Oxadiazoles, amino-, tautomerism

Oxadiazoles, amino-alkylation, 6, 431 as photographic sensitizer, 6, 446 ring cleavage, 6, 434 tautomerism, 6, 381... 

Routes to benzoxadiazoles are based on approaches to the tautomeric 6-diazo-2,4-cyclo-hexadienones (Scheme 11) <9UST(247)135>. The appropriate 2-aminophenol hydrochloride is diazo-tized using either sodium nitrite or isoamyl nitrite and the diazonium chloride is then carefully neutralized with sodium carbonate or potassium carbonate. An alternative approach is from the monotosylhydrazone of the appropriate o-benzoquinone. Naphthoxadiazole (6) was prepared, in a manner analogous to the first route of Scheme 11, from 3-amino-2-naphthol <91AG(E)1476>. A slightly modified preparation is described in a later paper the method was applied to the synthesis of [9a- C]naphth[2,3-( ]-l,2,3-oxadiazole from 3-amino-[2- C]-2-naphthol <92Mi 403-03>. 

Hydroxy- (8a), 2-mercapto- (8b) and 2-amino-l,3,4-oxadiazoles (8c) are in equilibrium with the tautomeric oxadiazolines (9a), (9b) and (9c) respectively. Evidence from UV (72CJC3079) and IR (Section 4.23.2.2.3) spectra supports structure (9a) for A2-l,3,4-oxadiazolin-5-ones and structure (9b) for A2-l,3,4-oxadiazoline-5-thiones. The UV and IR spectra, fluorescence and p/iT values of 2-amino-l,3,4-oxadiazoles indicate that the amine tautomer (8c or d) rather than the imine tautomer (9c or d) predominates (69BSF870, 874, 64CR(258)4579>. 

It was established by comparison of the UV, fluorescence, and IR spectra and the pKa values of the above alkylated products that 2-amino-5-phenyl-l,3,4-oxadiazole predominates in the tautomeric equilibrium [Eq. (10)].63 Alkylation of 2-amino-5-methyl(phenyl)-1,3,4-oxadiazoles with aromatic a-haloketones also takes place at... 

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