Scheme 1. Synthesis of compounds. i) TFA in DCM or HCl in dioxane, DCM ii) N-(tert-butoxycarbonyl)-l-leucine (R = H) or N-(tert-butoxycarbonyl)-4-methyl-l-leucine (R = CH3), HATU, NMM or Et3N, DMF, 94% yield (2 steps, R = H), 91% yield (2 steps, R = CH3); iii) corresponding (a–e) indolecarboxylic acid, HATU, NMM or Et3N, DMF, 30–98% yield (2 steps); iv) LiOH, THF, v) CDI, THF, NH3·H2O; vi) trifluoroacetic acid anhydride, Et3N, THF, 0 °C, 12–63% yield (3 steps).
