Coumestans: Synthesis, isolation, and bioactivity

Coumestans are tetracyclic heterocycles having a coumarin ring and a benzofuran ring fused together, sharing a common C=C bond. A large number of oxygenated coumarins have been isolated from nature. In this review, we have compiled information to date on the natural occurrences of coumestans, their biological activity, and methods for their synthesis. We have further categorized the synthetic methods according to how the tetracycle is built, whether using the preformed coumarin ring, the furan ring, the flavanoid or by other diverse pathways..