A new approach to phosphoserine,phosphothreonine and phosphotyrosine synthons and to thiophospho analogs

Several phosphoserine, phosphothreonine and phosphotyrosine synthons suitable for the stepwise synthesis of phosphopeptides were prepared. Treatment of methylthiomethyl (MTM) esters of either Z-, Boc-, Alloc-serine and threonine with phosphochloridate in pyridine followed by MgBr₂ cleavage of MTM in diethyl ether afforded the title compounds in good yield. Thiophosphoserine and phosphotyrosine synthons were also obtained by the phosphoramidite method using di-(2,2,2-trichloroethyl) -N.N-diisopropylphosphoramidite and MCPBA as oxidizing reagent. Trichloroethyl proved valuable as phosphate protecting group especially in phosphotyrosine derivatives owing to its stability in acidic conditions. These synthons were involved in the liquid-phase synthesis of several phospho and/or thiophosphopeptides related to either src-protein kinase or rat liver pyruvate kinase.