Synthesis of a triply phosphorylated pentapeptide from human tau-protein

Two different strategies for the synthesis of a triply phosphorylated pentapeptide are described. In both cases a monophosphorylated selectively N-deprotected tripeptide is employed as C-terminal fragment. Coupling of this building block with a C-terminally unmasked bis-phosphorylated seryl-dipeptide unexpectedly failed due to decomposition of this peptide upon activation with different coupling reagents. Instead stepwise N-terminal elongation of the peptide chain with serine derivatives and subsequent O-phosphorylation of the serine OH-groups was successful. These results indicate that assembly of multiply phosphorylated peptides from preformed multiply phosphorylated phosphopeptide building blocks in general may be problematic and that a stepwise elongation of the amino acid chain may be preferable.