A series of novel 3,5‐disubstituted hydantoins 3a‐1 were easily synthesized in one‐step from the reaction of 2‐phenyl‐1,3,4‐oxadiazolin‐5‐ones 1a‐h with various free L‐α‐amino acids 2a‐e in m‐cresol at 150°. An alternative route leading to the formation of 3‐benzamido‐5‐isopropylhydantoin 3c was also developed to make clear the reaction mechanism.

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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 pKb 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 pKb 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.

A novel thermally stable and semiconducting polyheterocycle, poly(l,3,4-thiadiazole amine), was synthesized from 2-(p-aminophenyl)-l,3,4-oxadiazoline-5-thione via ring-opening. The polymer is a new class of ordered alternating copoly(ani1ine) containing 1,3,4-thiadiazole heterocyclic units. An investigation of the reaction of 2-phenyl-l,3,4-oxadiazoline-5-thione with aniline was conducted as a model reaction for the polymerization, and poly(phosphoric acid) (PPA) and phosphorus pentoxide/methanesulfonic acid (PPMA) were found to be favorable both as condensing agent and solvent for the formation of 2-anilino-5-phenyl-l,3,4-thiadiazole as a model compound. The polymerization was carried out both by two-step procedure that included ring-opening self-polyaddition giving poly(1-benzoylthiosemicarbazide), followed by cyclodehydration to poly(l,3,4-thiadiazole amine), and by a one-step procedure including cyclodehydration in situ. The poly(1 -benzoylthiosemicarbazide) which was formed in the first step in m-cresol had reduced viscosities up to 0,42 dL . g-', and it was converted to poly(l,3,4-thiadiazole amine) by treating in PPA or PPMA. Poly(l,3,4-thiadiazole amine) having reduced viscosities up to 0,25 dL . g was also synthesized by the direct one-step polymerization in PPA or PPMA.The polymer is highly thermally stable and exhibited no weight loss up to 350 "C under nitrogen.Its electric conductivity was less than lo-'' S * cm-' at ambient temperature, but markedly increased to 2,9. lo-' S . cm-' upon doping with iodine.