Expedient Synthesis of Substituted Benzoheterocycles using 2-Butoxy-2,3-dihydrofurans as [4+2] Benzannulation Reagents

2-Alkoxy-2,3-dihydrofurans were found to be versatile benzannulation reagents. Indoles can be synthesized in good to excellent yields via the [4+2] annulation of 2-butoxy-2,3-dihydrofuran with pyrroles catalyzed by copper bromide. With the same protocol, carbazoles can also be obtained when indoles are used as starting material with the aid of p-toluenesulfonic acid. This type of benzannulation reagent can also be used to synthesize benzofuran, benzothiophene and naphthalene derivatives in the presence of a catalytic amount of triflic acid in moderate yields. This benzannulation protocol features wide substrate scope and mild reaction condition. Moreover, most of examples have a good regioslectivity.