The scope of the Heck arylation of enol ethers with arenediazonium salts: a new approach to the synthesis of flavonoids

The scope of the Heck arylation of cyclic and acyclic enol ethers with arenediazonium salts was evaluated. Arylation of 2,3-dihydrofuran yielded 2-aryl-2,5-dihydrofurans as the major adducts (>99:1) except when using n-Bu4NHSO4 as additive or 4-NO2PhN2BF4 as arenediazonium salt. 2,3-Dihydropyran provided mixtures of the three possible isomeric Heck adducts. Arylation of n-butylvinylether with arenediazonium bearing electron-donating groups resulted in substituted acetophenones as almost exclusive products in good overall yields. Substituted 4H-chromenes provided 2-aryl-2H-chromenes in moderate yield when applying the Pd(OAc)2/2,6-di-t-butyl-4-methylpyridine catalytic system, which were applied in the synthesis of flavonoids.