Mass Spectrometry of Aliphatic Macrolides, Important Semiochemicals or Pheromones.

Macrolides are a relatively common structural motif prevalent in Nature. However, the structures of these large ring lactones have been relatively difficult to elucidate via NMR spectroscopy due to the minute amounts of compounds that are sometimes obtainable from natural sources. Thus, GC-MS analysis of individual macrolactones has become the method of choice for the structural identification of these compounds. Here we discuss the mass spectrometric behavior of aliphatic macrolides, evaluating spectra from numerous compounds of various ring size, including derivatives containing methyl branches as well as double bonds. The specific fragmentation of these macrolactones under electron impact conditions allows for the development of a general rule set aimed at the identification of similar compounds by mass spectrometry. In addition, the mass spectra of dimethyl disulfide adducts of unsaturated macrolides are discussed. The mass spectra of almost 50 macrolides are presented.

A synthetic dodecanolide library for the identification of putative semiochemicals emitted by mantellid frogs.

Aliphatic macrolides are used by various organisms as semiochemicals or pheromones, e.g. by the endemic Madagascan frog family Mantellinae. Volatile, methyl-branched aliphatic macrolides occur in various species, but their identification by MS is difficult, while isolation is not possible due to the low amounts produced. Therefore, we synthesized a library of methyl-branched 11-dodecanolides to identify unknown macrolides secreted by Gephyromantis moseri, a mantelline frog restricted to the rainforest and characterized by largely terrestrial habitats. The syntheses used metal-organic epoxide opening and ring-closing-metathesis as key steps. All library members 2-, 4-, 6-, 8-, and 10-methyl-11-dodecanolide were thus identified as part of the secretion by comparison of their mass spectra and their gas chromatographic retention indices. These compounds comprise new natural products.

Identification, synthesis and mass spectrometry of a macrolide from the African reed frog Hyperolius cinnamomeoventris.

The contents of the gular glands of the male African reed frog Hyperolius cinnamomeoventris consist of a mixture of aliphatic macrolides and sesquiterpenes. While the known macrolide gephyromantolide A was readily identified, the structure of another major component was suggested to be a tetradecen-13-olide. The synthesis of the two candidate compounds (Z)-5- and (Z)-9-tetradecen-13-olide revealed the former to be the naturally occurring compound. The synthesis used ring-closing metathesis as key step. While the Hoveyda-Grubbs catalyst furnished a broad range of isomeric products, the (Z)-selective Grubbs catalyst lead to pure (Z)-products. Analysis by chiral GC revealed the natural frog compound to be (5Z,13S)-5-tetradecen-13-olide (1). This compound is also present in the secretion of other hyperoliid frogs as well as in femoral glands of male mantellid frogs such as Spinomantis aglavei. The mass spectra of the synthesized macrolides as well as their rearranged isomers obtained during ring-closing metathesis showed that it is possible to assign the location of the double bond in an unsaturated macrolide on the basis of its EI mass spectrum. The occurrence of characteristic ions can be explained by the fragmentation pathway proposed in the article. In contrast, the localization of a double bond in many aliphatic open-chain compounds like alkenes, alcohols or acetates, important structural classes of pheromones, is usually not possible from an EI mass spectrum. In the article, we present the synthesis and for the first time elucidate the structure of macrolides from the frog family Hyperoliidae.

Take time to smell the frogs: vocal sac glands of reed frogs (Anura: Hyperoliidae) contain species-specific chemical cocktails.

Males of all reed frog species (Anura: Hyperoliidae) have a prominent, often colourful, gular patch on their vocal sac, which is particularly conspicuous once the vocal sac is inflated. Although the presence, shape, and form of the gular patch are well-known diagnostic characters for these frogs, its function remains unknown. By integrating biochemical and histological methods, we found strong evidence that the gular patch is a gland producing volatile compounds, which might be emitted while calling. Volatile compounds were confirmed by gas chromatography-mass spectrometry in the gular glands in 11 species of the hyperoliid genera Afrixalus, Heterixalus, Hyperolius, and Phlyctimantis. Comparing the gular gland contents of 17 specimens of four sympatric Hyperolius species yielded a large variety of 65 compounds in species-specific combinations. We suggest that reed frogs might use a complex combination of at least acoustic and chemical signals in species recognition and mate choice.

Macrolides and alcohols as scent gland constituents of the Madagascan frog Mantidactylus femoralis and their intraspecific diversity.

Acoustic and, to a lesser degree, visual signals are the predominant means of signaling in frogs. Nevertheless, certain lineages such as the mantelline frogs from Madagascar use the chemical communication channel as well. Males possess femoral glands on the hind legs, which recently have been shown to contain volatile compounds used in communication as pheromones. Many mantelline species occur in sympatry, and so far species recognition is regarded to occur mainly by acoustic signals. The analysis of the gland constituents of Mantidactylus femoralis by GC/MS revealed the presence of volatile macrolides and secondary alcohols. The new natural products mantidactolides A (4) and B (6), as well as several methyl carbinols, were identified, and their structures were confirmed by synthesis. The analysis of individuals from different locations of Madagascar revealed the presence of two groups characterized by specific patterns of compounds. While one group contained the alcohols and mantidactolide B, the other showed specific presence of the macrolides phoracantholide I (1) and mantidactolide A (4). Genetic analysis of some individuals showed no congruence between genetic relatedness and gland constituents. Several other individuals from related species had different gland compositions. This suggests that a basic set of biosynthetic machinery might be available to a broader group of related species.