Scheme 4. Synthesis of Peptidomimetics 23c-ea.

aReagents and conditions: (a) methyl L-leucinate, HBTU, HOBt, DIPEA, CH2Cl2, rt, overnight, 86%. (b) (i) HNEt2, MeCN, rt, 30 min, quant.; (ii) 1-adamantanecarboxylic, HBTU, HOBt, DIPEA, CH2Cl2, rt, overnight, 87%. (c) (ii) LiOH, MeOH, rt, overnight; (ii) 2 N HCl, 84%. (d) corresponding L-valinate, HBTU, HOBt, DIPEA, CH2Cl2, rt, 48 ~ 95%. (e) (i) HNEt2, MeCN, rt, 30 min, quant.; (ii) HBTU, HOBt, DIPEA, CH2Cl2, rt, overnight, 72 ~ 80%.