Ring-opening reaction of 1,3,4-oxadiazolone and 1,3,4-oxadiazolinethione: Reaction of 2-phenyl-1,3,4-oxadiazolin-5-one and 2-phenyl-1,3,4-oxadiazoline-5-thione with amines

The ring-opening abilities of amines toward 1,3,4-oxadiazolines, 2-phenyl-1,3,4-oxadiazolin-5-one ( 1a ) and 2-phenyl-1,3,4-oxadiazoline-5-thione ( 1b ), were investigated with relation to their basicities or pK_b values. Oxadiazolines 1a and 1b were easily reacted with amines such as benzylamine and aniline, but not with p-nitroaniline, to form the corresponding ring-opening adducts. The reactions of both 1a and 1b with o-phenylenediamine produced benzodiazoles with the liberation of benzoylhydrazide, whereas the reactions with o-aminobenzamide furnished quinazolines with the liberation of ammonia. o-Aminophenol and o-aminothiophenol were also reacted with 1a and 1b both of them giving 1,5-dibenzoylcarbohydrazide from 1a and 1,2-dibenzoylhydrazine from 1b. From the conditions affording the corresponding ring-opening adducts or reaction products, the ring-opening abilities of the amines toward 1a and 1b are in good correlation with the strength of their basicities or pK_b values. The ring-opening of oxadiazolines were proved to occur with anilines. Therefore, the other reactions are also supposed to proceed via the ring-opening steps.