Three-component synthesis of 2-acyl-2,3-dihydro-4H-thiochromeno[4,3-b]furan-4-ones and their reductive rearrangement into 4H,5H-thiochromeno[4,3-b]pyran-5-ones

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.