Synthesis of Casein-Related Peptides and Phosphopeptides. IV. Phosphorotriester and 'Phosphite-Triester' Phosphorylation of Protected Serine-Containing Peptides

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Abstract
The phosphorylation of Ac-Ser- NHMe or Ac- Gly-Ser-Gly-NHMe with diphenyl phosphorochloridate /pyridine gave diphenyl hydrogen phosphate in high yield instead of the expected-Ser(PO3Ph2)-peptide. A 31P n.m.r . spectroscopy study of the phosphorylation reaction established that, with the use of diphenyl phosphorochloridate in pyridine, the intermediate diphenyl phosphoro-N-pyridinium chloride was the active phosphorylation species and that phosphorylation of the hydroxy group of Ac-Ser- NHMe or Ac- Gly-Ser-Gly-NHMe was followed by rapid dephosphorylation of the O-( diphenylphosphono ) seryl residue. While diethyl phosphorochloridate /pyridine phosphorylation of Ac-Ser- NHMe also did not give Ac-Ser(PO3Et2)- NHMe, both the two-step ( i ) diethyl phosphorochloridite /pyridine, (ii) iodine/water and the two-step (i) diethyl N,N-diethylphosphoramidite/1H-tetrazole, (ii) iodine/water 'phosphite-triester' phosphorylation of Ac-Ser- NHMe gave Ac- Ser(PO3Et2)- NHMe in high yield.