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Chemical Ecology: The Chemistry of Biotic Interaction
directly or indirectly from a dietary source; this is a subject which merits further study.
Perhaps the most interesting arthropodan defensive compounds from the point of view of structural diversity are the alkaloids. While alkaloids had long been believed to arise only as a consequence of plant secondary metabolism, it has become apparent over the last few decades that arthropods are both prolific and innovative alkaloid chemists. The millipede Polyzonium rosalbum, once thought to secrete camphor (20), in fact gives off a camphoraceous/earthy aroma produced by the spirocyclic isoprenoid imine polyzonimine (21).
The biosynthesis of this imine and its congener, nitropolyzonamine (22), would be another challenging area for future exploration.
We have recently characterized the heptacyclic alkaloid chilocorine from the ladybird (coccinellid) beetle Chilocorus cacti (23). In spite of its superficial complexity, this structure is easily dissected into two tricyclic moieties, A and B, each of which can be regarded as an acetogenin which has been elaborated from a straight chain of 13 carbon atoms stitched together at three points by a trivalent nitrogen atom.
It is intriguing to note that while the azaphenalene skeleton of part structure A is, in fact, commonly found among coccinellid alkaloids (24), the azaacenaphthylene skeleton of B is known, again combined with fragment A, only from the recently described hexacyclic alkaloid exochomine (25). Since pyrroles are not basic, we would expect tricyclic compounds resembling B to have been missed in a conventional alkaloid isolation scheme, and we anticipate that a targeted search for these novel pyrroles may well turn up additional examples of this otherwise unknown ring system.
Aside from yielding the most complex insect alkaloids so far characterized, coccinellid beetles are sources of a wide array of structural types.