stereoselectivity of the cyclocondensation are discussed. The potential of the methodology is illustrated by a protecting-group-free synthesis of the phlegmarine-type Lycopodium alkaloid (-)-cermizine B.

Theoretical Study of Nickel-Catalyzed Selective Alkenylation of Pyridine: Reaction Mechanism and Crucial Roles of Lewis Acid and Ligands in Determining the Selectivity

Selective alkenylation of pyridine is challenging in synthetic organic chemistry due to the poor reactivity and regioselectivity of the aromatic ring. We theoretically investigated Ni-catalyzed selective alkenylation of pyridine with DFT. The first step is coordination of the pyridine-AlMe₃ adduct with the active species Ni⁽⁰⁾(NHC)(C₂H₂) 1 in an η²-fashion to form an intermediate Int1. After the isomerization of Int1, the oxidative addition of the C-H bond of pyridine across the nickel-acetylene moiety occurs via a transition state TS2 to form a Ni^(II)(NHC) pyridyl vinyl intermediate Int3. This oxidative addition is rate-determining. The next step is C-C bond formation between pyridyl and vinyl groups leading to the formation of vinyl-pyridine (P1). One of the points at issue in this type of functionalization is how to control the regioselectivity. With the use of Ni(NHC)/AlMe₃ catalyst, the C⁴- and C³-alkenylated products (ΔG°^⧧ = 17.4 and 21.5 kcal mol^-1, respectively) are formed preferably to the C² one (ΔG°^⧧ = 22.0 kcal mol^-1). The higher selectivity of the C⁴-alkenylation over the C³ and the C² ones is attributed to the small steric repulsion between NHC and AlMe₃ in the C⁴-alkenylation. Interestingly, with Ni(P(i-Pr)₃)/AlMe₃ catalyst, the C²-alkenylation occurs more easily than the C³ and C⁴ ones. This regioselectivity arises from the smaller steric repulsion induced by P(i-Pr)₃ than by bulky NHC. It is notable that AlMe₃ accelerates the alkenylation by inducing the strong CT from Ni to pyridine-AlMe₃. In the absence of AlMe₃, pyridine strongly coordinates with the Ni atom through the N atom, which increases Gibbs activation energy (ΔG°^⧧ = ∼27 kcal mol^-1) of the C-H bond activation. In other words, AlMe₃ plays two important roles, acceleration of the reaction and enhancement of the regioselectivity for the C⁴-alkenylation.

Concise synthesis of alkaloid (-)-205B

Described herein is a short total synthesis of alkaloid (-)-205B (1) by means of an anti-selective SN2' alkylation of an attractively functionalized cyclopropanol and diastereoselective cyclization of the resulting aminoallene adduct for bicyclic ring formation. The synthesis features a general route to cis- or trans-2,6-disubstituted piperidines by lithium aluminum hydride reduction of the imine intermediate by an appropriate choice of solvent and cis- or trans-2,5-disubstituted pyrrolidines by an exceptional level of chirality transfer from a pendant allene. Particularly noteworthy are the brevity and convergence made possible by a segment-coupling strategy.

Rhodium-catalyzed hydroformylation in fused azapolycycles synthesis

N-Heterocycles, including fused ones, have proven to be an important class of compounds since they possess biological and pharmacological activities themselves and serve as valuable intermediates for synthetic drug discovery. My interest in the synthesis of these compounds stems from studies dealing with the hydroformylation (oxo) of olefins. The dihydroindolizines and benzofused ones are easily generated via rhodium-catalyzed hydroformylation of N-allylpyrroles and indoles: the butanal intermediate undergoes an intramolecular cyclodehydration giving the final polycyclic compound. This chapter reports my results in the area of the conversions of oxo aldehydes with additional C,C-bond-forming reactions together with relevant work from other laboratories on additional C,N-bond-forming reactions, encountered in the field of Azapolycycles synthesis over the last 5 years or so. The intramolecular sequences for polycylization will be especially emphasized using rhodium complexes to effect these transformations, under both conventional and microwave heating.

