Diacylhydrazines, dehydration

The dehydration of A,A -diacylhydrazines is a standard method for the formation of the 1,3,4-oxadiazole ring. 2,5-Dipicryl-l,3,4-oxadiazole (DPO) (19) is synthesized by treating 2,4,6-trinitrobenzoic acid (17) with phosphorous pentachloride, followed by treatment with hydrazine to give the A, A -diacylhydrazine (18) which undergoes dehydration on further reaction with phosphorous pentachloride in 1,2-dichloroethane. DPO exhibits high thermal stability but is very sensitive to impact and shock, making it useful in detonation transfer compositions. 

Treatment of acylhydrazines with diazonium salts gives rise to tetrazenes (199) which can be readily dehydrated with base to give 1,5-disubstituted tetrazoles (Scheme 33). The reaction is also effective with 1,2-diacylhydrazines since one of the acyl groups is cleaved during the dehydrative cyclization, which may be conveniently carried out in situ without isolation of the tetrazene. This reaction can be useful for the synthesis of 1-aryltetrazoles using 1,2-diformylhydrazine as substrate (B-67MI41300). Mesoionic tetrazolium 5-oxides (8a) are obtained on treatment of bis(alkylsulfonyl)methanes with diazonium salts. Hydra-zones of type (200) are also obtained from these reactions and the tetrazenes... 

An interesting application of the cyclodehydration approach is the synthesis of bis-oxadiazoles 65 by dehydration of bis-diacylhydrazines 64 [36,37], Similarly, reaction of perfluoroanhydride 66 leading to 67 is also reported [38]. Bis-oxadiazoles 69, which have a good thermal stability, are prepared by cyclodehydration of the corresponding tetrafluoroisophthaloylbis(perfluoroacyl-hydrazines) 68 (Scheme 20) [39]. 

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