Oxadiazol-2-ones

As detailed in CHEC-II(1996) <1996CHEG-II(4)179>, 4,5-dihydro-l,2,4-oxadiazol-5-ones are readily hydrolyzed, and further examples (see Equation 17) of this transformation have been reported involving the synthesis of amidoximes 126 from the dihydro-1,2,4-oxadiazol-5-one 115 <2004T10907>. 

Dihydro-1,2,4-oxadiazol-5-ones (74) cannot be 7V-acylated by either chlorocarbonyl isocyanate or trichloroacetyl chloride. However, preparation of 4-chlorocarbonyl compounds (73) can be achieved by cycloaddition of stable nitrile oxides to the C=N double bond of chlorocarbonyl isocyanate <888994, 90ZOR339). Compounds (73) decompose with ammonia, primary amines, or primary amides to isocyanates and (74) (Scheme 26). 

Ring closure of 7V-(carboxymethyl)amide oxime yields 4//-l,2,4-oxadiazin-6(5//)-ones 3. The required acids are obtained by treatment of 1,2,4-oxadiazol-5(4f/)-ones with ethyl chloroacetate in the presence of potassium carbonate, followed by hydrolysis. The cyclization proceeds in good yield in the presence of dicyclohexylcarbodiimide in pyridine or dioxane.12... 

Phenyl-3-ureido-l,2,4-oxadiazole (61) did not rearrange on refluxing with sodium hydroxide in ethanol (only hydrolysis products were obtained).50 5-Substituted 3-phenylureido-l,2,4-oxadiazoles (62) were unaffected by melting but on refluxing with aqueous potassium hydroxide in ethanol, rearrangement to 3-acyl amino-1,2,4-triazolin-5-ones (63) occurred,... 

Other heterocycles were employed for the synthesis of imidazoles. Reaction of various amines with oxazoles 73 led to an expedient synthesis of imidazoles 74 under microwave conditions (14CC1867). Palladium-catalyzed decarboxylative intramolecular condensation of 1,2,4-oxadiazol-5-(4fJ)-ones 75 furnished substituted imidazoles 76 in good-to-excellent yields (14SL1916). 

Oxadiazol - 5 - ones. Excess phosgene passed into a gently refluxing soln. of 1-p-chlorobenzoyl-2,2-dimethylhydrazine for 2-3 hrs. 

Ring syntheses of 1,2,4-oxadiazoles from a one-atom component and a four-atom component 272... 

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