2-aryl-l,3,4-oxadiazoles

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). 

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. 

Aryl-l,2,4-oxadiazole allgemeine Arbeitsvorschrift250 3 mmol 3-(l-Hydroximino-alkyl)-4-oxo-3,4-dihy-dro-chinazolin werden in 30 ml 15%igc Salzsaure lind 30ml Ethanol 8h gekocht. Man stellt dann mit Natriumcarbonat alkalisch und isoliert die kristallinen Produkte durch Absaugen. 

From 3-Aroylamino-5-methyl- and 3-Acetamido-5-aryl-l,2,4-oxadiazoles... 

Oxadiazoles. N-Aroyl-N N -dimethylformamidines, ArCON—CHNMea [which are prepared by condensing dimethylformamide dimethyl acetal, Me2NCH(OMe)2, with amides ArCONH2], are converted into 5-aryl-l,2,4-oxadiazoles (603) by the action of hydroxylamine. The 3-benzyl-5-methyl-derivative (604) is formed when iV-acetyl-A-nitrosophejiylalanine is irradiated in the presence of triethylamine. Nitrones R CH—N(Bu )—6 add isothiocyanates R NCS to form the thiones (605 R , R == alkyl or aryl). ... 

A palladium(II)-catalysed [3,3]-aza-Claisen formal sigmatropic rearrangement of 3-allyloxy-5-aryl-l,2,4-oxadiazoles indicates the intervention of a cationic palladacycle similar to the one postulated for the Cope rearrangement of 1,5-dienes (Scheme 141). " °... 

Buscemi, S., Macaluso, G., and Vivona, N., Heterocyclic photorearrangements — photochemical behavior of some 3-acetylamino-5-aryl-l,2,4-oxadiazoles. A photoinduced iso-heterocydic rearrangement, Heterocycles, 29,1301, 1989. 

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 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>. 

Aziridinylbenzaldoxime 340, formed from the reaction of a hydroximoyl chloride with aziridine 339 (Scheme 56), reacts with HG1 to form the chloroalkyl-substituted amidoxime 341. Reaction with sodium hydride affects ring closure to give the 3-aryl-4,5-dihydro-5-isopropyl-l,2,4-oxadiazoles 343. This latter reaction is proposed to proceed... 

When parameters of the Pariser-Parr-Pople configuration interaction molecular orbital (PPP-CI MO) method were modified so as to reproduce the Aol)s values for l,3-di(5-aryl-l,3,4-oxadiazol-2-yl)benzenes 16 and 17, the calculated HOMO and LUMO energy levels corresponded with the experimental ionization potential and electron affinity values. The relationships between the electrical properties and molecular structures for the dyes were investigated. The absorption maximum wavelengths for amorphous films were found to be nearly equal to those for solution samples <1997PCA2350>. 

More recently, the El and electrospray ionization (ESI) mass spectra of 2,5-diaryl- and 2-arylamino-5-aryl-l,3,4-oxadiazoles, as well as their complexes with copper cations, were studied. Under ESI conditions, loss of NH3 and HNCO, from complexes of 2,5-diphenyl-l,3,4-oxadiazole, 2,5-bis(2-pyridyl)-l,3,4-oxadiazole, or 2,5-bis(4-pyridyl)-1,3,4-oxadiazole with copper cation, was observed <2004JMP272>. An unusual elimination of isocyanic acid was found in fragmentation of some protonated 2,5-diaryl derivatives <2002RCM390>. 

Alkyl(or aryl)amino-5-aryl-l,3,4-oxadiazoles were earlier successfully obtained on solid support as shown in Scheme 32 <2001TL2583>. 

I.3.4.2.4. Heterocumulenes The 1,3-dipolar cycloaddition of substituted ben-zonitrile oxides to the C=N group of chlorocarbonyl isocyanate C1C(0)N=C=0 gives 3-aryl-4-chlorocarbonyl-5-oxo-4,5-dihydro-l,2,4-oxadiazoles 172 in 75%-80% yield (340). A similar reaction with chlorosulfonyl isocyanate, C1S02N=C=0, affords 4-unsubstituted oxadiazolinones 173 (341). 

The carbon-nitrogen triple bond of aryl thiocyanates acts as a dipolarophile in 1,3-dipolar cycloadditions. Reactions with nitrile oxides yield 5-arylthio-1,2,4-oxadiazoles 227 (X = O Y = S). Aryl selenocyanates behave similarly forming 5-arylseleno-l,2,4-oxadiazoles 227 (X = 0 Y = Se). Reactions of 5-aryl-... 

The reaction of 2-amino-5-aryl-l,3,4-oxadiazoles 449 with chloroacetic acid afford imidazo[2,l- ][l,3,4]oxadiazoles 450 (unreported yield) (Equation 101) <1996JIC702>. 

The cyclocondensation of 3-acyl/3-aroyl-5-aryl[l,3,4]oxadiazol-2(3//)-ones 98 with hydrazine hydrate gives the corresponding 3,6-disubstituted-[l,2,4]triazolo[3,4- ][l,3,4]oxadiazoles 99 (Equation 26) <1991H(32)237>. 

Alkylation of 2-amino-5-(alkyl, aryl, hetaryl)-l,3,4-oxadiazoles is said to occur at the ring N atom (N-3) (94HOU526), but this report remains doubtful. In addition, as others have pointed out, the very low basicity of imidazo[2,l-b][l,3,4]oxadiazoles precludes the isolation of hydrohalides of type 20. 2,6-Diaryl-imidazo[2,l-ft][l,3,4]oxadiazoles 22 can be obtained directly by treatment of 2-amino-5-aryl-l,3,4-oxadiazoles 21 with phenacyl-bromides in ethanol [reflux in a water bath at 70°C ( ) for 5 h] (84MI1). 

A similar procedure was applied in the production of 3-(3-aryl-l,2,4-oxadiazol-5-yl)propanoic acids succinic anhydride was used instead of the acid chloride and the yields of these reactions were similar to the previous trials (46-62%). 

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