Oxadiazolin-5-ones

A -l,3,4-Oxadiazolin-5-one, 4-alkyl-2-(4-pyridyl)-biological activity, 6, 445 A -l,3,4-Oxadiazolin-5-one, 2-aryl-alkylation, 6, 431... 

The synthesis and properties of heat-resistant polyazomethines containing 2,5-disubstituted oxadiazole fragments, being insulators convertible into semiconductors by doping with iodine, have been described. The radical copolymerization of alkenes with the fluorescent co-monomer 2-/-butyl-5-(4 -vinyl-4-biphenylyl)-l,3,4-oxadiazole has resulted in useful macromolecular scintillators. Anionic polymerization of 2-phenyl-l,3,4-oxadiazolin-5-one has produced a nylon-type product <1996CHEC-II(4)268>. 

Reaction of 2-mercaptomethyl-l,3,4-oxadiazolin-5-one 321 with o-aminophenol 322 and glycine, in refluxing isopropanol, gives 3-mercaptomethyl-[l,2,4]triazole[3,4-A benzoxazole 71, via intermediate 323 (Scheme 31), and 3-mercaptomethyl-6-oxo-oxazolo[2,3-c][l,2,4]triazole 324, respectively <1996M549>. 

The crystal structures of 5-methyl-5-(2-methylprop-l-enyl)-2-phenyl-4-(4-phenylthiazol-2-yl)-A -1,3,4-oxadiazoline <90ZC26> and the rubidium and silver salts of 2-phenyl-A -l,3,4-oxadiazolin-5-one <85JOC4461> have been analyzed. A crystal study of bis-[2-(5-phenyl-l,3,4-oxadiazol-2-yl)-methyl] ether produced bond lengths and angles and showed the rings to be nearly coplanar <87AX(C)2166>. 

Irradiation of A -cyclohexyl-C-phenylsydnone (1, R = cyclohexyl, R = phenyl) in benzene under a nitrogen atmosphere gave the 1,3,4-oxadiazolin-5-one (317, R = cyclohexyl, R = phenyl 15%), the bisazoethylene (318, R = cyclohexyl, R = phenyl 15%), and the triazole (320, R = cyclohexyl, R = phenyl 12%). When the nitrogen atmosphere was replaced by carbon dioxide, then the yield of the 1,3,4-oxadiazolin-5-one was increased from 15% to 47%. These results provide good support for the general mechanistic proposals given in Fig. 4. 

When the hydroxy proton in an amidoxime is replaced by a carbethoxy group (30), cyclization leads to an oxadiazolin-5-one (31) as shown in Eq. (32). An obvious extension is to use an A-alkyl-amidoxime118 to give an A(4)-alkyloxadiazolin-5-one. 

When ethyl iodide acts on the silver salt of the 5-hydroxy derivative (83), a 1 1 mixture of the O- and A-ethyl derivatives (84 and 85) are obtained. Evidently 85 is thermodynamically the more stable of the two isomers, since on standing for 3 days as much as 90% of 4-ethyl-3-methyl-l,2,4-oxadiazolin-5-one (85) is formed. Only 84 can be converted to the amino derivative 86. 

Diphenyl-1,2,4-oxadiazolin-5-one undergoes a similar cleavage with evolution of carbon dioxide on photolysis in dioxane [Eq. (18)].66 The reactive intermediate formed on photolysis of 2,4-diphenyl-l,3,4-oxadiazolin-5-one forms dihydropyrazole, pyrazole, and 1,2,4-triazole derivatives by cycloaddition to conjugated alkenes, acetylenes, and nitriles [Eq. (19)], respectively.66... 

A2-1,3,4-Oxadiazolin-5-ones acylation, 6, 439 alkylation, 6, 439 biological activity, 6, 445 melting point, 6, 430 synthesis, 6, 441, 442 tautomerism, 6, 430 A3- l,3,4-Oxadiazolin-2-ones 5,5-disubstituted IR spectra, 6, 429 PE spectra, 6, 430 A3-1,3,4-Oxadiazolin-5-ones ionization potential, 6, 428... 

The chemical shifts of the ring protons are lowered by quaternization as shown by the values of S (TFA) 10.7 and 9.3 p.p.m. for the 2- and 5-protons in 3-phenyl-l,3,4-oxadiazolium perchlorate. A S (CDC13) value of 2.58 p.p.m. for the methyl protons in 2-methyl-l,3,4-oxadiazole reflects the low electron density at the ring carbon atom. The value of the chemical shift of the proton or protons in the O-alkyl or A-alkyl group assists in differentiating between isomeric 2-alkoxy-l,3,4-oxadiazoles and 4-A-alkyl-A2-1,3,4-oxadiazolin-5-ones. 

Hydroxy- (8a), 2-mercapto- (8b) and 2-amino-l,3,4-oxadiazoles (8c) are in equilibrium with the tautomeric oxadiazolines (9a), (9b) and (9c) respectively. Evidence from UV (72CJC3079) and IR (Section 4.23.2.2.3) spectra supports structure (9a) for A2-l,3,4-oxadiazolin-5-ones and structure (9b) for A2-l,3,4-oxadiazoline-5-thiones. The UV and IR spectra, fluorescence and p/iT values of 2-amino-l,3,4-oxadiazoles indicate that the amine tautomer (8c or d) rather than the imine tautomer (9c or d) predominates (69BSF870, 874, 64CR(258)4579>. 

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