Organocatalytic Asymmetric Construction and Application of Axially Chiral Spiro-bisindoles

Abstract

Spiro skeletons have emerged as a privileged class of chiral carriers across various research fields, including asymmetric catalysis and functional materials, due to their remarkable configurational rigidity. However, limited structural diversity of spiro frameworks significantly restricts the expansion of their applications. Here we present a new class of axially chiral spiro-bisindole frameworks and report their first enantioselective construction via a chiral phosphoric acid-catalyzed intramolecular dehydrative cyclization reaction. Unlike the classical SPINOL backbone, incorporation of indole moieties in place of phenol enhances the nucleophilicity of ketone substrates, thereby eliminating the need for a tedious pre-activation process. By leveraging the retained active sites of indole, the resulting highly enantioenriched spiro-bisindoles can be rapidly transformed into other valuable structures. More importantly, axially chiral fluorescent molecules with good asymmetry factors and quantum fluorescence efficiency are readily accessed, opening a new avenue for developing chiral fluorescent materials. Control experiments demonstrate the pivotal role of both unmasked N-H bonds in achieving good efficiency and enantiocontrol.