Synthesis of (z)-10,10-difluoro-13-hexadecen-11-ynyl acetate, new difluoro analogue of the sex pheromone of the processionary moth

The Influence of Fluorinated Molecules (Semiochemicals and Enzyme Substrate Analogues) on the Insect Communication System

Can the introduction of fluorine atoms affect the bioactivity of natural semiochemicals? Can fluorine contribute in the creation of specific enzyme inhibitors to interrupt or disrupt the insect communication system? The first step for the bioactivity of a molecule is interaction with the biological sensor. Hydrogen and fluorine are almost bioisosteric and the receptor site of the enzyme can still recognize and accept the fluoro analogue of its natural substrate. However, the peculiar electronegativity of the fluorine atom can affect the binding, absorption, and transport of the molecule. The differences in the molecule's electronic properties can lead to differences in the chemical interactions between the receptor and the fluorinated substrate. Fluorine introduction can modify the metabolic stability and pathway of the semiochemicals in many different ways. Fluorinated analogues can show synergism, inhibition, or hyperagonism effects on insect behaviors, that is, the activity of the nonfluorinated parent compounds can be mimicked, lost, or increased. In any case, the fluorinated molecules can interact with the bioreceptors in a new and disrupting way. The semiochemicals are olfactory substances: fluorine can affect their volatility or smell. Production of semiochemicals from exogenous substances, perception at antennal receptors, and processing of biological responses are the main steps of communication among insects. In the production step, the fluorinated molecules can interact with enzymes that catalyze the biosynthesis of the natural pheromones. In the perception step, fluorinated semiochemicals can interact with the olfactory receptor cells; this often leads to totally unpredictable behaviors. Fluorinated molecules have been developed as probes to elucidate the complex chemorecognition processes of insects. Many of these molecules have been tested to find highly effective behavior-modifying chemicals. New analogues have been synthesized to investigate the metabolic pathway of a pheromone molecule and many of them are promising disrupting agents. Despite such titanic research efforts, the results have often been random, rational trends in the induced behaviors have sometimes been impossible to find, and practical applications of the fluorinated semiochemicals are still uncertain.

Synthesis of fluorinated analogs of myristic acid as potential inhibitors of egyptian armyworm (Spodoptera littorialis) Δ11 desaturasedesaturase

To study the activity of the different desaturases present in the pheromone biosynthetic pathway of the Egyptian armyworm, Spodoptera littoralis, we prepared a series of mono- and gem-difluorinated analogs of myristic acid with halogen substitution at the C8-C11 positions of the aliphatic chain via specifically positioned dithiane precursors. Thus, transformation of dithianes by treatment with N-bromosuccinimide in the presence of H2O followed by reduction with LiAlH4 afforded the appropriate alcohols, which reacted with diethylaminosulfur trifluoride to give rise to the corresponding monofluoroderivative intermediates. Alternatively, the introduction of the gem-difluoro functionality was carried out by reaction of the appropriate dithiane intermediate with 1,3-dibromo-5,5-dimethylhydantoin in the presence of HF/pyridine. The activity of these fluorinated FA as substrates and inhibitors of the desaturases involved in the biosynthesis of the sex pheromonal blend of S. littoralis has been studied. In this case, 11-fluorotetradecanoic acid elicited a moderate inhibitory activity of delta11 desaturase.

Stereoselective synthesis of trifluoro- and monofluoro-analogues of frontalin and evaluation of their biological activity

The stereoselective synthesis of both enantiomers of trifluoro frontalin (-)-(1S,5R)- and (+)-(1R,5S)-8, as well as of diastereomeric monofluoro frontalines (-)-(1R,2R,5R)-18 and (-)-(1R,2S,5R)-20, analogues of the bioactive component of the aggregation pheromone of the Scolytidae insect family, has been accomplished starting from (-)-(1R)- and (+)-(1S)-menthyl (S)-toluene-4-sulfinate as a source of chirality and methyl trifluoroacetate or fluoroacetate, respectively, as sources of fluorine. The C-1 stereocenters were installed via stereoselective epoxidation of beta-sulfinyl ketones 2 and 13 with diazomethane. The bicyclic core was obtained by totally stereocontrolled and chemoselective tandem Wacker oxidation/intramolecular ketalization of the intermediate unsatured sulfinyl diols 5, 15, and 19. Axially fluorinated (-)-20 elicited a strong electroantennographic response in laboratory tests on females of Dendroctonus micans, whereas equatorially fluorinated (-)-18 and the trifluoroanalogue (-)-8 showed modest responses. Field trials using (-)-20 were not indicative owing to the locally scarce population of D. micans, but it showed some attractiveness for other Coleoptera families.

Insect parapheromones in olfaction research and semiochemical-based pest control strategies

The possibility of disrupting the chemical communication of insect pests has initiated the development of new semiochemicals, parapheromones, which are anthropogenic compounds structurally related to natural pheromone components. Modification at the chain and/or at the polar group, isosteric replacements, halogenation or introduction of labeled atoms have been the most common modifications of the pheromone structure. Parapheromones have shown a large variety of effects, and accordingly have been called agonists, pheromone mimics, synergists and hyperagonists, or else pheromone antagonists, antipheromones and inhibitors. Pheromone analogues have been used in quantitative structure-activity relationship studies of insect olfaction, and from a practical point of view they can replace pheromones when these are costly to prepare or unstable under field conditions.

Ligand specificity of pheromone-binding proteins of the processionary moth

Photoaffinity labeling of proteins extracted from sensory hairs and antennal branches of the processionary moth Thaumetopoea pityocampa with a tritium-labeled diazoacetate analogue of the sex pheromone (Z)-13-hexadecen-11-ynyl acetate revealed a 15-kDa pheromone-binding protein in male moth sensory hairs (SH-15). A different 15-kDa protein in male antennal branches (B-15) was not photolabeled. All extracts except male sensory hairs showed a photolabeled 20-kDa protein; a photolabeled male 30-kDa protein in the branches (B-30) was also observed. The 20-kDa proteins in the sensory hairs (SH-20) and branches (B-20) showed differing affinities for the photoaffinity analogues; moreover, SH-15 exhibits higher affinity for the natural pheromone, (Z)-13-hexadecen-11-ynyl acetate, than for its alcohol metabolite and other analogues in competitive displacement experiments. The affinity shown by the pheromone-binding protein for the metabolic product suggests that the alcohol may be also transported by the binding protein. Interestingly, a shift in labeling from SH-15 to SH-20 was produced in the presence of an excess of the natural pheromone, its alcohol and other analogues. The binding showed little discrimination among structurally similar analogues of the pheromone, while saturated and aromatic molecules showed little affinity for the proteins of either sensory hairs or antennal branches.