Scheme 1. The synthesis of derivatives 5a–f and 6a–f. Reagents and conditions: (a) DMP, TsOH, dry acetone, 60 min, r.t., 82%; (b) BOP, DIEA, amino acid ester hydrochlorides, 3–8 h, r.t., 86% (5a), 81% (5b), 72% (5c), 78% (5d), 81% (5e), 67% (5f); (c) TFA, DCM, H2O (9 : 1 : 1), 4 h, r.t., 89% (6a), 83% (6b), 79% (6c), 82% (6d), 83% (6e), 80% (6f).
