Expedient Single Step Access to Strained Tricyclic Ketals

The synthesis of cyclopropyl methyl ketones and highly strained fused substituted dihydrobenzopyran cyclopropyl lactones derived from coumarin carboxylates and chloroacetone in the presence of potassium carbonate is reported. One step synthetic access to the previously unknown dihydrobenzopyran cyclopropyl tricyclic ketals is achieved with wide substrate scope. Substituted coumarin carboxylates, phenylamides or a quinolin-2(1H)-one possessing alkyl electron donating (methyl, t-butyl) and electron withdrawing groups (F, Cl, Br, NO2) in the 4 or 6 positions formed the highly strained dihydrobenzopyran cyclopropyl tricyclic ketals in moderate yield alongside the expected coumarin carboxylate. Saturation or substitution at the 5-position or 6-OMe afforded no tricyclic ketal compound but solely coumarin carboxylates. The formation of both structures in selected derivatives was confirmed by X-ray crystallography. A plausible mechanism is proposed for the formation of the fused lactone; via intramolecular rearrangement of cis cyclopropyl methyl ketones with phenolic acetate via the formation of a hemiacetal.