A new method for faster and simple preparation of 3,3',4,4'-tetrahydro-3,3'-disubstituted-4,4'-biscoumarins with participation of an organozinc reagent is reported. The reaction is promoted by ultrasound irradiation and it offers a simple experimental setup and excellent reproducibility of the results. Moreover, homodimers were isolated in yields of 45⁻92%. The dimerization conditions are applicable to coumarins with electron-withdrawing groups at third position.
Keywords: aci-nitro tautomer; biscoumarins; coumarins; homodimerization; organometallic reagents; radical coupling; ultrasound irradiation.