Access to Highly Strained Tricyclic Ketals Derived from Coumarins

Synthesis of highly strained fused substituted dihydrobenzopyran cyclopropyl lactones derived from coumarin carboxylates are reported. The substrate scope tolerates a variety of 6- and 8-substituents on the coumarin ring. Substitution at the 5- or 7-position is resistant to tricyclic lactone formation except with 7-methyl substitution. Benzamide-containing coumarins afford the tricyclic ketal. A plausible mechanism is proposed for the formation of the fused lactone: intramolecular rearrangement of trans cyclopropyl methyl ketones with phenolic acetate via the formation of a hemiacetal.