Blue-light-promoted carbon–carbon double bond isomerization and its application in the syntheses of quinolines

Abstract

A blue-light-promoted carbon-carbon double bond isomerization in the absence of any photoredox catalyst is reported. It provides rapid access to a series of quinolines in good to excellent yields under simple aerobic conditions. The protocol is direct, catalyst-free and operationally convenient.

Continuous Flow Synthesis of Quinolines via a Scalable Tandem Photoisomerization‐Cyclization Process

“…a base‐mediated aldol condensation between 2‐nitrobenzaldehyde and selected methyl ketones (see SI for full details). [18a] The corresponding aldol products were obtained in generally high yield and allowed for further elaboration of the nitro group into the desired amine species. To achieve this chemoselective reduction we opted to employ iron powder (ca.…”

“…in the presence of catalytic amounts of acid (HCl aq.) in hot ethanol,[18a] which provided the target compounds not only in a simple and short procedure but moreover in generally high yield as the E ‐isomer (see SI for full details). Interestingly, our studies also identified small amounts (<5 %) of 2‐acylindole species (e.g.…”

“…In view of the effectiveness of this flow process, we next wished to evaluate its scalability over a period of several hours. We selected substrate 3a for this study as a previous report[18a] had evaluated its scaled preparation in a photochemical batch operation yielding 1.49 g of the desired quinoline product ( 1a , 10.4 mmol, 93 % yield) over a period of 5 days using a blue LED lamp (5 W power). To generate sufficient quantities of substrate 3a we opted to use a modified 2‐step approach as reported by List to avoid the formation of indigo dye as a consequence of a competing Baeyer–Drewson reaction .…”

“…To this end, we decided to exploit a variation of the Friedländer quinoline synthesis in which a Z ‐configured enone undergoes intramolecular cyclocondensation with a pendent amine group (Scheme ). Importantly, this approach requires the isomerization of the E ‐configured enone ( 3 ) in a process that can either be achieved under acidic or photochemical conditions. As we were targeting the latter approach, we identified 2‐aminophenyl‐enones ( 3 ) as key intermediates, which would be generated from their corresponding nitro‐precursors ( 4 ).…”

Visible-Light Photocatalytic E to Z Isomerization of Activated Olefins and Its Application for the Syntheses of Coumarins

“…The transformation was successfully achieved via a selective UV-light irradiation at 402 nm. More recently, Wang and Zhai disclosed a blue-light-promoted carbon-carbon double bond isomerization and its application in the syntheses of quinolines in the absence of photocatalyst [22].…”