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Synthetic Scheme of 14a-14d. Reagents and conditions: (a) (1) TFA, anhydrous DCM, RT, 3 h, add TEA to adjust pH to 7.0; (2) various Boc-l-amino acid, EDCI, HOBt and TEA, anhydrous DCM, 25 °C, 12 h, 41–49% for two steps; (b) (1) TFA, anhydrous DCM, RT, 3 h, add TEA to adjust pH to 7.0; (2) Cinnamic acid, EDCI, HOBt and TEA, anhydrous DCM, 25 °C, 12 h; 31–49%. (c) NaBH4, MeOH, 0 °C to RT, 25 °C; 41–57%; (d) Dess-Martin Reagent, DCM, 25 °C, 2 h; 68–81%.