Fast molecular evolution associated with high active metabolic rates in poison frogs

Molecular evolution is simultaneously paced by mutation rate, genetic drift, and natural selection. Life history traits also affect the speed of accumulation of nucleotide changes. For instance, small body size, rapid generation time, production of reactive oxygen species (ROS), and high resting metabolic rate (RMR) are suggested to be associated with faster rates of molecular evolution. However, phylogenetic correlation analyses failed to support a relationship between RMR and molecular evolution in ectotherms. In addition, RMR might underestimate the metabolic budget (e.g., digestion, reproduction, or escaping predation). An alternative is to test other metabolic rates, such as active metabolic rate (AMR), and their association with molecular evolution. Here, I present comparative analyses of the associations between life history traits (i.e., AMR, RMR, body mass, and fecundity) with rates of molecular evolution of and mitochondrial loci from a large ectotherm clade, the poison frogs (Dendrobatidae). My results support a strong positive association between mass-specific AMR and rates of molecular evolution for both mitochondrial and nuclear loci. In addition, I found weaker and genome-specific covariates such as body mass and fecundity for mitochondrial and nuclear loci, respectively. No direct association was found between mass-specific RMR and rates of molecular evolution. Thus, I provide a mechanistic hypothesis of the link between AMRs and the rate of molecular evolution based on an increase in ROS within germ line cells during periodic bouts of hypoxia/hyperoxia related to aerobic exercise. Finally, I propose a multifactorial model that includes AMR as a predictor of the rate of molecular evolution in ectothermic lineages.

Organometallic enantiomeric scaffolding: a strategy for the enantiocontrolled construction of regio- and stereodivergent trisubstituted piperidines from a common precursor

Reported herein is a general and efficient method to construct 2,3,6-trisubstituted piperidines in a substituent-independent fashion. From the high enantiopurity organometallic scaffold (-)-Tp(CO)(2)[(η-2,3,4)-(1S,2S)-1-benzyloxycarbonyl-5-oxo-5,6-dihydro-2H-pyridin-2-yl)molybdenum (Tp = hydridotrispyrazolylborato), a variety of TpMo(CO)(2)-based 2,3,6-trifunctionalized complexes of the (η-3,4,5-dihydropyridinyl) ligand were easily obtained in 5 steps through a sequence of highly regio- and stereospecific metal-influenced transformations (15 examples). From the 2,3,6-trifunctionalized molybdenum complexes, either 2,6-cis-3-trans or 2,3,6-cis systems were selectively obtained through the choice of an appropriate stereodivergent demetalation protocol. The potential of this strategy in synthetic chemistry was demonstrated by the short total synthesis of four natural and one non-natural alkaloids: indolizidines (±)-209I and (±)-8-epi-219F in the racemic series, and enantiocontrolled syntheses of (-)-indolizidine 251N, (-)-quinolizidine 251AA, and (-)-dehydroindolizidine 233E.

Roughing it: a mantellid poison frog shows greater alkaloid diversity in some disturbed habitats

Four five-skin alkaloid extracts of the Madagascan poison frog Mantella baroni from three disturbed collection sites were compared with four five-skin extracts from three undisturbed sites. The number of alkaloids (diversity) was significantly different in M. baroni between undisturbed and disturbed collection sites, with more alkaloids generally being found in frogs from disturbed sites. Two undisturbed sites did not differ from two disturbed sites, but the third disturbed site (coded 6) had more than twice the alkaloid diversity found in frogs from the third undisturbed site (coded 5a/5b). There was no difference in the quantity of alkaloids in M. baroni between undisturbed and disturbed collection sites. The hypothesis that an undisturbed habitat confers a benefit to poison frogs dwelling therein, in allowing for the sequestration of greater alkaloid diversity and amounts, is challenged by our results. In the course of our study, we found that collections of frogs separated by an interval of three months at an undisturbed site differed by only 4% in alkaloid composition over this period, whereas frogs collected at a disturbed site and collected approximately three months later already had a 26% difference in alkaloid composition between the two collections. This constancy of skin alkaloid composition likely reflects a constancy of dietary prey items consumed by frogs at undisturbed sites.

Intramolecular pyridine activation-dearomatization reaction: highly stereoselective synthesis of polysubstituted indolizidines and quinolizidines

An unprecedented intramolecular pyridine activation-asymmetric dearomatization reaction is described. This process produces 5-substituted indolizidines and 6-substituted quinolizidines in excellent yields and in a highly regio- and diastereoselective fashion. Formal syntheses of trans-indolizidine alkaloids are presented along with some preliminary results in the formation of C-5 quaternary centers.

Analogues of amphibian alkaloids: total synthesis of (5R,8S,8aS)-(-)-8-methyl-5-pentyloctahydroindolizine (8-epi-indolizidine 209B) and [(1S,4R,9aS)-(-)-4-pentyloctahydro-2H-quinolizin-1-yl]methanol

BACKGROUND

Prior work from these laboratories has centred on the development of enaminones as versatile intermediates for the synthesis of alkaloids and other nitrogen-containing heterocycles. In this paper we describe the enantioselective synthesis of indolizidine and quinolizidine analogues of bicyclic amphibian alkaloids via pyrrolidinylidene- and piperidinylidene-containing enaminones.

