1,3,4-Oxadiazoles from tetrazoles

Efimova YA, Karabanovich GG et al (2009) Tetrazoles LV. Perparation of 2-anilino-5-aryl(hetaryl)-1,3,4-oxadiazoles from 5-substituted tetrazoles under microwave activation. Russ J Org Chem 45 1241-1243... 

Oxadiazoles are available by cyclodehydration of A.A -diacylhydrazines or their equivalents. They are also available from tetrazoles (section 26.1.1.3) or by oxidative cyclisation of acyl hydrazones. ... 

Poly(aryl-l,3,4-oxadiazoles) from nitriles via tetrazoles... 

A similar intramolecular trapping of the intermediate (511) from the photolysis of the corresponding methyl tetrazole-l,5-dicarboxylate (510) gave methyl 5-methoxy-l,2,4-oxadiazole-3-carboxylate (512). 

Interesting oxadiazole-substituted benzonitriles prepared from the respective aryl iodides were transformed into the corresponding (dioxadiazolephenyl)tetrazoles. Despite the high temperature and other drastic conditions, the oxadiazole ring stayed intact during both reactions (Scheme 20) <1999JOC6425>. 

Two variations of the transformation of 3-acyltetrazoles into oxadiazoles are useful from a synthetic point of view. The first transformation involves the reaction of tetrazole with diketene. In the second, the sodium salt of the tetrazole is treated with oxalyl chloride. UV irradiation of some 3-amino-l,2,4-oxadiazoles leads to the formation of the corresponding 2-amino-l,3,4-oxadiazoles <1996CHEC-II(4)268>. 

The energetic 1,3,4-oxadiazole (22) is synthesized from the reaction of the tetrazole (20) with oxalyl chloride. In this reaction the tetrazole (20) undergoes a reverse cycloaddition with the expulsion of nitrogen and the formation of the 1,3-dipolar diazoalkane (21) which reacts with the carbonyl groups of oxalyl chloride to form the 1,3,4-oxadiazole rings. 

Abstract Synthesis methods of various C- and /V-nitroderivativcs of five-membered azoles - pyrazoles, imidazoles, 1,2,3-triazoles, 1,2,4-triazoles, oxazoles, oxadiazoles, isoxazoles, thiazoles, thiadiazoles, isothiazoles, selenazoles and tetrazoles - are summarized and critically discussed. The special attention focuses on the nitration reaction of azoles with nitric acid or sulfuric-nitric acid mixture, one of the main synthetic routes to nitroazoles. The nitration reactions with such nitrating agents as acetylnitrate, nitric acid/trifluoroacetic anhydride, nitrogen dioxide, nitrogen tetrox-ide, nitronium tetrafluoroborate, V-nitropicolinium tetrafluoroborate are reported. General information on the theory of electrophilic nitration of aromatic compounds is included in the chapter covering synthetic methods. The kinetics and mechanisms of nitration of five-membered azoles are considered. The nitroazole preparation from different cyclic systems or from aminoazoles or based on heterocyclization is the subject of wide speculation. The particular section is devoted to the chemistry of extraordinary class of nitroazoles - polynitroazoles. Vicarious nucleophilic substitution (VNS) reaction in nitroazoles is reviewed in detail. 

There seem to be few examples of organometallics derived from higher azoles (triazoles, tetrazole, oxadiazoles etc.) being used in coupling reactions, possibly due to the relative instability of boronates and stannanes, although there is an example of a coupling of a 5-stannyltetrazole."" ... 

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