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

The phosphorylated derivative Nα-(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 lH-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(P03Me2)-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/CH2C12treatments 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 CF3S03H, (CH3)3SiBr, (CH3)3SiBr, or CF3S03Si(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. © 1990, American Chemical Society. All rights reserved.