An Efficient Process for the Synthesis of trans-2,3-Disubstituted-2,3-dihydro-4H-1-benzopyran-4-ones (Chroman-4-ones)

The piperidine catalyzed Knoevenagel condensation of 2′-aryl/alkyl-2-hydroxyacetophenones 7 and aryl/alkylaldehydes 8 in refluxing isopropyl alcohol with azeotropic removal of water affords, in high yield, equilibrium mixtures of E- and Z-chalcones 9 and cis- and trans-chroman-4-ones 10 which are effectively isomerized in situ to the trans-2,3-diaryl/alkylchroman-4-ones, following the addition of 1,8-diazabicyclo[5,4,0]undec-7-ene (DBU) to the cooled reaction mixture and may be isolated in high purities (91–99%) by simple filtration, with good to excellent yields (70–95%).