Synthesis and reactivity of 1,2,4-oxadiazolium salts
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Synthesis and reactivity of 1,2,4-oxadiazolium salts Mikhail V. Il'in1, Dmitrii S. Bolotin1* 1
Institute of Chemistry, Saint Petersburg State University, 7/9 University Embankment, Saint Petersburg 199034, Russia; e-mail: [email protected] Submitted April 6, 2020 Accepted May 13, 2020
Published in Khimiya Geterotsiklicheskikh Soedinenii, 2020, 56(7), 824–828
This review summarizes the reports related to the chemistry of 1,2,4-oxadiazolium salts and published until 2020. Emphasis has been put on the information revealing convenient synthetic methods for the preparation of these specific organic compounds. Importantly, this review sheds the light on reactivity and thereby synthetic potential of 1,2,4-oxadiazolium salts. Keywords: 1,2,4-oxadiazole, 1,2,4-oxadiazolium salt, electrophilic reactant, heterocycle, nucleophilic substitution.
1,2,4-Oxadiazoles represent an important class of fivemembered heterocycles. Synthesis and wide scope of application of these compounds in pharmaceutical chemistry, materials chemistry, and synthetic organic chemistry have been comprehensively reviewed.1 Although, in organic chemistry, 1,2,4-oxadiazole ring serves as a convenient precursor for generation of other heterocyclic and acyclic compounds via thermal and photochemical ring cleavages1a–c or reactions with various nucleophilic reagents, these processes typically proceed under harsh conditions and, in many cases, are characterized by low or moderate yields of products.1a To achieve the synthetic transformations of 1,2,4-oxadiazole ring using different nucleophiles, additional electrophilic activation is required. One of the most useful options for the activation of the 1,2,4oxadiazole ring is utilization of 1,2,4-oxadiazolium salts instead of parent 1,2,4-oxadiazoles (Fig. 1). In spite of the continuously increasing number of publications related to the application of 1,2,4-oxadiazoles in organic synthesis, only small part of the articles is focused on the preparation and reactivity of 1,2,4oxadiazolium salts. This review covers the possible approaches toward these specific heterocyclic systems and points out the advantages of their utilization in different synthetic strategies. 0009-3122/20/56(7)-0824©2020 Springer Science+Business Media, LLC
Figure 1. Structures of 1,2,4-oxadiazolium salts.
1. SYNTHESIS OF 1,2,4-OXADIAZOLIUM SALTS 1.1. Functionalization of 1,2,4-oxadiazoles One of the possible synthetic routes for obtaining 1,2,4oxadiazolium salts is based on the functionalization of preliminary prepared uncharged 1,2,4-oxadiazole system. Inspection of the literature data revealed two general approaches toward 1,2,4-oxadiazoles 1. The first route includes the reaction of amidoximes 2 and acid chlorides 31a or other reactive derivatives of carboxylic acids2 in basic medium. This chemical transformation proceeds via generation of O-acyl amidoximes, which further undergo heterocyclization. The second route is based on 1,3-dipolar 824
Chemistry of Heterocyclic Compounds 2020, 56(7), 824–828
cycloaddition of nitrile oxides t
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