Triazoles, from rearrangement 1,2,4-oxadiazoles

Examples include the synthesis of 3-amino-l,2,4-oxadiazoles starting from 3-acylamino-5-methyl-l,2,4-oxadiazole <2002H811> and 2-aryl-l,2,3-triazoles from l,2,4-oxadiazole-3-ketone arylhydrazones <1999T12885, 2006JOC5616>. Oximes, hydrazones, formamidines, and thioureas of the furazan series also undergo base-catalyzed mononuclear rearrangements <2004RCB1121>. Nucleophilic attack at N(3) takes place in the benzofuroxan series. For example, reaction with secondary amines leads to o-nitroarylhydrazines (Scheme 55). 

Other non-traditional preparations of 1,2,3-triazoles have been reported. The rearrangement in dioxane/water of (Z)-arylhydrazones of 5-amino-3-benzoyl-l,2,4-oxadiazole into (2-aryl-5-phenyl-27/-l,2,3-triazol-4-yl)ureas was investigated mechanistically in terms of substituents on different pathways <06JOC5616>. A general and efficient method for the preparation of 2,4-diary 1-1,2,3-triazoles 140 from a-hydroxyacetophenones 139 and arylhydrazines is reported <06SC2461>. 5-Alkylamino-] //-], 2,3-triazoles were obtained by base-mediated cleavage of cycloadducts of azides to cyclic ketene acetals <06S1943>. Oxidation of N-... 

From Other Heterocyclic Systems 4-Arylazoisoxazoles can rearrange to 2-aryltriazoles, and 4-amino-2//-triazoles can be prepared by reaction of 4-nitrosoimidazoles with hydrazines (Scheme 34). The hydrazones of 3-benzoyloxadiazoles are intermediates in the latter reaction.The generality of rearrangements of this type has been discussed, and a further example, involving the rearrangement of a 1,2,5-oxadiazole to a triazole, has been described. 

Amino-1,2,3-triazoles with a substituent at the 4-position have been prepared (i) from azides and active methylene nitriles (ii) from azides and ynamines (iii) from diazomethane and carbo-diimides (iv) from azides and 1,1-diaminoethenes and (v) from the rearrangement of 3-hydrazono-1,2,4-oxadiazoles. Among these, the first method, a regiospecific process, is the most versatile and convenient although it is suitable only for 5-NH2-substituted triazoles. Other methods are used to prepare 5-NHR , 5-NR R - and 5-NHCOR-substituted triazoles. Intramolecular cyclization of suitable precursors also gives 5-aminotriazoles. For example, a-imino-a-piperidyl phenylhydrazones (838), in the presence of copper acetate, give 5-piperidyl-triazoles (839) (Equation (85)) <94H(38)739>. 

Hydrazones (33) rearrange to 1,2,3-triazoles (34) from a geometry in which the oxadiazole and N are cis oriented with respect to each other (Scheme 11) <81JHC723>, Electron-donating as well as... 

Examples of the rearrangement of 1,2,4-oxadiazoles into 1,2,3-triazoles were discovered by Ruccia and co-workers178, 179 and earlier by Gramantieri180 as shown in Eq. (68).179 In the example selected, the product 93 was obtained directly from the reaction with phenyl-... 

---