Lewis acid catalyzed benzannulation

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The reaction of o-alkynyl(oxo)benzenes 1 with alkynes 2 in the presence of a catalytic amount of AuCl_3 in (CH_2Cl)_2 at 80℃ gave the [4+2] benzannulation products, naphthyl ketone derivatives 3 and 4, in high yields. When the reaction was carried out using AuBr_3 instead of AuCl_3, the reaction speed was enhanced and the chemical yield was increased. On the other hand, when the reaction was carried out in the presence of a catalytic amount of Cu(OTf)_2 and 1 equivalent of a Bronsted acid, such as CF_2HCO_2H, in (CH_2Cl)_2 at 100℃, the decarbonylated naphthalene products 5 were obtained in high yields. Similarly, the Cu(OTf)_2-H_2O promoted reaction of the enynals 6 with an alkyne 2 afforded the corresponding [4+2] benzannulation products, decarbonylated benzene derivatives 7, in good, yields. Both AuX_<3^-> and Cu(OTf)_2-catalyzed benzannulations proceed most probably through the formation of the benzo[c]pyrylium ate complex 8, the Diels-Alder addition of alkynes 2 to the ate complex, and the resulting bicyclic pyrylium ion intermediate 9. Interestingly, the reaction of enynals 6, including o-alkynylbenzaldehydes 1, and carbonyl compounds 10 in the presence of a catalytic amount of AuBr_3 in 1,4-dioxane at 100℃ gave the functionalized aromatic compounds 3 in high yields. The AuBr_3-catalyzed formal [4+2] benzannulation proceeds most probably through the coordination of the triple bond of 1 to AuBr_3, the formation of pyrylium auric ate complex via the nucleophilic addition of the carbonyl oxygen atom, the reverse electron demand type Diels-Alder addition of the enols, derived from 10, to the auric ate complex, and subsequent dehydration and bond rearrangement. Similarly, the AuBr_3-catalyzed reactions of 1 with acetal compounds afforded the corresponding aromatic compounds in good yields.