AuBr₃-catalyzed [4 + 2] benzannulation between an enynal unit and enol

The reaction of enynals 1, including o-alkynylbenzaldehydes, and carbonyl compounds 2 in the presence of a catalytic amount of AuBr₃ in 1,4-dioxane at 100 °C gave the functionalized aromatic compounds 3 in high yields. The AuBr₃-catalyzed formal [4 + 2] benzannulation proceeds most probably through the coordination of the triple bond of 1 to AuBr₃, the formation of a 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 2, to the auric ate complex, and subsequent dehydration and bond rearrangement. Similarly, the AuBr₃-catalyzed reactions of 1 with acetal compounds afforded the corresponding aromatic compounds in good yields.