THREE-COMPONENT SYNTHESIS OF 2-ACYL-2,3-DIHYDRO-4<i>H</i>-THIOCHROMENO[4,3-<i>b</i>]FURAN-4-ONES AND THEIR REDUCTIVE REARRANGEMENT INTO 4<i>H</i>,5<i>H</i>-THIOCHROMENO[4,3-<i>b</i>]PYRAN-5-ONES

Abstract

Three-component condensation of in situ generated pyridinium acyl methylides with aromatic aldehydes and 4-hydroxythiocoumarin led to a series of 2-acyl-2,3-dihydro-4H-thiochromeno[4,3-b]furan-4-ones. The reaction proceeds diastereoselectively with the formation of trans-isomers and represents a cascade process involving the Knoevenagel condensation, carbo-Michael reaction, and intramolecular nucleophilic substitution. The subsequent redox rearrangement of 2-acyl-2,3-dihydro-4H-thiochromeno[4,3-b]furan-4-ones by the action of Zn and ZrCl₄ grants access to 4H,5H-thiochromeno[4,3-b]pyran-5-ones.