1.2.5- Oxadiazole 2-oxides, cleavage

Oxidative cleavage of pyrazol-4,5-dione-4-[7V-(pyrazol-4-yl)]hydrazone 658 occurred when this compound was oxidized with 4-chloroperbenzoic acid (73G179) (Scheme 193) to give pyrazole diazonium salt 659 in 72% yield, oxadiazol-5-one 660 in 0,5% yield, hydrazone derivative 661 in 10% yield and the bispyrazole 662 in 17% yield. Oxidation of hydrazone 663 under similar conditions... 

Other non-traditional preparations of 1,2,3-triazoles have been reported. The rearrangement in dioxane/water of (Z)-arylhydrazones of 5-amino-3-benzoyl-l,2,4-oxadiazole into (2-aryl-5-phenyl-27/-l,2,3-triazol-4-yl)ureas was investigated mechanistically in terms of substituents on different pathways <06JOC5616>. A general and efficient method for the preparation of 2,4-diary 1-1,2,3-triazoles 140 from a-hydroxyacetophenones 139 and arylhydrazines is reported <06SC2461>. 5-Alkylamino-] //-], 2,3-triazoles were obtained by base-mediated cleavage of cycloadducts of azides to cyclic ketene acetals <06S1943>. Oxidation of N-... 

Sealants obtained by curing polysulfide liquid polymers with aryl bis(nitrile oxides) possess stmctural feature of thiohydroximic acid ester. These materials exhibit poor thermal stability when heated at 60°C they soften within days and liquefy in 3 weeks. Products obtained with excess nitrile oxide degrade faster than those produced with equimolar amounts of reagents. Spectroscopic studies demonstrate that, after an initial rapid addition between nitrile oxide and thiol, a second slower reaction occurs which consumes additional nitrile oxide. Thiohydroximic acid derivatives have been shown to react with nitrile oxides at ambient temperature to form 1,2,4-oxadiazole 4-oxides and alkyl thiol. In the case of a polysulfide sealant, the rupture of a C-S bond to form the thiol involves cleavage of the polymer backbone. Continuation of the process leads to degradation of the sealant. These observations have been supported by thermal analysis studies on the poly sulfide sealants and model polymers (511). 

Substituted 1,2,4-oxadiazoles were prepared by addition of nitrile oxides to imines or hydrazones. It has been reported that interaction of hydroximoyl chlorides 262 with chiral hydrazones 263 in the presence of EtsN leads to intermediates 264 with diastereoselectivity up to 97%. A subsequent N-N bond cleavage to remove chiral auxiliary by formic acid leads to 1,2,4-oxadiazolines 265 with ee up to 91% (equation 113). ... 

Oxadiazoles undergo thermal and photochemical ring cleavage at the 0(1)-N(2) and C(3)-C(4) bonds to yield nitrile and nitrile oxide fragments, and products derived therefrom. Thus, diphenylfurazan (30, X = 0) decomposes under flash vacuum pyrolysis conditions (600°C, 10-3 mm... 

Reaction with alkynic 1,3-dipolarophiles gives the primary adducts (74) which are not isolated. Thermal cleavage of the oxadiazole ring gives the nitrile oxide valence tautomer... 

The mass spectrometry of 1,2,4-oxadiazoles is dominated by stepwise 1,3-dipolar cycloreversion, i.e., fragmentation 191. This fragmentation is particularly useful for 1,2,4-oxadiazole characterization and a wide selection of derivatives undergo cleavage to a nitrile oxide fragment <2003H(60)2287, CHEC-III(5.04.3.3)249>. Mass spectrometric analysis of 1,2,4-oxadiazoles and dihydro-1,2,4-oxadiazoles has been reviewed <2005MI328>. 

A new method for the synthesis of 1,1-dialkyldiazomethanes (2.101) has been developed by Warkentin s group (Majchrzak et al., 1989). A-Acylhydrazones of ketones (2.99) are oxidized with lead tetraacetate to 2,5-dihydro-l,3,4-oxadiazoles (2.100). This intermediate undergoes a photolytic cleavage if irradiated with UV light (300 nm) at room temperature in benzene (2-45). 

Oxadiazoles.—1,2,3-Oxadiazoles. 3-Phenylsydnone (594 R = H) is anodically chlorinated at position 4, while anodic oxidation yields mainly phenol and benzaldehyde. The foregoing sydnone reacts with tetracyanoethylene to give the ene-hydrazone (596), presumably via the cyclo-adduct (595), by loss of carbon dioxide and cleavage of the strained carbon-carbon single bond. Irradiation of diphenylsydnone (594 R = Ph) in the presence of methyl pro-piolate affords the pyrazole (597) by way of the nitrile imine PhC=N—NPh with benzonitrile, a mixture of the 1,2,3- and 1,2,4-triazoles (598) and (599), together with the 1,3,4-oxadiazolinone (600) and 7s/ -benzoyl-iV -phenylhydrazine, is formed. ATV -Dibenzoylphenylhydrazine is produced by photo-oxygenation of diphenylsydnone. 

Diaryl-1,2,4-oxadiazole 4-oxides undergo photochemical cleavage to give, in addition to an arylni-trile, a nitrosocarbonyl intermediate that can be trapped by dienes and 2,2-dimethyl-2H-benzLmidazole 1,3-dioxide cleaves to 0-(2-nitrosophenyl)acetone oxime. ... 

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