Insights into the unexpected chemoselectivity in Bronsted acid catalyzed cyclization of isatins with enaminones: convenient synthesis of pyrrolo[3,4-c]quinolin-1-ones and spirooxindoles

Divergent cascade syntheses constitute a highly attractive and challenging area in synthetic chemistry, and can exhibit unexpected chemoselectivity. Herein, a Bronsted acid-controlled protocol is described for the efficient catalysis of two different reactions, namely acylation cascade- and [1+2+3]-type cyclization of enaminones and isatins for the concise synthesis of highly functionalized pyrrolo[3,4-c]quinolin-1-ones and spirooxindoles in the presence of carboxylic acids and KHSO4, respectively. The observed chemoselectivity was reasonably explained by trapping the intermediate alpha,beta-unsaturated 2-oxindoles, and the source of the hydroxyl group, carbocation intermediate, and amination reaction were also evaluated.