BX3-Mediated Intermolecular Formation of Functionalized 3-Halo-1H-indenes via Cascade Halo-Nazarov-Type Cyclization

Abstract

A BX3-promoted, intermolecular regioselective synthesis of 3-halo-functionalized 1H-indenes from 4-oxo-4H-chromene-3-carb­aldehydes and alkynes has been developed. BX3 displays a dual role of Lewis acid catalyst and halide source for haloallyl cation formation for the intended halo-Nazarov-type cyclization. The overall transformation represents an efficient cascade annulation that employs readily available starting materials, inexpensive reagents and a convenient and mild reaction procedure to generate halo-functionalized indenes (45 examples). The reaction was also extended to 8-formylcoumarins to deliver coumarin-based 3-halo-1H-indenes in 79–95% yield (6 examples). The reaction involves conversion of the aldehyde into an sp3 carbon with two new C–C bonds and additionally a C–X bond is formed (X = halide).