The reactions of phenols with α,β-unsaturated aromatic acids in presence of polyphosphoric acid : synthetic and mechanistic studies

Abstract

The reactions of cinnamic acid with phenol itself, catechol, hydroquinone, pyrogallol and 2-naphthol in presence PPA were studied and that with resorcinol was reinvestigated. With phenol itself and hydroquinone were obtained 3,4-dihydro-4-phenylcoumarin (1r) and 6-hydroxy-3,4-dihydro-4-phenylcoumarin (Is), respectively, as the sole products. Catechol and pyrogallol, on the other hand, afforded, besides the corresponding 3,4-dihydrocoumarins, viz. 8-hydroxy-3,4-dihydro-4-phenylcoumarin (It) and 7,8-dihydroxy-3,4-dihydro-4-phenylcoumarin (lu), as the major products, 5,6-dihydroxy-3-phenylindanone (5b) and the chalcone 1-(2',3',4'-trihydroxy)-3-phenylpropan-2-en-1-one (3d), respectively, as minor produets. But contrary to earlier observation, resorcinol was found to give 7-hydroxy-3,4-dihydro-4-phenylcoumarin (Iv) instead of 7-hydroxyflavanone (4e). 2-Naphthol, on the other hand, afforded the chalcone derivative 1-[2'-hydroxynaphthyl]-3-phenylpropan-2-ene-1-one (7) as the exclusive product. The yields of the major products in the above reactions were 50-67% except that with phenol itself yielding Ir in 40% yield. The structures of all the products 1r, Is, It, lu, 1v, 3d, 5b and 7 were established from their various spectral (1R, 1 H and 1 3 C NMR and mass) data. Evidence for plausible mechanisms of formation of the above products was provided.