B.R.S.M. Yield isn't everything


Unnatural Products 3: Pentaprismane

For the last planned post in my Unnatural Products series, I’m going to write about Eaton’s 1981 synthesis of pentaprismane.[1] At the time, unnatural hydrocarbons were hot targets, and as the next largest prismane on the list this target was the subject of much research by groups around the world. Perhaps Eaton's biggest rivals were the groups of Paquette and Petit, and in fact all three had, at various times, synthesised hypostrophene as an intended precursor to the target.

Unfortunately, the ‘obvious’ [2 + 2] disconnection from pentaprismane turned out to be a dead end and the photochemical ring closure was unsuccessful. The 1970s and early 1980s saw the publication of a number of other similarly creative, but sadly ill-fated, approaches based on various ring contractions, and the compound gained a well-earned reputation for extraordinary synthetic inaccessibility.

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Eaton’s other reagents

On Friday, I found myself discussing the use of Eaton's reagent with a coworker. Many people know it as a handy alternative to polyphosphoric acid (PPA) for acylations, Friedel-Crafts reactions and the like. It's even endorsed by Milkshake. A simple 7.7 wt% solution of phosphorus pentoxide in methanesulfonic acid, it's not as viscid, viscous or  unpleasant to work up as PPA.[1] And it's commercially available. But neither my coworker, nor many other people I know, are aware of Eaton's other contributions to the synthetic world. He's still an emeritus professor at the University of Chicago, and you can read about his interests on his website, but what he's most famous for, unbeknownst to many younger chemists, is his synthesis of the cubanes.

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