3- Amino-5-phenyl-l,2,4-oxadiazole

The irradiation of 3-amino-5-phenyl-l,2,4-oxadiazole (98) gave the corresponding 1,3,4-oxadiazole (99) (Scheme 41) [88JCS(P1)1313 88JHC931 95FI(41)2095). The formation of 99 has been explained by assuming a RCRE mechanism. 

Fig. 21. Relative energy of the exeited states of 3-amino-5-phenyl-l,2,4-oxadiazole and of some reaetive intermediates.

Amino-5-phenyl-l,2,4-oxadiazole and 1 gave enamino-ketone 188 that on irradiation with a low-pressure mercury lamp gave benzimidazole 189 as a result of a photoinduced rearrangement of the 1,2,4-oxadiazole ring (88JHC1551) (Scheme 32). 

A literature report57 indicated that 3-amino-5-phenyl-l,2,4-oxadiazole 76 (R = Ph) reacted with ethoxycarbonylisothiocyanate in refluxing ethyl acetate to give the stable thiourea 79. A reexamination30 of this reaction showed that the product was the rearranged 1,2,4-thiadiazole 80 probably obtained through the intermediate formation of 79. 

Zur photo-katalysierlcn Ringoffnung von 3-Acylamino-furazanen in Gegenwart prim, und sek. Amine zu 3-Amino-l,2,4-oxadiazolen s.Lit 356 (z.B. 3-Methylamino-5-phenyl-l,2,4-oxadiazol, 45%, aus 4-Ben-zoylamino-3-methyl-furazan und Methylamin). 

Amino derivatives of 1,2,4-oxadiazoles, isoxazoles, and 1,2,5-oxadiazoles interact with phenyl isocyanate to produce various 3-substituted 5-amino-l,2,4-thiadiazoles, via intermediate thioureides which can be isolated. The tendency to rearrange follows the order 1,2,4-oxadiazoles, isoxazoles, and 1,2,5-oxadiazoles <1996CHEC-II(4)307>. 

Najer and co-workers130,131 reported a higher frequency (1660 cm-1) for the C=N band in 5-amino-3-phenyl-l,2,4-oxadiazole. 

Buscemi, S., Vivona, N. and Caronna, T. (1995) A generalized and efficient synthesis of 3-amino-, 3-(N-alkylamino)-, 3-(N, N-dialkylamino)-5-alkyl-l, 2,4-oxadiazoles by irradiation of 3-alkanoylamino-4-phenyl-l, 2,5-oxadiazoles (furazans). Synthesis, (8), 917-919. 

Enough 1,2,4-oxadiazoles and their reduced derivatives have been made that there is general agreement on where to expect the C=N absorption in the IR spectrum, i.e. 1560-1590 cm-1. In the partially reduced (oxadiazoline) ring the A2 double bond appears at 1550-1565 cm-1 and the NH bands at 3320 and 3220-3250 cm-1. In an amino derivative, 5-amino-3-phenyl-l,2,4-oxadiazole, the C=N bond absorbs at higher frequency (1660 cm"1) (63PMH(2)229>. 

The formation of l-amino-s-triazoline-5-ones in low yield on thermolysis of 3-phenyl-5-hydrazino-l,2,4-oxadiazoles [Eq, (50)] [81JCS(Pl)1703] is known. The best method of synthesizing 1-aminotriazolinones includes direct amination of 3-R-triazolinones. Thus, amine 193 was obtained in 76% yield [81 JCS(P1) 1703]. 

A 3 2 mixture of coned. H2SO2 and HNO3 (d. 1.52) added slowly at —5 to 0° to a soln. of 3-phenyl-5-acetamidomethyl-l,2,4-oxadiazole in coned. H2SO4, and the mixture dil. with ice 3-(m-nitrophenyl)-5-acetamidomethyl-l,2,4-oxadia-zole. Y 90%.—Nitration of the free aminomethyl derivative gave a mixture of m- and p-nitrophenyl isomers, whereas only the m-nitrophenyl derivative was obtained, when the amino group was acetylated or further removed from the ring. F. e. s. G. Palazzo and G. Cor si, G. 93, 1196 (1963). 

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-3-aminoisoxazoles, 5-alkyl-3-amino-l,2,4-oxadiazoles, and 5-alky 1-3-amino-1,2,5-oxa-diazoles, when heated with phenyl isothiocyanate produce the corresponding thioureas (308) (Equation (46)), (310) (Equation (47)) and (312) (Equation (48)) which subsequently rearrange by a common mechanism to yield 1,2,4-thiadiazoles (309), (311), and (313), respectively <84CHEC-I(6)463>. 

R = —Cl [3-(4-C,hlor-phenyl)-l, 2,4-axadiazol-5-yl]- [3-( 4-chlor-phenyl)-1,2,4-oxadiazol-5-yl]-methyl-amino -methan-,... 

Eine andere Methode ist die Reaktion von 2-Amino-5-phenyl-l,3,4-oxadiazol mit Brom zum Perbromid, das bei Diazotierung zu 2-Brom-5-phenyl-l,3,4-oxadiazol (31% Schmp. 106 107°)62Onach dem Craig-Verfahren621 reagiert. Als Nebenprodukt erhalt man 2-Hydr-oxy-5-phenyl-l,3,4-oxadiazol. 

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