A metal‐free, photoinduced aerobic tandem amine dehydrogenation/Povarov cyclization/aromatization reaction between N‐aryl glycine esters and indoles leads to tetracyclic 11H‐indolo[3,2‐c]quinolines under mild conditions and with high yields. The reaction can be performed by using molecular iodine along with visible light, or by combining an organic photoredox catalyst with a halide anion. Mechanistic studies reveal that product formation occurs through a combination of radical‐mediated oxidation steps with an iminium ion or N‐haloiminium ion [4+2]‐cycloaddition, and the N‐heterocyclic products constitute new analogues of the antiplasmodial natural alkaloid isocryptolepine.
A common synthetic route to indoxyl and 2‐oxindole alkaloids utilizes the oxidation of indoles to 3‐hydroxyindolenines, followed by acid‐mediated 1,2‐rearrangement. However, controlling the regioselectivity is often challenging and there is an ongoing need for new reaction conditions allowing to steer product selectivity. We report herein that phosphoric acids are ideal organocatalysts for the highly regioselective 1,2‐rearrangement of 3‐hydroxyindolenines to 2‐oxindoles, with predictable product selectivity arising from an efficient dual activation mode.
Aerobic oxidative dehydrogenation reactions of benzylamines to imines were studied and the efficiencies of various ground‐ and excited state quinone organocatalysts were compared. Long wave‐absorbing anthraquinones readily catalyze the aerobic photodehydrogenation of primary and secondary benzylamines to benzylidenebenzylamines with high rate and selectivity, and the reaction mechanism was studied by laser flash photolysis. Further, branched α‐benzyl dibenzyl‐amines undergo a regioselective photocatalytic dehydrogenation to branched aldimines, which can efficiently be converted into cis‐1,3‐diaryl tetrahydro‐isoquinolines through diastereoselective super acid‐mediated Pictet‐Spengler cyclizations.magnified image
We report metal-free, photoinduced aerobic tandem dehydrogenative Povarov cyclisation / Csp3−H oxygenation reactions between N-aryl glycine esters and α-substituted styrenes, which efficiently lead to 4,4-disubstituted dihydroquinoline-3-ones under mild conditions. The...
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