Ethyl chloroformate → 5-(4-nitrobenzyl)-1,3,4-oxadiazol-2(3H)-one

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A mixture of 2-(4-nitrophenyl)acetohydrazide (0.50 g), ethyl chlorocarbonate (0.34 g) and N,N-dimethylacetamide (10 mL) was stirred at room temperature for 1 hr. Water was added to the reaction mixture, and the mixture was extracted with ethyl acetate. The organic layer was washed successively with saturated aqueous sodium hydrogen carbonate and saturated brine, dried over anhydrous magnesium sulfate, and concentrated to give colorless crystals. A mixture of the obtained crystals, diphosphorus pentaoxide (1.50 g), hexamethyldisiloxane (2.96 g) and 1,2-dichlorobenzene (10 mL) was stirred at 160° C. for 2 hrs. Water was added to the reaction mixture, and the mixture was extracted with ethyl acetate. The organic layer was washed successively with saturated aqueous sodium hydrogen carbonate and saturated brine, dried over anhydrous magnesium sulfate, and concentrated to give 5-(4-nitrobenzyl)-1,3,4-oxadiazol-2(3H)-one as pale-yellow crystals (0.29 g, yield 50%). Recrystallization thereof from ethyl acetate-hexane gave pale-yellow prism crystals. melting point: 170-171° C.

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