Coumarins and related compounds. Part IX. A new synthesis of benzocoumarins and 3-methylbenzocoumarins from naphthols and acrylic esters

Abstract

A direct synthesis of 5,6- and 7,8-benzodihydrocoumarins (1,2-dihydronaphtho[2,1-b]pyran-3-one and 3,4-di-hydronaphtho[1,2-b]pyran-2-one)(IIa and Va) and their corresponding 3-methyl derivatives (IIb and Vb), obtained by aluminium chloride-catalysed reaction of β- and α-naphthols with the appropriate acrylic ester derivative is reported. Dehydrogenation of the dihydrocoumarin derivatives with palladium–charcoal to give the corresponding coumarins (IIIa, IIIb, VIa, and VIb) is also described. The structure of 3,4-dihydro-7,8-benzo-coumarin-6-propionic acid (3,4-dihydro-2-oxonaphtho[1,2-b]pyran-6-propionic acid)(IV), obtained as a by-product, has been proved by n.m.r. and mass spectrometry.