Aldehydes as Carbon Radical Acceptors: Silver Nitrate Catalyzed Cascade Decarboxylation and Oxidative Cyclization toward Dihydroflavonoid Derivatives

Abstract

Silver nitrate-catalyzed cascade decarboxylation and oxidative cyclization of a-oxocarboxylic acids, alkenes, and aldehydes is demonstrated for the first time. With ammonium persulfate as the oxidant, the cascade reactions afford dihydroflavonoid derivatives as products in moderate to good yields, exhibiting a broad substrate tolerance. Control experiments indicated that the mechanism includes a radical pathway with aldehydes as the carbon radical acceptors.

A Novel and Facile Synthesis of Chroman‐4‐one Derivatives via Cascade Radical Cyclization Under Metal‐free Condition

“…Hetero‐arylaldehyde, for example, thiophene‐2‐carbaldehyde was also reacted with 2 a well in this system and gave the expected product in 55% yield ( 3 ah ). To our delight, aliphatic aldehydes such as butyraldehyde, propionaldehyde and 3‐methylbutanal were also compatible in this transformation and led to different chroman‐4‐one derivatives which are hard to prepare using previous methods . However, formaldehyde failed to afford any of the desired product under current condition.…”

“…Recently, Li and co‐workers reported an elegant method toward phosphorylmethyl‐substituted chroman‐4‐ones via phosphoryl radical‐initiated cascade cyclization using an easy available o ‐(allyloxy)arylaldehydes as starting materials (Scheme a) . Subsequently, Wu et al developed a silver‐catalyzed cascade decarboxylation and oxidative cyclization of a ‐oxocarboxylic acids with 2‐(allyloxy)arylaldehydes to 1,4‐dicarbonyl‐containing chroman‐4‐ones as shown in Scheme b . Lately, Chen and co‐workers expanded this kind of radical cascade cyclization using o ‐(allyloxy)arylaldehydes as starting materials with 1,3‐dicarbonyl compounds or tert ‐carboxylic acids to produce other valuable and complex chroman‐4‐one derivatives in the presence of silver catalyst (Scheme c and 1 d) .…”

Visible‐Light‐Induced Difluoroacetylation of O‐(Allyloxy)Aryl‐Aldehydes: Access to Difluoroacetylated Chroman‐4‐ones

“…For examples, Li group employed a silver‐catalyzed phosphoryl radical‐initiated cascade cyclization of 2‐(allyloxy)arylaldehydes to form phosphorylmethyl‐substituted chroman‐4‐ones in 2016 (Scheme a) . Then, Wu group developed a silver nitrate‐catalyzed cascade decarboxylation to dihydroflavonoid derivatives (Scheme b) . In 2018, Zhao group reported a silver‐catalyzed alkyl radical‐trigged cascade cyclization reaction to provide 1,5‐dicarbonyl‐containing chroman‐4‐ones (Scheme c) .…”

Radical Reactions for the Synthesis of 3‐Substituted Chroman‐4‐ones

“…In 2017, Wu and co‐workers reported a silver‐catalyzed cascade decarboxylation and oxidative cyclization of 2‐(allyloxy)arylaldehydes 36 with α‐oxocarboxylic acids 37 towards 1,4‐dicarbonyl‐containing chroman‐4‐ones 38 (Scheme ) . Control experiments of 36a and 37a under the optimized conditions with 2,2,6,6‐tetramethyl‐1‐piperidinyloxy (TEMPO) as a radical scavenger did not give any desired product 38a , suggesting that this reaction is a radical process.…”

“…[57] Two possible reaction pathways were proposed: In path A, the aldehyde group first led to acyl radical 30 in the presence of Ag + /S 2 In 2017, Wu and co-workers reported a silver-catalyzed cascade decarboxylation and oxidative cyclization of 2-(allyloxy)arylaldehydes 36 with α-oxocarboxylic acids 37 towards 1,4dicarbonyl-containing chroman-4-ones 38 (Scheme 9). [58] Control experiments of 36a and 37a under the optimized conditions with 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) as a radical scavenger did not give any desired product 38a, suggesting that this reaction is a radical process. Additionally, only the adduct 40, rather than 39, was detected in 39 % yields, demonstrating that the acyl radical was produced from 2-oxo-2-phenylacetic acid (37a) then trapped by TEMPO.…”