1.2.4- Oxadiazol-4-oxides

The removal of oxygen from 1,2,4-oxadiazole 4-oxides and polymer-supported analogues 187 can be achieved with triethyl- or trimethylphosphite, as the example in Equation (30) shows <2005JC0887>. 

The cycloaddition of nitrile oxides to amidoximes 234 leads to 1,2,4-oxadiazole 4-oxides which can then be deoxygenated with trimethyl phosphite (Equation 48) <1997T1787>. 

The reaction of hydroximoyl chlorides 403 with amidoximes 402 in the presence of TEA leads to 1,2,4-oxadiazole 4-oxides 404 via 1,3-dipolar cycloaddition and elimination of an amine (Equation 77) <1997T1787, 2005JC0887>. 

These routes are dimerization to furoxans 2 proceeding at ambient and lower temperatures for all nitrile oxides excluding those, in which the fulmido group is sterically shielded, isomerization to isocyanates 3, which proceeds at elevated temperature, is practically the only reaction of sterically stabilized nitrile oxides. Dimerizations to 1,2,4-oxadiazole 4-oxides 4 in the presence of trimethylamine (4) or BF3 (1 BF3 = 2 1) (24) and to 1,4,2,5-dioxadiazines 5 in excess BF3 (1, 24) or in the presence of pyridine (4) are of lesser importance. Strong reactivity of nitrile oxides is based mainly on their ability to add nucleophiles and particularly enter 1,3-dipolar cycloaddition reactions with various dipolarophiles (see Sections 1.3 and 1.4). 

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). 

Bis-[2,4,6-trimethyl-phcnyl]-1,2,4-oxadiazol-4-oxid-Trifluorboran-Komplex 67% Schmp. 177I ... 

Unter dem EinfluB iiberschiissiger Mengen Triethylamin dimerisieren die Carbonsaure-nitril-oxide zu den Furoxanen II130. Uberraschend ist hier jedoch der EinfluB von Tris-[2-hydroxy-ethyl]-amin, das die Dimerisierung wieder in Richtung der 1,2,4-Oxadiazol-4-oxide als Haupt-produkt verschiebt130 z. B. ... 

Durch Reduktionsmittel, z. B. Phosphor(V)-chlorid128 oder mit Zinkstaub in Eisessig5fI werden 1,2,4-Oxadiazol-4-oxide deoxygeniert ... 

Oxadiazole 4-oxides 157 (Ar, Ai = Ph or/ -Tol) are formed from amidoximes 155 and nitrile oxides 156 <97X1089, 97T1787>. l,2,4-Oxadiazolidine-3,5-dione 158 is benzylated at N-2 by benzyl bromide <97SL263>. 3,6-Dimethylpyridazinol4,5-c]furoxan 159 is converted into 3-acetyl-4-(l,l-dinitroethyl)furoxan 160 by the action of nitric acid <96ZOR957>. 

Potassium salt 1,2,4-Oxadiazole 4-oxides from nitrosolic acids and halides... 

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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