RESULTS

Our previously reported synthesis of racemic 8-epi-indolizidine 209B has been extended to the laevorotatory enantiomer, (-)-9. Attempts to adapt the synthetic route in order to obtain quinolizidine analogues revealed that a key piperidinylidene-containing enaminone intermediate (+)-28 was less tractable than its pyrrolidinylidene counterpart, thereby necessitating modifications that included timing changes and additional protection-deprotection steps. A successful synthesis of [(1S,4R,9aS)-4-pentyloctahydro-2H-quinolizin-1-yl]methanol (-)-41 from the chiral amine tert-butyl (3R)-3-{benzyl [(1R)-1-phenylethyl]amino}octanoate (+)-14 was achieved in 14 steps and an overall yield of 20.4%.

CONCLUSION

The methodology reported in this article was successfully applied to the enantioselective synthesis of the title compounds. It paves the way for the total synthesis of a range of cis-5,8-disubstituted indolizidines and cis-1,4-disubstituted quinolizidines, as well as the naturally occurring trans-disubstituted alkaloids.

Synthesis of (-)-Indolizidine 167B based on domino hydroformylation/cyclization reactions

The synthesis of (-)-Indolizidine 167B has been achieved from optically active (R)-3-(pyrrol-1-yl)hex-1-ene. The key step is a highly regioselective hydroformylation reaction and a one-pot intramolecular cyclization providing a general approach to the indolizine nucleus.

Intramolecular Conjugate Displacement: A General Route to Hexahydroquinolizines, Hexahydroindolizines, and Related [m,n,0]-Bicyclic Structures with Nitrogen at a Bridgehead

N-Protected amino aldehydes can be converted into allylic alcohols by the classical Morita-Baylis-Hillman reaction (cf. 2 --> 3) or by condensation with selenium-stabilized carbanions, followed by oxidation (cf. 2 --> 8 --> 3). The derived acetates undergo cyclization when the nitrogen protecting group is removed, affording [m,n,0]-bicyclic structures with nitrogen at a bridgehead (cf. 4 --> 5 --> 6). Formation of bicyclic structures via the reactions of Schemes 1 and 2 is general, and the stereochemistry of the starting amino aldehyde is preserved.

General Strategy for the Construction of Enantiopure Pyrrolidine-Based Alkaloids. Total Synthesis of (−)-Monomorine

An enantiopure cis-2,5-disubstituted pyrrolidine building block was prepared from cocaine. The synthetic utility of this compound as a chiral building block was demonstrated by a short and efficient synthesis of the pyrrolidine-based alkaloid (-)-monomorine (six steps, 37% overall yield).

Predator learning favours mimicry of a less-toxic model in poison frogs

Batesian mimicry--resemblance of a toxic model by an edible mimic--depends on deceiving predators. Mimetic advantage is considered to be dependent on frequency because an increase in mimic abundance leads to breakdown of the warning signal. Where multiple toxic species are available, batesian polymorphism is predicted--that is, mimics diversify to match sympatric models. Despite the prevalence of batesian mimicry in nature, batesian polymorphism is relatively rare. Here we explore a poison-frog mimicry complex comprising two parapatric models and a geographically dimorphic mimic that shows monomorphism where models co-occur. Contrary to classical predictions, our toxicity assays, field observations and spectral reflectances show that mimics resemble the less-toxic and less-abundant model. We examine "stimulus generalization" as a mechanism for this non-intuitive result with learning experiments using naive avian predators and live poison frogs. We find that predators differ in avoidance generalization depending on toxicity of the model, conferring greater protection to mimics resembling the less-toxic model owing to overlap of generalized avoidance curves. Our work supports a mechanism of toxicity-dependent stimulus generalization, revealing an additional solution for batesian mimicry where multiple models coexist.

A General, Convergent Strategy for the Construction of Indolizidine Alkaloids: Total Syntheses of (−)-Indolizidine 223AB and Alkaloid (−)-205B

N-Toluenesulfonyl aziridines comprise effective second electrophiles in the solvent controlled three-component linchpin union of silyl dithianes for the stereocontrolled convergent elaboration of protected 1,5-amino alcohols. This tactic, in conjunction with a one-flask sequential cyclization, constitutes an effective general strategy for the construction of indolizidine and related alkaloids, illustrated here with the total syntheses of (-)-indolizidine 223AB (1) and alkaloid (-)-205B (2).

