Synthesis and bioactivity studies of harmonine : the defense alkaloid of the asian lady beetle Harmonia axyridis
The Asian lady beetle Harmonia axyridis shows voracious feeding behavior which qualified the beetle as biological control agent. Populations established and H. axyridis became an invasive species. The invasive success derives from various factors among others from its resistance to pathogens which most likely deduces from harmonine. This alkaloid is present in the hemolymph and displays a variety of biological properties, e.g., activity against 12 bacterial strains. To study the hemolymph, a beetle rearing was established. The samples were derivatized and analyzed via GC-MS. Harmonine was the major component. One minor component present in only 4 ‰ showed a similar fragmentation pattern. To elucidate the structure, the AcN-CI-MS/MS technique was used. A Δ9,12-dehydro-harmonine structure is assumed. A synthesis was developed to obtain natural (R) harmonine, the racemic form and non-natural (S)-enantiomer based on a “Wittig-type one-pot” reaction. The reaction yielded the carbon skeleton of harmonine with high Z-selectivity (≥98/2). Through FGI harmonine was available. To obtain both enantiomers, horse liver esterase was used for saponification. This yielded optically pure precursors which were used to synthesize (R)- and (S)-harmonine. The protocol was altered to obtain an OH-analog and a C17 harmonine. Microbial growth inhibition assays confirmed an antibacterial activity of both enantiomers and C17-harmonine. The OH-analog was less active. The AChE inhibitory activity was evaluated. Both enantiomers showed the same, moderate activity. C17-harmonine was less active. The compounds were tested against L. major. The antileishmanial activity of (R)-harmonine was demonstrated. The IC50 value was lower than the one for the drug in use. Both harmonine enantiomers showed the same activity. The possibility of a “leishmanial AChE” was examined. Candidates were modeled. Docking studies confirmed that harmonine binds close to the active center of the “leishmanial AChE”.
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