Chapter 6 Alkaloids from Ants and Other Insects

On the Biological Diversity of Ant Alkaloids

Ants have outstanding capacity to mediate inter- and intraspecific interactions by producing structurally diverse metabolites from numerous secretory glands. Since Murray Blum's pioneering studies dating from the 1950s, there has been a growing interest in arthropod toxins as natural products. Over a dozen different alkaloid classes have been reported from approximately 40 ant genera in five subfamilies, with peak diversity within the Myrmicinae tribe Solenopsidini. Most ant alkaloids function as venom, but some derive from other glands with alternative functions. They are used in defense (e.g., alarm, repellants) or offense (e.g., toxins) but also serve as antimicrobials and pheromones. We provide an overview of ant alkaloid diversity and function with an evolutionary perspective. We conclude that more directed integrative research is needed. We suggest that comparative phylogenetics will illuminate compound diversification, while molecular approaches will elucidate genetic origins. Biological context, informed by natural history, remains critical not only for research about focal species, but also to guide applied research.

Venom Function of a New Species of Megalomyrmex Forel, 1885 (Hymenoptera: Formicidae)

Alkaloids are important metabolites found across a variety of organisms with diverse ecological functions. Of particular interest are alkaloids found in ants, organisms well known for dominating the ecosystems they dwell in. Within ants, alkaloids are found in venom and function as potent weapons against heterospecific species. However, research is often limited to pest species or species with parasitic lifestyles and thus fails to address the broader ecological function of ant venom alkaloids. Here we describe a new species of free-living Megalomyrmex ant: Megalomyrmex peetersi sp. n. In addition, we identify its singular venom alkaloid (trans-2-butyl-5-heptylpyrrolidine) and elucidate the antibiotic and insecticidal functions of its venom. Our results show that Megalomyrmex peetersi sp. n. venom is an effective antibiotic and insecticide. These results are comparable to venom alkaloids found in other ant species, such as Solenopsis invicta. This research provides great insight into venom alkaloid function, and it is the first study to explore these ideas in the Megalomyrmex system.

Naturally Occurring Lumazines

Natural products containing a lumazine motif were first isolated from natural sources in 1940. These natural products are relatively rare, with fewer than 100 lumazines known to occur in Nature. This review discusses the isolation of lumazines, their biological activity, and their biosynthesis, where known.

Caste-specific tyramides from Myrmicine ants

Analysis of the extracts of male ants of Monomorium minimum and Monomorium ebeninum by GC-MS and GC-FTIR revealed the presence of tyramides 2 and 4c, for which the structures were established by comparison with synthetic samples. These compounds and their analogues 1 and 3 were also found in males of other Monomorium species, males of Myrmicaria opaciventris, and males of several Solenopsis (Diplorhoptrum) species. Vapor-phase FTIR spectra revealed critically important structural clues to two of the tyramides, which had methyl branching in the tyramide acyl moiety. Tyramide 4c exhibited a strong intramolecular amide NH hydrogen bond where an alpha-keto group was deduced to be present in the acyl moiety and also showed the overlap of this ketone group frequency with that of the amide nu(C horizontal lineO). The biological function of these compounds is uncertain; however, their role in ant-mating behavior may be suggested by a large body of evidence.

Asymmetric Synthesis of 6-Alkyl- and 6-Arylpiperidin-2-ones. Enantioselective Synthesis of (S)-(+)-Coniine

A variety of diolefinic hydrazides (1) have been assembled in a highly diastereoselective manner by addition of allyllithium to chiral SAMP hydrazones followed by N-acylation with acryloyl chloride. Substrates 1 undergo ring-closing metathesis to give the cyclic enehydrazides (5) which can be easily converted into virtually enantiopure 6-alkyl- or 6-arylpiperidin-2-ones (7). The versatility of this hydrazone addition-RCM protocol has been further exemplified by the conversion of the unsaturated heterocycle 5b into the piperidine alkaloid (S)-(+)-coniine.

