A one-pot ultrasound-assisted regio and stereoselective synthesis of indenoquinoxaline engrafted spiropyrrolidines

A catalyst free ultrasound-assisted regio-/stereoselective modular approach was accomplished for the synthesis of highly constrained indenoquinoxaline engrafted spiro pyrrolidines from easily available substrates. This one-pot strategy utilizes 1,3-dipolar cycloaddition from a four component reaction of ninhydrin, 1,2-phenylenediamine, β-nitrostyrene and benzylamine or amino acids under ultrasound irradiation. The transformation is mild and operationally simple, providing architecturally complex fused spiro polycyclic heterocycles. This synthesis was confined to follow the group-assistant-purification (GAP) chemistry process, which can avoid chromatographic purifications and use of catalysts and allows easy access to a novel class of spiro engrafted polyheterocyclic scaffolds, which may be beneficial in biomedical research/materials science in the near future. This opens an era for the formation of a single exo product, when compared with reported protocols, by merely switching over reaction conditions to US irradiation. We have demonstrated a catalyst free ultrasound-assisted multicomponent synthesis of highly regio and stereoselective spiropyrrolidines containing the indenoquinoxaline moiety via a 1,3-dipolar cycloaddition strategy.