The chemistry of coumarin derivatives, part 2. Reaction of 4‐hydroxycoumarin with α,β‐unsaturated aldehydes

Abstract 4‐Hydroxycoumarin (= 4‐hydroxy‐2H‐1‐benzopyran‐2‐one) reacts with enals to give 1,2‐ or 1,4‐addition products, depending on the nature and relative location of the substituents on the olefinic double bond (Scheme 2). The resulting adducts further react intra‐ or intermolecularly, affording dimeric coumarins or pyranocoumarins in the case of 1,2‐addition and acetalic pyranocoumarins in the case of 1,4‐addition. With enals bearing alkyl groups at C(β), 2H‐pyrano[3,2‐c]coumarins are the only products formed, and the reaction represents an easy and straightforward entry into this class of recently described biologically active natural products.