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aReagents and conditions: (i) 1-Boc-3-oxopiperazine, K2CO3, DMF, 55°C; (ii) trifluoroacetic acid, dichloromethane, r.t.; (iii) corresponding substituted benzenesulfonyl chloride, TEA, dichloromethane, 0°C to r.t.; (iv) H2, 10% Pd●C, dichloromethane, methanol, r.t..
