1.2.4- Oxadiazole recyclization

Amino-l,3,4-oxadiazoles (423) ring-open and the products immediately recyclize to triazolinones (424) (66AHC(7)183). 

Thermal recyclization of the 3-diazenofuroxanyl unit to form the 4-nitro-l,2,3-triazole fragment has been found in noncondensed 1,2,5-oxadiazole 2-oxide derivatives (3,3 -azofuroxans) with acetamido substituents in the 4,4 -posi-tions <1999MC17>. 

Nitrosoimidazoles are also hydrolyzed and recyclized to oxadiazoles, apparently by the pathway given in Eq. (18) although the JV-acyl-amidoxime was not isolated.67 An aminooxadiazole has also been syn-... 

In contrast to uncharged TAs, such protonated forms as perchlorates 206 easily react with hydrazines and hydroxylamine at the 2 position to give recyclization products—2-mercaptophenyl triazoles and oxadiazoles 241—which show promise for use as ligands (91KGS1220) (Scheme 94). 

Recyclization of 5-R-2-amino-l,3,4-oxadiazoles, on heating with hydrazine hydrate, leads to the corresponding 3,4-diaminotriazoles, whose yields varied from 21% to 44% [Eq. (49)] (63LAI19). The reaction of oxadiazoles 186 with semicarbazide was used to synthesize 3,4-diaminotriazoline-5-one in several steps [67LA(702)101]. 

Trifluoromethyl-l-oxa-2,4-diazole of 92 (Rf = CF3) is prepared by base-catalyzed condensation of 93 with trifluoroacetic anhydride in excellent yield (Scheme 12.13). The same method is applicable for the preparation of monofluor-omethyl, difluoromethyl, and 1,1-difluoroethyl compounds although their yields are low. The reaction of aminooxime 93 with 2-(trifluoroethylidene)-l,3-dithiane 95 under acid-catalyzed conditions, followed by ring-opening of the 1,3-dithiane ring and recyclization of oxadiazole ring provides 5-(2,2,2-trifluoroethyl)-l-oxa-2,4-diazole 96 in 38% yield (Scheme 12.13). °... 

Drach and co-workers acylated 1640 (X = P(0)(0CH3)2) to afford 1644 in excellent yield. Refluxing 1644 in aqueous ethanol effected hydrolysis and recyclization to the 2,5-disubstituted 1,3,4-oxadiazoles 1646, presumably via the A, A -diacyUiydrazide 1645 (Scheme 1.423). 

An interesting approach to chain-fluorinated quinazolines rehed on photochemical recyclization of 1,2,4-oxadiazole derivatives 697 (Scheme 146) [410]. The corresponding quinazolones 698 were obtained in 40-75 % yields when pyrene was used as a sensitizer. This is contrary to the data obtained for simple alkyl-substituted oxadiazoles, which gave highest yields of the products upon irradiation in the presence of triethylamine. 

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