Cascade reaction of β,γ-unsaturated α-ketoesters with phenols in trityl chloride/TFA system. Highly selective synthesis of 4-aryl-2H-chromenes and their applications
The treatment of β,γ-unsaturatedα-ketoesters with phenols in the presence of trityl chloride and 4 Å molecular sieves in refluxing trifluoroacetic acid afforded 4-aryl-2H-chromenes in high yields, in which a reverse of the regiochemistry of Jørgensen–Rutjes chromane synthesis was observed. The isolation of 4H-chromene intermediates, confirmed by single-crystal X-ray analysis, indicates that the early