A revised structure for alkaloid 235C isolated from skin extracts of mantellid (Mantella) frogs of Madagascar

Madagascan frogs of the mantellid genus Mantella have been a rich source of alkaloids derived from dietary arthropods. Two species of frogs, inhabiting swamp forest, contain a unique set of alkaloids, previously proposed, based only on GC-MS and GC-FTIR data, to represent dehydro analogues of the homopumiliotoxins. The major alkaloid of this set, alkaloid 235C (2), now has been isolated in sufficient quantities (ca. 0.3 mg) to allow determination of the structure by NMR analysis. The structure of alkaloid 235C proved to be a 7,8-dehydro-8-desmethylpumiliotoxin. A comparison is presented between the mass, infrared, and (1)H NMR spectra of 235C (2) and a synthetic dehydrohomopumiliotoxin (1), initially proposed incorrectly as the structure for 235C.

Alkaloids from amphibian skin: a tabulation of over eight-hundred compounds

A diverse array of biologically active, lipid-soluble alkaloids have been discovered in amphibian skin. Such alkaloids include the following: the steroidal samandarines from salamanders, the batrachotoxins, histrionicotoxins, gephyrotoxins, and epibatidine from neotropical poison frogs (Dendrobatidae), the pumiliotoxins, allopumiliotoxins, homopumiliotoxins, and decahydroquinolines from certain genera of anurans from four families (Dendrobatidae, Mantellidae, Bufonidae, and Myobatrachidae), a variety of izidines (pyrrolizidines, indolizidines, quinolizidines, lehmizidines), pyrrolidines, piperidines, various tricyclics (related in structures to the coccinellines), and spiropyrrolizidines from the first three of these four families, the pseudophrynamines from one genus of Australian frogs, and a variety of unclassified alkaloids as yet of undetermined structure. With the exception of the samandarines and the pseudophrynamines, all alkaloids appear to be derived from dietary sources. Although only a few of the over 800 amphibian skin alkaloids have been detected in arthropods, putative arthropod sources for the batrachotoxins and coccinelline-like tricyclics (beetles), the pumiliotoxins (ants, mites), the decahydroquinolines, izidines, pyrrolidines, and piperidines (ants), and the spiropyrrolizidines (millipedes) have been discovered. Ants are likely sources for histrionicotoxins, lehmizidines, and tricyclic gephyrotoxins. Epibatidines represent an important alkaloid class without a putative dietary source. The structures for many of these alkaloids have been rigorously established, while the structures of others represent tentative proposals, based only on mass spectral and FTIR spectral data, along with analogies to structures of well-defined alkaloids.

Convergent evolution of chemical defense in poison frogs and arthropod prey between Madagascar and the Neotropics

With few exceptions, aposematically colored poison frogs sequester defensive alkaloids, unchanged, from dietary arthropods. In the Neotropics, myrmicine and formicine ants and the siphonotid millipede Rhinotus purpureus are dietary sources for alkaloids in dendrobatid poison frogs, yet the arthropod sources for Mantella poison frogs in Madagascar remained unknown. We report GC-MS analyses of extracts of arthropods and microsympatric Malagasy poison frogs (Mantella) collected from Ranomafana, Madagascar. Arthropod sources for 11 "poison frog" alkaloids were discovered, 7 of which were also detected in microsympatric Mantella. These arthropod sources include three endemic Malagasy ants, Tetramorium electrum, Anochetus grandidieri, and Paratrechina amblyops (subfamilies Myrmicinae, Ponerinae, and Formicinae, respectively), and the pantropical tramp millipede R. purpureus. Two of these ant species, A. grandidieri and T. electrum, were also found in Mantella stomachs, and ants represented the dominant prey type (67.3% of 609 identified stomach arthropods). To our knowledge, detection of 5,8-disubstituted (ds) indolizidine iso-217B in T. electrum represents the first izidine having a branch point in its carbon skeleton to be identified from ants, and detection of 3,5-ds pyrrolizidine 251O in A. grandidieri represents the first ponerine ant proposed as a dietary source of poison frog alkaloids. Endemic Malagasy ants with defensive alkaloids (with the exception of Paratrechina) are not closely related to any Neotropical species sharing similar chemical defenses. Our results suggest convergent evolution for the acquisition of defensive alkaloids in these dietary ants, which may have been the critical prerequisite for subsequent convergence in poison frogs between Madagascar and the Neotropics.