What is "carpanone"

Carpanone is a naturally occurring lignan-type natural product most widely known for the remarkably complex way nature prepares it, and the similarly remarkable success that an early chemistry group, that of Orville L. Chapman, had at mimicking nature's pathway. Carpanone is an organic compound first isolated from the carpano trees (Cinnamomum sp.) of Bougainville Island by Brophy and coworkers, trees from which the natural product derives its name. The hexacyclic lignan is one of a class of related diastereomers isolated from carpano bark as mixtures of equal proportion of the "handedness" of its components (i.e., racemic mixtures), and is notable in its stereochemical complexity, because it contains five contiguous stereogenic centers. The route by which this complex structure is achieved through biosynthesis involves a series of reactions that, almost instantly, take a molecule with little three-dimensionality to the complex final structure. Notably, Brophy and coworkers isolated the simpler carpacin, a phenylpropanoid with a 9-carbon framework, recognized its substructure as being dimerized within the complex carpanone structure, and proposed an hypothesis of how carpacin was converted to carpanone in plant cells:

• carpacin underwent loss of a methyl (-CH) group from the ring methoxy (-OCH) group to provide the phenol, desmethylcarpacin,
• this phenol intermediate then underwent a phenolic coupling to generate a dimeric intermediate, which was
• followed immediately by a Diels-Alder (4+2) cycloaddition reaction to create 2 new rings, to give the final carpanone product.

Remarkably, within two years, Chapman and coworkers were able to chemically design a route to mimic this proposed biosynthetic route, and achieved the synthesis of carpanone from carpacin in a single "pot", in about 50% yield.

Carpanone itself is limited in its pharmacologic and biologic activities, but related analogs arrived at by variations of the Brophy-Chapman approach have shown activities as tool compounds relevant to mammalian exocytosis and vesicular traffic, and provided therapeutic "hits" in antiinfective, antihypertensive, and hepatoprotective areas.

The original Chapman design and synthesis is considered a classic in total synthesis, and one that highlights the power of biomimetic synthesis.