There are elements in the periodic table that, despite their abundance on earth, appear extremely infrequently in natural products. Fluorine is a good example: there are only a handful of natural products that contain monofluoroalkyl substituents and there are none with CF2 and CF3 groups. The rarity of the latter two tends to surprise students when they hear it for the first time. I suppose this has to do with the omnipresence of CF2 and CF3 substituents in therapeutic agents, which is why some people assume that the inspiration for their emergence must have come from some natural product. There is a tendency to forget that if we think about how C-F bonds are biosynthetically introduced into organic compounds, it is almost impossible to place more than one fluorine atom on the same carbon.
Boron is another enigma. There is a lot of it on our planet, yet we rarely see boron in natural products. A marine natural product borophycin is one of those rarities. Earlier today we had Professor Jon Clardy of Harvard Medical School give one of University of Toronto’s annual Gordon lectures (this is a set of three lectures given by one person and the topics vary from talk to talk). While the theme of today’s lecture was not related to boron, I caught myself reminiscing on some of the work Jon had done 20 years ago. He is responsible for a spectacular collection of isolated and characterized natural products, and one of them is borophycin (see above). I am showing the structure of this intriguing compound and a link to Jon’s 1994 paper together with Moore and their colleagues. Isolated from green-blue algae, borophycin is a potent cytotoxin which was characterized by both NMR and X-ray in the 1994 JOC report. I don’t think there is unequivocal evidence supporting or refuting the essential nature of boron to the biological activity of this molecule, but it is a really nice example of a rare boron-containing compound found in nature. By the way, this molecule might also provide a clue as to why boron is NOT (contrary to some popular belief) benign from the standpoint of toxicity. Boron-containing waste is a big problem and it is known that it causes blue algae to grow. Perhaps there is a link there with borophycin, but I am not sure.