Photoredox-Catalyzed Redox-Neutral Decarboxylative C–H Acylations of Coumarins with α-Keto Acid

Abstract

A novel and green photocatalytic strategy for the synthesis of C-4-acylated coumarins with α-keto acids and 3nitrocoumarin has been developed. This operationally simple protocol works under mild reaction conditions, providing convenient access to 4-acyl coumarin derivatives. The control experimental results showed that the nitro radical produced by the cleavage of the C−N bond acts as an electron acceptor to complete the photocatalytic cycle, achieving a redox-neutral reaction.

Syntheses, reactivity, and biological applications of coumarins

“…(Shankar and Kumara Swamy, 2022) 3.2.3 3-Nitro, 3-cyano, 3-acetamido, and N-methoxy-3-carboxamide coumarins 3-Nitrocoumarins represent an intriguing class of compounds, and their versatility in organic synthesis stems from the ability to produce diverse derivatives of coumarins, particularly those with 4acyl substitutions. Jin et al discovered a novel green approach for the synthesis of C-4-acylated coumarins (130) starting from 3nitrocoumarins (127) in the presence of α-keto acids (128) (Sun et al, 2023). The protocol demonstrated to efficiently work under mild conditions using photocatalysis, mediated by 4-CzIPN (129, 1,2,3,5-tetrakis (carbazol-9-yl)-4,6-dicyanobenzene) when the light source was a 365-370 nm LED (Scheme 7-path a).…”

Recent advances in catalytic decarboxylative transformations of carboxylic acid groups attached to a non-aromatic sp2or sp carbon

“…Very recently, Jin et al described an elegant method for the synthesis of C-4-acylated coumarins 252 from α-keto acids and 3-nitrocoumarin 250 in a redox-neutral fashion. 100 Importantly, the reaction proceeds in the presence of 2,4,5,6-tetra(9 H -carbazol-9-yl)isophthalonitrile (4CzIPN, 251 ) as the organophotocatalyst under metal free conditions. Control experiments suggest that the nitro radical generated upon the C–N bond cleavage of 3-nitrocoumarin serves as an electron acceptor to finish the photocatalytic cycle under external oxidant free conditions (Scheme 88).…”

Copper-Catalyzed Synthesis of Difluoromethylated/C-4- and C-5-Functionalized Polycyclic Coumarin Derivatives

“…In addition, we have established a facile silver-catalyzed double decarboxylation reaction of arylthiodifluorinated acetic acid with coumarin-3-carboxylic acid to synthesize a series of C-4 arylthiodifluoromethyl 3,4-dihydrocoumarin derivatives (Scheme A) . Very recently, Jin and co-workers synthesized C-4 acyl coumarins via a decarboxylation coupling reaction of α-ketoacetic acid and 3-nitro coumarin (Scheme A) . So far, the functionalization of the C-3 and C-4 positions of coumarins has been extensively reported; nevertheless, it is noteworthy that the generation of C-4 and C-5 highly functionalized polycyclic coumarins by the direct free-radical cascade cyclization has not been reported.…”