Imidazo oxadiazole salts

Treatment of 2-amino-l,3,4-oxadiazoles 13 with bromoketones in boiling ethanol gives imidazo[2,l-h][l,3,4]oxadiazoles 20 directly (isolated as salts (82JIC1170).2... 

Only a few reactions of imidazo[2,l-h][l,3,4]oxadiazoles have been reported. The basicity of these compounds is rather low hydrohalides cannot be isolated (69ZC337). Protonation of 23 with strong acids occurs at position 7, giving, for example, 24 (mp 212°C with explosion ). With aqueous base 24 is converted back to 23 (72BSF3968). On alkylation of 23 the quaternary salts 25 are obtained (72BSF3968). Reaction of 25 (R = Et) with ethyl cyanoacetate (Et3N/EtOH) yields 26, albeit in low yield (3.8%) (72BSF3968). 

In contrast to imidazo[2,l-h][l,3,4]oxadiazoles (see Section III), im-idazo[2,l-h][l,3,4]thiadiazoles form stable salts (hydrohalides, perchlorates, picrates, picronolates). Hydrohalides are often the primary products of the cyclization reaction. 

Amino-l,3,4-oxadiazoles (205) on treatment with cr-halogeno-ketones gave intermediate quaternary salts 206 that did not cyclize directly to imidazo[2,l-6]oxadiazoles (208) with base. Hydrolysis of 206 with aqueous potassium carbonate caused ring-opening at C-2 with subsequent closure to the imidazolone 207. These latter compounds could be cyclized to 208 with phosphorus oxychloride.204c 21 246... 

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