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.
The peptide Ac-Ser(P)- NHMe and the multiple-Ser(P)-containing peptides Ac-Ser(P)-Ser(P)- NHMe and Ac-Ser(P)-Ser(P)-Ser(P)- NHMe were prepared in good yield by a global phosphorylation approach which employed benzyl phosphate protection. The three serine-containing peptides were phosphorylated by the use of dibenzyl N,N-diethylphosphoramidite/1H-tetrazole followed by in situ oxidation of the resultant dibenzyl phosphite-triesters with m- chloroperoxybenzoic acid. The protected Ser(PO3Bzl2)-containing peptides were purified by C18 reverse-phase silica flash column chromatography and the benzyl phosphate groups were cleaved from the protected Ser(PO3Bzl2) peptides by palladium- catalysed hydrogenolysis.