1,3,4-Oxadiazolium

More than forty x-ray diffraction analyzes for 1,2,4-oxadiazoles and a 1,2,4-oxadiazolium salt <86JMC1540> have been reported and data for compounds (I)- 5) are presented as representitive. 

Nitrilium-Salze addieren Nitriloxidc unter Bildung von 1,2,4-Oxadiazolium-Salzen z. B. erhiilt man aus N-tert.-Butyl-bcnzoni trilium-hexachloroantimonal und 2,4,6-Trimethoxy-benzonitriloxid (Chloroform — 70°) 85% 4-Isopropyl-5-phenyl-3-( 2,4,6-trimethoxy-phenyl)-1,2 A-oxadiazolium-hexachloroantimonat... 

Bei der Umsetzung von Bis-[diphenyl-methylen]-ammonium-Salzen (hergestellt aus geminalen Dihalogeniden und Carbonsaure-nitrilen in Gegenwart von Antimon(V)-chlorid) mit N-Phenyl-hydroxylamin erhalt man 2,3,5-Triphenyl-1,2,4-oxadiazolium-hexachloroantimonat( /) (70% Schmp. 146-1480)276 ... 

In Gegenwart von Perchlorsaure cyclisieren 3-(3-Oxo-l-alkenylamino)-l,2,4-oxadiazole unter Anellierung zu 1,2,4-Oxadiazolium-perchloraten, die in speziellen Fallen als Salze isoliert wer-den konnen278 z.B. ... 

Voneinigen 1 2 4-Oxadiazolium-Salzen ist ihre leichte Hydrolyse in Wasser278 bzw. in waBriger Salzsaure253 oder die Isomerisierung mit Triethylamin zu Carbodiimidcn bckannt252 z.B. ... 

Triazolium, Oxadiazolium, Thiadiazolium, Dioxazolium, Oxathiazolium, Dithiazolium ... 

Reaction of the A-nitrosoglycine (394) with acetic anhydride gave the anhydro-5-hydroxy-l,2,3-oxadiazolium hydroxide (395). Reaction with DMAD resulted in formation of the intermediate 1 1 cycloadduct (396) which was not isolated and which lost CO2 under the thermal reaction conditions to give dimethyl l-phenylpyrazole-3,4-dicarboxylate (397) (83MI40300). This reaction is capable of considerable variation in terms of the substituents... 

Oxadiazolium-2-aminides mass spectra, 6, 429 ring cleavage, 6, 435... 

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

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

Disubstituted triazolium salts are prepared from alkyl or aryl hydrazines via an oxadiazolium salt 28 (Scheme 16). Addition of a chiral amine on this salt resulted in a ring opening - ring closure reaction affording the triazolium salts 29. 

We have found that the chemical sulfation of N-methyl-N-(2-hydroxyethyl)nitrosamine resulted in the direct formation of the oxadiazolium ion. 

The reactions of the oxadiazolium ions with nucleophiles such as thiophenolSy guanine, and guanosine resulted in methylation of the nucleophiles (53). Thus, the formation of the oxadiazolium ion creates an electrophilic species not... 

The reaction of nitrile oxides with nitrilium salts, which are more reactive than the corresponding nitriles, as discussed in CHEC-II(1996) <1996CHEC-II(4)179>, is the main route through to oxadiazolium salts. Further advances in this area have not been reported. 

The 170 NMR spectrum of 4,5-diphenyl-l,3,4-oxadiazolium-2-olate was reported earlier <1996CHEC-II(4)268>. Since then, no report on 170 NMR spectra of 1,3,4-oxadiazoles has been published. 

Substituted tetrazoles reacting in the mass spectrometer with acyl ions afforded 2,5-disubstituted 1,3,4-oxadiazoles with nitrogen loss. Tandem mass spectrometry allowed for the collision-induced dissociation of the products. Chemical ionization was the better method to make the transformation. A scheme for the transformation of 5-substituted tetrazoles into 2,5-disubstituted 1,3,4-oxadiazoles was proposed (Scheme 1) <2001JMP1069>. The fragmentation patterns of monocyclic l,3,4-oxadiazolium-2-thiolates have been proposed by Ollis and Ramsden <1974J(P1)645>. 

The reactions of substituents on ring nitrogen atoms in 1,3,4-oxadiazolium cations 2 and exocyclically conjugated mesoionic 1,3,4-oxadiazoles 3 are uncommon. Similar reactions of compounds 4 and 5 were reported occasionally. Biphenylmethylation of 2-trifluoroacetamido-l,3-4-oxadiazole 105 occurred regioselectively (Scheme 23), affording only the 2-acylimino derivative instead of the expected product 106 <1997H(44)133>. 

Chloro(hydroxyimino)methyl]-3-phenyl-l,2,3-oxadiazolium-5-olate-(3-phenylsydnone-4-carbohydroximoyl chloride) reacts in situ (through nitrile oxide) with N-arylmaleimides or 2-methyl-N-phenylmaleimide to give 5-aryl-3-(3-phenylsydnon-4-yl)-3a,6a-dihydropyrrolo[3,4-r/]isoxazole-4,6-diones or dame thyl-5-pheny l-3-(3-phenylsydnon-4-yl)-3a,6a-dihydropyrrolo[3,4-d]isoxazole-4,6-diones, respectively (265). 

There is an early report by Westphal and Henklein that oxadiazolium salts 17 cyclize in boiling water to give salts of imidazo[2,l-6][l,3,4]oxadia-zoles (18), which on treatment with sodium hydroxide yield the corresponding free bases (69ZC25). But Hetzheim notes that these results are in error... 

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