Stereoselective Synthesis of Atropisomeric Acridinium Salts by the Catalyst‐Controlled Cyclization of ortho‐Quinone Methide Iminiums

Abstract

Quinone methides are fundamental intermediates for a wide range of reactions in which catalyst stereocontrol is often achieved by hydrogen bonding. Herein, we describe the feasibility of an intramolecular Friedel–Crafts 6π electrocyclization through ortho‐quinone methide iminiums stereocontrolled by a contact ion pair. A disulfonimide catalyst activates racemic trichloroacetimidate substrates and imparts stereocontrol in the cyclization step, providing a new avenue for selective ortho‐quinone methide iminium functionalization. A highly stereospecific oxidation readily transforms the enantioenriched acridanes into rotationally restricted acridiniums. Upon ion exchange, the method selectively affords atropisomeric acridinium tetrafluoroborate salts in high yields and an enantioenrichment of up to 93 : 7 e.r. We envision that ion‐pairing catalysis over ortho‐quinone methide iminiums enables the selective synthesis of a diversity of heterocycles and aniline derivatives with distinct stereogenic units.

Enantioconvergent 6π Electrocyclization Enabled by Photoredox Racemization

“…The 6π electrocyclization is a well-established transformation, but enantioselective examples remain scarce − and are challenging to develop for systems involving a terminal oxygen atom . The difficulties in promoting enantioselective oxa-6π electrocyclic transformations are related to absolute orbital control, as unlike in systems with a substituted carbon or nitrogen, the terminal oxygen lacks substituents that could aid in stereoinduction.…”