Scheme 1. Syntheses of the branched BBB-shuttle peptides. (a) Synthesis of THRre_2m (X = Mal) and THRre_2f (X = CF). The Alloc group was selectively removed before the cleavage to introduce other functional groups, such as 6-maleimidohexanoic acid (Mal) or carboxyfluorescein (CF). (b) Synthesis of THRre_2c.