New Facile Synthesis of 3,5‐Dihydro‐6 H ‐imidazo[1,2‐ b ]‐1,2,4‐triazol‐6‐ones by an Iminophosphorane‐Mediated Annulation

Iminophosphorane 1 reacted with an aromatic isocyanate to unexpectedly give a mixture of carbodiimides 2 , 3 and 4 through both the normal and the abnormal aza‐Wittig reactions. 3‐Aminoimidazolone 10 was obtained from the reaction of hydrazine hydrate with carbodiimide 2 . Reaction of 10 with triphenyphosphane, hexachloroethane and triethylamine produced iminophosphorane 11 in good yield. A tandem aza‐Wittig reaction of iminophosphorane 11 with isocyanate or CS 2 generated 3,5‐dihydro‐6 H ‐imidazo[1,2‐ b ]‐1,2,4‐triazol‐6‐ones 13 or 15 in satisfactory yield. Carbodiimides 18 , obtained from normal aza‐Wittig reactions of vinyl iminophosphorane 17 with aromatic isocyanates, reacted with hydrazine to give 2‐arylamino‐3‐amino‐4 H ‐imidazol‐4‐ones 20 . One‐pot reactions of 2‐arylamino‐3‐amino‐4 H ‐imidazol‐4‐ones 20 with isocyanates (or acyl chlorides), triphenyphosphane, hexachloroethane and triethylamine produced 3,5‐dihydro‐6 H ‐imidazo[1,2‐ b ]‐1,2,4‐triazol‐6‐ones 22 (or 23 ) in good yields. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)