2- Aryl-1,3,4-oxadiazoles

Table 47 Reaction of 2-aryl-1,3,4-oxadiazoles with arylamines to give 3,4-diaryl-1,2,4-triazoles (Equation 70)...

Most of the freely soluble 2-aryl-1,3,4-oxadiazoles can be hydrolyzed in acid or alkaline solution even more readily than the 2-alkyl-5-aryl-... 

The acid hydrolysis of the 2-aryl-1,3,4-oxadiazoles can be used for their analytical determination. The method used is either to break down the compound by heating with hydrochloric acid under reflux to give the acid hydrazide and then to titrate the hydrazide with iodide in bicarbonate solution,68, 69 or to titrate potentiometrically directly with sodium nitrite in a hydrochloric acid medium.128 In this way the acid hydrazide is formed in the first reaction step and is then converted into the insoluble azide by the sodium nitrite. 

Catalytic amounts of Co(OAc)2 or Mn(OAc)2 in combination with ferf-butyl hydrogen peroxide (1.2 equiv.) as oxidant proved to be effective for amination of benzoxazoles, benzothiazoles, and 2-phenyl-1,3,4-oxadiazole [95]. It is worth noting that in amination of benzoxazoles with secondary aliphatic amines the cobalt catalyst provides better results, while the manganese catalyst is more effective for a similar reaction with primary alkylamines and ammonia. Also the reactions of benzoxazoles with secondary aliphatic amines or formamides have been performed successfully with Cu(OAc)2 (20 mol.%) and oxygen as oxidant [96]. Other azoles do not react with primary alkylamines in the presence of this oxidant. In contrast, when FeCls was used in 0.25-1 equiv. amounts, amination of benzoxazoles and 2-aryl-1,3,4-oxadiazoles (with the exception of their nitro derivatives) proved to... 

Grignard reagents or alkyllithium addition to methyl 2-aryl-l,3,4-oxadiazol-5-yl formates afforded a series of 2-acyl-5-aryl-oxadiazoles (Equation 9) <2005BML1423>. 

Title Soluble Poly(Aryl-Oxadiazole) Conjugated Polymers... 

Kambe [3] prepared electroluminescent devices containing two or more stacked organic layers, one of which consisted of an electron injecting an organic layer of conjugated poly(aryl-oxadiazole) derivatives, (111) and (IV). 

Oxadiazole, 2-amino-5-aryl-biological activity, 6, 445 reactions, 6, 439... 

Oxadiazole, 5-aryl-2-(2-carboxyphenyl)-plant growth regulator, 6, 445... 

Recently, the reaction of 3-methoxy-5-aryl-l,2,4-oxadiazoles in the presence of diphenylacetylene to give the corresponding quinazolinones has been reinvestigated and an electron transfer mechanism was proposed (99JOC7028). 

Finally, a series of 2-chloromethyl-5-aryl-1,3,4-oxadiazoles 82 were prepared by reaction of aromatic hydrazides 81 and a chloromethylorthofor-mate used as the solvent under microwave activation [62]. Potentially, the chloromethyl group could imdergo nucleophiUc substitution expanding the scope of this reaction (Scheme 28). 

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-... 

Alkyl- and 5-aryl-2-amino-1,3,4-oxadiazoles were prepared by tosyl chloride/pyridine-mediated cyclization of thiosemicarbazides in good yields (79-99%). Interestingly, thiosemicarbazides exhibited a higher rate of cyclization than the corresponding semicarbazides. For example, 171 (X-S) was converted to oxadiazole 172 within 5 h <06JOC9548>. 

Electrochemical reduction of 3-phenylsydnone 89 and its 3-(4-methoxy)phenyl and 3(4-methyl)phenyl analogues represents a new method for the preparation of 2,4-dihydro-3-aryl-l,2,3-oxadiazole-5-ones <2006CCA273, 2006MI776>. The products were isolated in 82 to 88% yield, and their proposed structures are supported by melting point, elemental analysis, IR, proton NMR, and mass spectral data. 

Aryl-4,5-dihydro-l,2,4-oxadiazoles 123 produced the quinazolines 125 on heating in acetic anhydride this process is proposed to proceed via the acetylated diaza-1,3-butadiene intermediate 124 (Scheme 13) C1999AXC2158, 2002PJC1137, 2003TL2015>. 

The reaction of the stable and readily available N-protected (a-aminoacyl)benzotriazoles 215 (Equation 36) with amidoximes 206 in ethanol gave the N-protected 5-amino-substituted 1,2,4-oxadiazoles 216 in high yield, under mild conditions and with good (>97%) retention of chirality <2005ARK36>. The method is also applicable to aromatic (V-acylbenzotriazoles, giving access to 5-aryl-l,2,4-oxadiazoles in 73-82% yield. 

Young and DeVita have shown that 1,2,4-oxadiazoles can be prepared by a new route in a novel one-pot procedure by the palladium-mediated coupling of an aryl iodide with an amidoxime in the presence of carbon monoxide (Equation 40) <1998TL3931>. 

The oxidation of aromatic aldoximes with ceric ammonium nitrate produces nitrile oxides which undergo subsequent cycloaddition to nitriles to produce 1,2,4-oxadiazoles (Equation 47) <1997PJC1093>. The anodic oxidation of aromatic aldoximes in the presence of acetonitrile has been reported to give low yields of either 3-aryl-5-methyl-1,2,4-oxadiazoles (2-25%) or 3,5-bis-aryl-l,2,4-oxadiazoles (6-28%), although the synthetic utility of this route is limited by competitive deoximation to the carbonyl being the major reaction pathway <1997MI3509>. 

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