Copper-catalyzed alkyne oxidation/Büchner-type ring-expansion to access benzo[6,7]azepino[2,3-b]quinolines and pyridine-based diones

Abstract

General access to highly valuable seven-membered rings via Büchner-type reaction remains a formidable challenge. Here we report a Cu-catalyzed intermolecular oxidation of alkynes using N-oxides as oxidants, which enables expedient preparation of valuable benzo[6,7]azepino[2,3-b]quinolines and pyridine-based diones. Importantly, in contrast to the well-established gold-catalyzed intermolecular alkyne oxidation, the dissociated pyridine or quinoline partner could be further utilized to construct N-heterocycles in this system and the reaction most likely proceeds by a Büchner-type ring expansion pathway. A mechanistic rationale for this cascade cyclization is supported by DFT calculations.

A copper-catalyzed annulation reaction of diazo esters with propargyl amines for the synthesis of 2,5-dihydropyrroles

“…Based on preliminary mechanistic studies and previous reports, 32 a plausible mechanism is proposed in Scheme 5. Initially, the diazo compound ( 2b ) is decomposed by reacting with the Cu( i ) cation, resulting in the release of molecular nitrogen and the formation of a copper carbene species A .…”

Recent advances in transition-metal-catalyzed Büchner reaction of alkynes

“…Recently, Shen and co-workers developed a copper-catalyzed intermolecular oxidation of alkynes using N -oxides as oxidants, via a Büchner-type ring expansion pathway, which enables expedient synthesis of valuable benzo[6,7]azepino[2,3- b ]quinolines. 21 Of note, in contrast to the well-established gold-catalyzed alkyne oxidation, the dissociated pyridine or quinoline partner could be further utilized to construct N-heterocycles in such a tandem sequence. This transformation provides the first example of a non-noble metal catalyzed Büchner reaction of alkynes.…”