Selective Synthesis of Dihydrophenanthridine and Phenanthridine Derivatives from the Cascade Reactions of o-Arylanilines with Alkynoates through C-H/N-H/C-C Bond Cleavage

In this paper, an unprecedented selective synthesis of dihydrophenanthridine and phenanthridine derivatives through the cascade reactions of 2-arylanilines with alkynoates is presented. Mechanistic studies showed that the formation of the dihydrophenanthridine scaffold involves an initial C(sp(2))-H alkenylation of 2-arylaniline with alkynoate followed by an intramolecular aza-Michael addition. When this reaction is carried out at elevated temperature, the in situ formed substituted dihydrophenanthridine readily undergoes a retro-Mannich-type reaction to give the corresponding phenanthridine through C-C bond cleavage. Compared with literature methods, this novel protocol has advantages such as easily obtainable substrates with a free amino group, pharmaceutically privileged products, cheap catalysts, and conveniently controllable selectivity.