Oxadiazolium salts rearrangement

Benzyloxadiazolium salts (67a) deprotonate in alkali to form coloured anhydro bases (68). Treatment of salts (67a) with acetyl chloride, or salts (67b) with triethylamine, results in a bimolecular reaction, with rearrangement, to give 3-(2,2-diacylhydrazino)pyrazoles (71JCS(C)2314). 2-Methyl-3-aryl-l,3,4-oxadiazolium salts condense with aldehydes to yield 2-styryl-l,3,4-oxadiazolium salts but no reaction occurs between aldehydes and the isomeric 5-methyl-3-aryl-l,3,4-oxadiazolium salts. 

An oxadiazolium inner salt was found to be an intermediate in the rearrangement of a nitrone (106) to an amide (108). The red salt (107) was isolated. In accord with the mechanism as shown, only when the nitrone function was substituted at C-3 did the rearrangement occur (79JHC1477). 

A brief comparison of rearrangements of oxadiazolium and thiadiazolium salts deserves mention. The sequence of reactions represented in Scheme 105 has been well studied. The oxodiazolium salt (231) reacts with aniline to afford the acylamidrazone (232) (71JCS(C)409)... 

Benzamidinc combines with 2-amino-3-phenacyl-l,3,4-oxadiazolium bromides to give l-acylamino-2-benzimidoylamino-4-arylimidazolcs. Yields arc only moderate (14-43%), but the reaction works for a variety of 4-arylimidazoles f22. Reactions of /V-methyl-JV-(JV -phenylbenzimidoyl)amino-acetonitrile (13) under acidic conditions lead to imidazolium salts which have amino (14) or amido (15) groups in the 4-position (Scheme 2.2.6). The 4-amino salt (14) undergoes Dimroth rcaaangement to the 4-phenylaminoimidazole (16) direct conversion of (13) into (16) also occurs in warm alkali [8]. A Claisen rearrangement of the adduct (17), which forms from interaction of an arylamidoxime and a propiolate ester, provides a method... 

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