Squaric acid ester-based total synthesis of echinochrome A

The total synthesis of echinochrome A is described. Both key intermediates 5 and 8 were efficiently prepared from diisopropyl squarate 7. Nucleophilic addition of aryllithium 8 to 5, followed by thermal ring-expansion/ cyclization of the 1,2-adduct 4, furnished hydroquinone 3. Oxidation and full deprotection of 3 gave the title compound.