Trifluoroacetic acid → (S)-(4-chlorophenyl)-[4-(2-methyl-6-nitro-2,3-dihydroimidazo[2,1-b]oxazol-2-ylmethyl)piperazin-1-yl]methanone

A mixture of tert-butyl (S)-4-(2-methyl-6-nitro-2,3-dihydroimidazo[2,1-b]oxazol-2-ylmethyl)-piperazine-1-carboxylate prepared in Example 393 (300 mg, 0.82 mmol) and trifluoroacetic acid (6 ml) was stirred at room temperature for 3 hours. The reaction mixture was concentrated under reduced pressure. The residue was dissolved in methylene chloride (15 ml), and the solution was added triethylamine (2 ml, 14.35 mmol). To the solution, 4-chlorobenzoyl chloride (150.7 mg, 0.86 mmol) was added with cooling on ice-bath followed by stirring at room temperature for 1 hour. The reaction mixture was washed with a saturated sodium hydrogencarbonate solution, water and a saturated saline solution in this order, dried over sodium sulfate and then filtered. The filtrate was concentrated under reduced pressure, and the residue was treated with diethyl ether-ethyl acetate to afford (S)-(4-chlorophenyl)-[4-(2-methyl-6-nitro-2,3-dihydroimidazo[2,1-b]oxazol-2-ylmethyl)piperazin-1-yl]methanone (282 mg, yield 85%) as a yellow powder.