Organocatalytic Enantioselective Mannich Reaction of Cyclic <i>N</i> -Sulfonyl Ketimines to Tryptanthrin-Derived Ketimines

An organocatalytic asymmetric addition of cyclic <i>N</i>-sulfonyl ketimines to tryptanthrin-derived ketimines has been developed with moderate-to-excellent yields and excellent enantioselectivities (up to 99% yield, 90-99% ee), with a chiral bifunctional tertiary amine-squaramide as the catalyst. This protocol provides an effective method to construct enantiomerically pure 6,6-disubstituted indolo[2,1-<i>b</i>]quinazolinone derivatives containing a cyclic <i>N</i>-sulfonyl ketimine scaffold.