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?

Organic & Biomolecular Chemistry Pub Date: 2011-03-09 DOI: 10.1039/C0OB01143F

Abstract

The treatment of β,γ-unsaturated α-ketoesters with phenols in the presence of trityl chloride and 4 ? molecular sieves in refluxing trifluoroacetic acid afforded 4-aryl-2H-chromenes in high yields, in which a reverse of the regiochemistry of J?rgensen–Rutjes chromane synthesis was observed. The isolation of 4H-chromene intermediates, confirmed by single-crystal X-ray analysis, indicates that the early stage of the reaction involves a Friedel–Crafts alkylation/cyclodehydration processes. Stirring of the 4H-chromene intermediate with trityl chloride in deuterotrifluoroacetic acid under reflux afforded the 2H-chromene and triphenylmethane in high yields, which implies the late stage of the reaction involves a hydrogen transfer process. Highly selective derivation of the hydroxyl esters to the corresponding hydroxyl amides, amino acids, amino esters and Friedel–Crafts adducts was further accomplished. Our endeavors will lead to a better understanding of the controlling elements behind their structural motifs. The products were confirmed unambiguously from their spectra and by single-crystal X-ray analysis.

Graphical abstract: 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
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