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

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-pyrano3,2-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.