Efficient synthesis of pyrano[2,3-c]coumarins via intramolecular domino Knoevenagel hetero-Diels–Alder reactions

The domino Knoevenagel hetero-Diels–Alder reaction of the O-propargylated salicylaldehydes and 4-hydroxycoumarin leads to pyrano[2,3-c]coumarins 3 and pyrano[2,3-c]chromones 4 in high yield in the presence of CuI as a Lewis acid. In all cases, the reaction was shown to exhibit high regioselectivity and form product 3 as main product.