Catalyst-Free Visible-Light-Promoted Cyclization of Aldehydes: Access to 2,5-Disubstituted 1,3,4-Oxadiazole Derivatives

Abstract

An efficient synthesis of a variety of 2,5-disubstituted 1,3,4-oxadiazole derivatives via a cyclization reaction by photoredox catalysis between aldehydes and hypervalent iodine­(III) reagents is described. The reaction proceeds under mild conditions and affords various target compounds in excellent yields. The commercially available aldehydes without preactivation and a simple visible-light-promoted procedure without any catalysts make this strategy an alternative to the conventional methods.

Diazotrifluoroethyl Radical: A CF3-Containing Building Block in [3 + 2] Cycloaddition

“…On the basis of the above experiments and related literature precedents, , a plausible mechanism for this reaction is proposed in Scheme . Initially, the homolysis of the C–I bond of hypervalent iodine reagent 2 is induced by blue LED irradiation to give diazotrifluoroethyl radical D and iodanyl radical E .…”

“…Despite the fact that several powerful approaches to access these units have been achieved (Figure d), such as the dehydrative cycloaddition of 1,2-diacylhydrazines, the oxidative cycloaddition of N -acylhydrazones, the ring-opening/cycloaddition cascade of 5-trifluoromethyltetrazole or the corresponding sodium salt, the direct C–H oxidative trifluoromethylation of 2-aryl-1,3,4-oxadiazoles with TMSCF 3 , and the trifluoromethylation of activated 2-bromo-5-phenyl-1,3,4-oxadiazole with complex [Ph 4 P] + [Cu­(CF 3 ) 2 ] − , the development of a novel strategy for the construction of 5-CF 3 -1,3,4-oxadiazoles under mild conditions still remains highly desirable. Herein we present a visible-light-induced [3 + 2] cycloaddition between hypervalent iodine­(III) reagent ( 2 ) and α-ketoacids ( 3 ) for the construction of 2-aryl-5-(trifluoromethyl)-1,3,4-oxadiazoles ( 4 ) (Figure d) . This process with readily accessible substrates features operational simplicity and is photocatalyst-, metal-, and additive-free, enabling access to synthetically valuable CF 3 -containing 1,3,4-oxadiazoles under mild conditions.…”

Synthesis of [1,2,3]Triazolo-[1,5-a]quinoxalin-4(5H)-ones through Photoredox-Catalyzed [3 + 2] Cyclization Reactions with Hypervalent Iodine(III) Reagents

“…Recently, a visible-light-induced [3 + 2] cyclization reaction of hydrazones with hypervalent iodine diazo reagents has been reported to synthesize 1-amino-1,2,3-triazoles by Li and Wang . Inspired by our catalytic photoredox synthesis of 2,5-disubstituted 1,3,4-oxadiazoles derivatives by a cyclization reaction between α-oxocarboxylic acids/aldehydes and a hypervalent iodine­(III) reagent, we expect to construct a 1,2,3-triazole fused heterocyclic skeleton through [3 + 2] cycloaddition with hypervalent iodine diazo reagents and imine via the [3 + 2] cyclization. Herein, we wish to report our study on the development of a visible-light mediated [3 + 2] annulation process of quinoxalinones with hypervalent iodine diazo reagents to construct [1,2,3]­triazolo­[1,5- a ]­quinoxalin-4­(5 H )-ones, in which one C–C bond and one N–N bond are sequentially formed under mild conditions.…”

Synthesis of 2-Imino-1,3,4-oxadiazolines from Acylhydrazides and Isothiocyanates via Aerobic Oxidation and a DMAP-Mediated Annulation Sequence

“…A number of protocols to synthesize 1,3,4-oxadiazoles have been developed over the years . These protocols include the dehydrative cyclization of 1,2-diacylhydrazines, desulfurative cyclization of thiosemicarbazides, and oxidative cyclization of N -acylhydrazones or their analogues, among others . However, the construction of 2-imino-1,3,4-oxadiazolines, which can have potent biological activity, has been much less investigated.…”