Reagents and conditions:
(a) N-Cbz-d-leucine, DCC, DMAP, CH2Cl2, rt, 96%; (b) Pd/C (10% wt/wt), H2 (1 atm),
THF,
rt; DCC, formic acid, CH2Cl2, rt, 82%; (c) N-formyl-l-leucine, DIAD, PPh3, THF,
0 °C to rt, 82%; (d) N-Cbz-d-leucine,
DIAD, PPh3, THF, 0 °C, 93%; (e) Pd/C (10% wt/wt),
H2 (1 atm), THF, rt; DCC, formic acid, CH2Cl2, rt, 67%.