Metal-free synthesis of sulfonylated indolo[2,1-a]isoquinolines from sulfur dioxide

Abstract

Access to sulfonylated indolo[2,1-a]isoquinolines through an efficient three-component reaction of 2-aryl-N-acryloyl indoles, sulfur dioxide and aryldiazonium tetrafluoroborates is developed. This transformation is performed under metal-free and mild conditions using DABCO·(SO₂)₂ as the sulfur dioxide surrogate, giving rise to a variety of sulfonylated indolo[2,1-a]isoquinoline derivatives in good to excellent yields. A plausible mechanism is proposed, which involves cascade sulfur dioxide insertion, radical addition and cyclization.