New Syntheses of Coumarins

Article abstract of DOI:10.1021/jo00345a009

New, versatile coumarin syntheses have been developed which are based on the Claisen rearrangement of allylic or propargylic aryl ethers in which the allylic or propargylic α-carbon is further oxygenated.One proceeds by first ester exchange to the aryl dialkyl orthoacrylate which then thermally rearranges.Hydrolytic treatment gives the o-hydroxydihydrocinnamic acid from which the coumarin is obtained by ring closure and dehydrogenation.In parallel fashion, an orthopropiolate may be used, eliminating the dehydrogenation step.These processes have the adventage ofavoiding the drastic acid conditions and orientation limitations of previous coumarin syntheses.They have been applied to syntheses in good yields of coumarins obtainable by previous methods in very poor yield or not at all.