1.2.4- Triazolo benzoxazoles

Addition of hydroxylamines to C=N double bonds produces stable compounds only for endocyclic C=N bonds. Addition of hydroxylamine to azirines of type 59 results in stable adducts 60 as seen in equation 40. Similar reactions have been observed in benzoxazoles and l,2,3-triazolo[4.5-rf]pyrimidones ° ... 

Treatment of 2- or 4-(p-formylphenyl)-2/f-l,2,3-triazoles with p-tolyl-substituted heterocycles such as benzo[6]furans,58,63 benzol-thiophenes,56,63 benzoxazoles,63,64 benzisoxazoles,54 oxazoles,63 isoxa-zoles,63 oxadiazoles,63,65 benzotriazoles,63 and y-triazolo [ 1,5-a]-pyri-dines19 has led to the formation of a number of new stilbene derivatives some of which are shown in Table XI. 

Treatment of hydrazones of general structure (273) with NBS leads to o-triazolo[5,l-6]thiazolium and imidazolium salts. The imidazolium salts (X = NH) lose HBr in aqueous pyridine to give v- triazolo[ 1,5-a Jimidazoles (equation 61) (67AG(E)261). The benzoxazole analogous to (273) failed to cyclize. 

Of the three possible l,2,4-triazolo-l,3-oxazoles (80-82) which may be found in this review, only the synthesis of l,2,4-triazolo[3,4-(>]l,3-benzoxazoles (82) has been recorded. 

Triazolo[3,4-6] 1,3-benzoxazole-3-thione (85) has been prepared by cyclizing 2-hydrazino-l,3-benzoxazole (83) with carbon disulfide (58USP2861076) or phenyl isothiocyanate (59JOC1478). The alternative approach (62TL1193) of fusing the 1,3-benzoxazole onto a 1,2,4-triazole was also used in the synthesis of 3-phenyl-1,2,4-triazolo[3,4- ]-benzoxazole (88) from 3-hydroxy-5-phenyl-4-(2-methoxyphenyl)-1,2,4-triazole (86). 

Oxidative cyclization of A(-(benzoxazol-2-yl)benzamidines (211) with lead tetraacetate leads to the formation of 2-aryl[l,2,4]triazolo[5,l-( ]benzoxazoles (212) (Equation (53)) <90S422>. 

Condensation of 2-hydrazinobenzoxazoles (286) with aromatic aldehydes in glacial acetic acid yields 3-aryl[l,2,4]triazolo[3,4-Z>]benzoxazoles (288) <88BCJ1339). Similarly, 3-aryl-9/7-[ 1,2,4]tri-azolo[4,3-Z>]benzimidazoles (287) are obtained from 2-hydrazinobenzimidazoles (289) (Equation (80)) <88BCJ1339>. 

The reaction of 2-hydrazinobenzoxazole (286) with cyanogen bromide gives 4-amino-[l,2,4]triazolo[3,4-Z>]benzoxazole hydrobromide (294-HBr) the free base (294) is obtained with potassium hydrogen carbonate (Equation (82)) <89H(29)925>. 

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