Venom chemistry of the ant Myrmicaria melanogaster from Brunei

Analysis of the extracts of the ant Myrmicaria melanogaster from Brunei in the Indonesian archipelago by GC-MS and GC-IR revealed the presence of five new alkaloids, identified as (9Z)-3-propylindolizidine (1), cis- and trans-2-butyl-5-propylpyrrolidine (2 and 3, respectively), (10E)-3-butyllehmizidine (7), and (5Z,8Z,9Z)-3-butyl-5-propyl-8-hydroxyindolizidine (10a), whose structures were established by comparison with synthetic samples. In addition the monoterpene hydrocarbons beta-pinene, myrcene, and limonene were detected along with all four isomers of 3-butyl-5-methylindolizidine (4a-d), cis- and trans-2-butyl-5-(4-pentenyl)pyrrolidine (5a and 5b), trans-2-butyl-5-pentylpyrrolidine (6), (5Z,9Z)-3-butyl-5-propylindolizidine (8), and (5Z,9E)-3-butyl-5-propylindolizidine (9), alkaloids well known from ants and frogs, whose structures were established on the basis of published spectra or comparison with authentic samples. This study utilized vapor-phase infrared analysis for the assignment of stereochemistry using Bohlmann bands for the bicyclic alkaloids and, in the case of 10a, the detection of an intramolecular hydrogen bond. A biogenetic relationship between the mono- and bicyclic ring systems is proposed.

Asymmetric Halo-Mannich-Type Reaction Provides Access to Pyrrolidines and β-Proline Derivatives

[reaction: see text] A new halo-Mannich-type reaction is reported using cyclopropyl carbonyl-derived enolates and sulfonyl-protected imines. Chiral oxazolidinones auxiliaries were found to be effective for completely controlling the stereochemistry of the products. Variations in the oxazolidinone, protecting group, and imine components show this to be a quite general reaction. The initial iodo-Mannich products were found to be readily cyclized in the presence of triethylamine to afford the resulting protected pyrrolidines, which could be readily deprotected under standard conditions.

Highly Selective Synthesis of Bicyclic Quinolizidine Alkaloids and Their Analogues via Double RCM Reaction of N-Alkynyl-N-(1,ω)-alkadienyl Acrylamides

The double ring-closing metathesis reaction of N-alkynyl-N-(1,omega)-alkadienyl acrylamides 1 using first- or second-generation Grubbs' catalyst afforded, in a highly selectively manner, the fused bicyclic quinolizidine alkaloid derivatives and their analogues bearing a 1,3-diene moiety, which may further undergo a Diels-Alder reaction with a dienophile to afford N-containing polycyclic compounds. The excellent selectivity of fused/dumbbell-mode cyclization has been realized by the higher reactivity of the electron-rich C=C bond or carbon-carbon triple bond combined with the lower reactivity of the electron-deficient C=C bond toward metallocarbenes and the thermodynamically more stable nature of fused bicyclic compounds 3 vs dumbbell-type bicyclic compounds 4.

Enantioselective synthesis of piperidine, indolizidine, and quinolizidine alkaloids from a phenylglycinol-derived delta-lactam

Starting from a common lactam, (3R,8aS)-5-oxo-3-phenyl-2,3,6,7,8,8a-hexahydro-5H-oxazolo[3,2-a]pyridine (1), or its enantiomer, the enantioselective synthesis of 2-alkylpiperidines and cis- and trans-2,6-dialkylpiperidines is reported. The potential of this approach is illustrated by the synthesis of the piperidine alkaloids (R)-coniine, (2R,6S)-dihydropinidine, (2R,6R)-lupetidine, and (2R,6R)-solenopsin A, the indolizidine alkaloids (5R,8aR)-indolizidine 167B and (3R,5S,8aS)-monomorine I, and the nonnatural base (4R,9aS)-4-methylquinolizidine.

Synthesis of Aryl-Substituted Piperidines by Superacid Activation of Piperidones

Diarylpiperidines (8-12) may be prepared in good to excellent yields (80-99%) by the reaction of piperidones (3d-h) with benzene and the Bronsted superacid, trifluoromethanesulfonic acid (CF(3)SO(3)H, TfOH). Tropinone (6) and quinuclidone (7) also react in good yields with benzene in TfOH to give the condensation products (13 and 14). Ketal and acetal derivatives also give condensation products (8 and 24) upon reaction with C(6)H(6) in TfOH. The conversion of 3g to 11 is sensitive to both acid quantity and acid strength; a mechanism is proposed for the conversion that invokes dicationic intermediates.