Synthesis of O-phosphotyrosine-containing peptides: 3. Synthesis of H-Pro-Try(P)-Val-OH via dimethyl phosphate protection and the use of improved deprotection procedures

Article abstract of DOI:10.1021/jo00300a041

The phosphorylated derivative N(α)-(tert-butoxycarbonyl)-O-(dimethylphosphono)tyrosine anthraquinon-2-ylmethyl ester, Boc-Tyr(PO₃Me₂)-OMaq (2), was prepared in high yields by either phosphotriester or ''phosphite-triester'' phosphorylation of Boc-Tyr-OMaq (1). In the former case, either sodium hydride or lithium diisopropylamide was used to generate the phenoxide ion which was then treated with dimethyl phosphorochloridate (MeO)₂P(O)Cl. Alternatively, dimethyl N,N-diethylphosphoramidite (MeO)₂PNEt₂ (15) and 1H-tetrazole were used in the phosphite-triester approach, to effect the quantitative, acid-catalyzed phosphitylation followed by m-chloroperoxybenzoic acid oxidation of the phosphite-triester intermediate. Removal of the 2-methylanthraquinone group (Maq) by reduction with sodium dithionite afforded Boc-Tyr(PO₃Me₂)-OH (3) in 70-75% overall yield. This derivative was used in the solution-phase synthesis of the tripeptide Boc-Pro-Tyr(PO₃Me₂)-Val-OMaq (9) by the Boc mode of peptide synthesis, which was followed by the removal of the Maq and Boc groups by successive dithionite reduction and 40% CF₃CO₂H/CH₂Cl₂ treatments respectively. Six procedures for the cleavage of the methyl phosphate group by acidolysis or silylolysis from H-Pro-Tyr(PO₃Me₂)-Val-OH·TFA (11) were examined with the source of hard acid being CF₃SO₃H, (CH₃)₃SiBr, (CH₃)₃SiBr, or CF₃SO₃Si(CH₃)₃ and either thioanisole or dimethyl sulfide as the soft nucleophile. While deprotection treatments gave H-Pro-Tyr(P)-Val-OH in modest isolated yields, monitoring with ³¹P NMR and ¹³C NMR indicated quantitative deprotection. Enhanced rates of methyl cleavage from the protecting phosphate group were observed with the use of thioanisole.