Metal-free syntheses of oxazoles and their analogues based on λ3-iodane-mediated cycloisomerization/functionalization reactions or [2+2+1] cycloaddition type reactions

Abstract

As a metal-free construction of oxazoles and furans concomitant with the introduction of oxygen functional groups or fluorine atoms into the side chains, we have developed λ 3 -iodane-mediated cycloisomerization/functionalization reactions of propargyl compounds. In these reactions, aryl-λ 3 -iodane ArI(X)Y works not only as a donor of heteroatomic functional groups but also as an activator of carbon-carbon triple bonds. Therefore, this methodology is not required any transition metal catalysts, which are frequently used in previous methods. Furthermore, this methodology can be extended to λ 3 -iodane-mediated [2+2+1] cycloaddition type reactions of alkynes, nitriles and heteroatoms for metal-free formation of oxazoles and imidazoles.

Recent Progress in Iodine‐Catalysed C−O/C−N Bond Formation of 1,3‐Oxazoles: A Comprehensive Review

“…With the C−C triple bond activation [25] advantage by Aryl λ 3 ‐iodanes over metal‐catalysed reaction, Akio Saito [26] reported a novel methodology of oxazoles 17 synthesis from propargyl species 16 via [2+2+1] cycloaddition [27] type reaction with introduced an acyloxy group (Scheme 7). The substituted amides with aromatic and aliphatic groups were well‐tolerated under standardised reaction conditions.…”

Synthesis of Oxazoline and Oxazole Derivatives by Hypervalent-Iodine-Mediated Oxidative Cycloaddition Reactions

“…The oxazoline nucleus is present in many heterocyclic compounds that have found important applications in medicinal and materials chemistry. 14,51,53,54 Numerous synthetic strategies for the construction of the oxazoline ring have been explored. Particularly important are synthetic approaches using hypervalent iodine reagents, such as (diacetoxyiodo)arenes, [55][56][57][58][59][60][61][62][63][64][65] iodosylarenes, 66 (difluoroiodo)arenes, 67 in situ generated iodine(III) species, [68][69][70][71][72][73] or iodonium ylides.…”

“…Oxazoles have found numerous applications in chemical and biomedical sciences, and many synthetic procedures for oxazole ring formation from various precursors have been developed. 10,16,28,30,51 Particularly important are methods based on the use of hypervalent iodine reagents such as (diacetoxyiodo)benzene, [76][77][78][79][80][81][82][83][84][85][86][87] [bis(trifluoroacetoxy)iodo]benzene, 79,[88][89][90][91] iodosylbenzene, [92][93][94][95] [hydroxy(tosyloxy)iodo]arenes, [96][97][98][99] recyclable iodine(III) compounds, 100 in situ generated iodine(III) species, 76,[101][102][103][104][105][106] and iodonium ylides. [107][108][109][110][111][112][113] The hypervalent-iodine(III)-mediated cyclization of Npropargylamides represents a useful synthetic methodology for the construction of the oxazole nucleus.…”

“…Numerous classes of heterocyclic compounds have been prepared under mild conditions using hypervalent iodine reagents. [50][51][52] In the present review, synthetic approaches to the azole derivatives, oxazolines 1, oxazoles 2, isoxazolines 3, and isoxaz-oles 4 (Figure 1), starting from various substrates using hypervalent iodine species are summarized, and recent developments in chemical and electrochemical synthetic procedures toward these heterocycles are discussed.…”

Oxidative cycloaddition of hydroxamic acids with dienes or guaiacols mediated by iodine(III) reagents

“…As part of our research on the syntheses of heterocycles by iodine(III)-mediated/catalyzed oxidative cycloaddition reactions [ 17 – 19 ], we have found that iodine(III) reagents are effective in the oxidation of N–O bonds of oximes in the cycloaddition reaction of in situ formed nitrile oxides [ 20 – 21 ]. Although Adam and Bottke’s group have demonstrated that (diacetoxyiodo)benzene (DIB) and iodosylbenzene are applicable to the ene reactions of acylnitroso species derived from hydroxamic acids [ 22 ], the iodine(III)-mediated oxidative cycloaddition reaction of hydroxamic acids with dienes is still unknown.…”