Influence of ultrasound on the Diels-Alder cyclization reaction: synthesis of some hydroquinone derivatives and lonapalene, an anti-psoriatic agent

Ultrasonic irradiation provides efficient promotion of the reaction between substituted buta-1,3-dienes with substituted 2,3-dimethoxycyclohexadiene-1,4-diones in a Diels-Alder cycloaddition which affords a variety of bicyclo[4.4.0] fused-ring systems in one step in high yields. Sonochemistry provides an increase in yield over a much shorter period than conventional methodology for reactions performed in benzene, toluene and methylene chloride. It provides a convenient route to naphthaquinols and lonapalene, a 5-lipooxygenase inhibitor in the treatment of psoriasis.