The design, synthesis and activity of pentapeptide pp60c‐src inhibitors containing L‐phosphotyrosine mimics

Abstract
Efficient syntheses of 4‐(R,S‐hydroxyphosphonomethyl)‐l‐phenylalanine and 4‐carboxy‐l‐phenylalanine within the context of the pentapeptide Ac‐Ile‐X‐Gly‐Glu‐Phe‐NH2 (wherein X = the unnatural amino acid) illustrate the use of a divergent synthetic strategy from an advanced common peptide intermediate to more readily access peptide‐based tyrosine kinase inhibitors. The key intermediate, Ac‐Ile‐Phe(4‐formyl)‐Gly‐Glu(O‐tBu)‐Phe‐NH2, was synthesized by a facile palladium‐catalyzed carbonylation of Ac‐Ile‐Phe(4‐iodo)‐Gly‐Glu(O‐tBu)‐Phe‐NH2. Oxidation of Ac‐Ile‐Phe(4‐formyl)‐Gly‐Glu(O‐tBu)‐Phe‐NH2 with tetrabutylammonium permanganate or addition of di‐t‐butylphosphite, both followed by trifluoroacetic acid deprotection, gave the target pentapeptide inhibitors wherein X = 4‐carboxy‐l‐phenylalanine or 4‐(R,S‐hydroxyphosphonomethyl)‐l‐phenylalanine, respectively. These two peptides gave somewhat more potent inhibition of the tyrosine kinase pp60c‐src than the corresponding pentapeptide wherein X =l‐phenylalanine, demonstrating that appended functionalities at the 4‐position are accepted and can enhance binding through added interactions within the catalytic region of the active site.