1.3.4- Oxadiazoles acylhydrazines

Acylhydrazine derivatives of this general type provide one of the most convenient routes to a variety of 1,3,4-oxadiazoles, and the scope and limitations of this method are discussed in Chapter 4.23. 

The malonate derivative (93) reacted with acylhydrazine (94) to give a mixture of diacylhydrazine monoenamine (95) and oxadiazole (88c), (Scheme 6) which was also formed from (95) on heating <88LA909). 

Representatives of this novel class of meso-ionic compounds in which the exocyclic substituent f (see Table I) is a stabilized carbanionoid residue [-C(CN)C02Me or -C(CN)2l have been recently synthesized. Base-catalyzed (potassium carbonate or triethylamine) condensation of AT-acylhydrazines (147) with 3,3-dichloroacrylonitriles (165) yield the greenish-yellow meso-ionic l,3,4-oxadiazol-2-enes (164) directly. 

Substituted acylhydrazones (95), formally derivatives of 1,2-diacylhydrazines (75), also cyclize to oxadiazoles (76), with loss of HX. Imidol ethers (95a R2 = H), prepared from acylhydrazines R1CONHNH2 and ortho esters HC(OR)3, cyclize on heating to monosub-stituted oxadiazoles (76 R1 = H). This reaction is an important synthesis of monosubstituted... 

Oxadiazoles 165 result when W.W-diacylhydrazines are treated with the triphenylphosphiiie-carbon tetrachloride adduct in the presence of triethylamine <97PJC77>. Acylhydrazines 166 (R = Me, i-Pr, n-Bti, Ph or Bn) react with ketenylidenetriphenyl-phosphine 167 in boiling xylene to yield the 1,3,4-oxadiazoles 168 (Scheme 9) <97SL283>. 

Oxadiazoles from N-acylhydrazines and carboxylic acids Thermal conversion... 

Synthesis from Acylhydrazines. The condensation of acyl hydrazines and ethyl dithioacetate in boiling ethanol furnishes 5-aryl-2-methyl-l,3,4-thiadiazoles, but 1,3,4-oxadiazoles are formed as main products in some cases. Under suitable conditions, the intermediate thioacetylacylhydrazines may be isolated. ... 

Oxadiazoles 8 [465] are obtained by acid-promoted cyclocondensation of carboxylic hydrazides (acylhydrazines) (a) with acid chlorides in the presence of P4O10 [466] and (b) with orthoesters catalyzed by alum (KA1(S04)2 12 H2O) [467] ... 

Historical examples of syntheses by cyclodehydration of bis-perfluoro-acylhydrazines with P2O3 were reported by Brown et al. [31 ] as well as by Chambers and Coffman [32]. By using the same approach, a series of symmetrically and asymmetrically substituted 2,5-bis(polyfluoroaryl)-l,3,4-oxadiazoles 60 can be prepared in excellent yields (Scheme 18) [33]. 

Scheme 18 Synthesis of 2,5-bis(polyfluoroaryl)-1,3,4-oxadiazoles 60 by cyclodehydration of bis-perfluoro-acylhydrazines with P2O5...

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