Asymmetric synthesis of bis-spiro cyclopropane skeletons via bifunctional phosphonium salt-catalyzed [2 + 1] annulation

Abstract

A new approach for constructing enantiopure spiro[cyclopropane-oxindole] and bispiro[oxindole-cyclopropane-cyclohexone] skeletons featuring three vicinal stereocenters was developed. In this approach, 3-alkenyl-oxindoles and α-bromoketones served as substrates for an asymmetric [2 + 1] cyclopropanation using a chiral bifunctional phosphonium salt catalyst. The reaction afforded the desired products in high yields (up to 97%) and with excellent stereoselectivities (up to 97% ee and >20 : 1 dr).