Scheme 2.
Reagents and conditions: (a) TFA:DCM = 1:1, r.t. 1 h; (b) EDC·HCl (1.1 eq), HOBt (1.1 eq), Boc-Val-OH or 9, or Boc-D-Tyr-OH (1 eq) for 10, DIPEA (3 eq), in DMF, r.t. 12 h.
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