1, 3, 4-Oxadiazolium salts

Oxadiazolium salts cycloaddition reaction, 6, 435 ring cleavage, 6, 434 synthesis, 6, 440... 

Oxadiazolium salts, 3-aryl-2-methyl-reaaions with aldehydes, 6, 439... 

Very little work has been reported on the 1,3,4-selenadiazole system. Ring-opening of 1,3,4-oxadiazolium salts (132) with hydrogen selenide gives 1,2-disubstituted hydrazines (133) which are cyclized with perchloric or fluoroboric acid to give 1,3,4-selenadiazolium salts (134) (Scheme 9)... 

On treatment with ethyl cyanoacetate and base, the fused 1,3,4-oxadiazolium salt 141 underwent ring opening with subsequent closure to the fused pyrazole 143 in low yield, probably via the intermediate 142.146 This reaction proceeds by analogy with the reactions of mono-cyclic oxadiazolium salts,147 but a similar intermediate 145 obtained from the fused thiadiazolium salt 144 failed to cyclize to a bicyclic system under a variety of conditions.146... 

The 1,3,4-oxadiazolium salts (288) react with hydrazines to give the dihydro-1,2,4,5-tetrazines (290) and 4-amino-1,2,4-triazoles (76KGS629, 74ZOR377, 7lDOK(200)134,... 

The 1,3,4-oxadiazolium salts (288) react with hydrazines to give the dihydro-1,2,4,5-tetrazines (290) and 4-amino-l,2,4-triazoles (76KGS629, 74ZOR377, 71DOK(200)134, 71MIP22100). This reaction probably proceeds via the intermediates (289). Similarly, 2-methylmercapto-l,3,4-oxadiazolium salts (291) and hydrazines afford the 1,4-dihydro-l,2,4,5-tetrazin-6(5//)-ones (292) (76CZ496). 

Related studies employing (V-protonated vinyl-165 or aryliminium salts,64 161 Lewis salts of aryl-63a and vinylimines,63b 1,3,4-oxadiazolium salts,166 and nitrilium salts167 constitute additional examples of reaction processes proceeding through cationic azadienes which formally participate in [4+ + 2] cycloaddition reactions.168... 

Amino-1,2,4-triazoles (50) or their derivatives are usually starting material for the synthesis. They are readily available and already contain the exocyclic N-N bond required for heteroaromatic A-imines. 4-Amino-1,2,4-triazoles can be quaternized by alkyl halides or tosylates at the N-l atom to give the salt 52.79-82 (Scheme 5) The orientation of quatemization is proved by the reactions in Scheme 6 for the example of the quaternary acylamino salts 52. Quaternary salts of the type 52 can also be prepared by reaction of 1,3,4-oxadiazolium salts (51)83 with aryl hydrazines84 and from aryl hydrazine hydrohalides and orthoesters.86 With alkali, the 1-alkyl-s-triazole-4-imines8(7-82 86 can be obtained in the normal manner from these salts (Scheme 5). The free A-imines are all stable except the A-unsubstituted compound itself.87 Recently, other structures were tentatively reported 88 for the deprotonation products of analogous quaternary salts (52) with hydrazine. 

Replacement of cyclic oxygen by cyclic nitrogen in 1,3,4-oxadiazolium salts... 

A suspension of the startg. 1,3,4-oxadiazolium salt in abs. ethanol treated with ethyl cyanoacetate and triethylamine, refluxed overnight ethyl 3-amino-l,5-diphenylp5 azole-4-carboxylate. Y 78%. F. e. s. G. V. Boyd and S. R. Dando, Soc. (C) 1971, 225. 

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