4‐(Acetylthio)‐2,2‐dimethyl‐3‐oxobutyl and 4‐(tert‐Butyldisulfanyl)‐2,2‐dimethyl‐3‐oxobutyl as Protecting Groups for Nucleoside 5′‐Phosphoramidates Derived from L‐Alanine Methyl Ester

Abstract

AbstractPhosphoramidates 1 and 2 were synthesized by H‐phosphonate methodology and subsequent oxidative amination with L‐alanine methyl ester. The removal of the protecting groups at pH = 7.5 and 37 °C in the absence and presence of porcine liver esterase (PLE) or glutathione (GSH) was monitored by HPLC. The stability of phosphoramidate 1 was additionally studied at pH = 9 and 10. The reduction of the disulfide bond with glutathione from 2 triggers the removal of the protecting group by cyclization releasing quantitatively nucleoside 5′‐{N‐[(1S)‐2‐oxo‐2‐methoxy‐1‐methylethyl]phosphoramidate} (7) as the desired product. With 1, enzymatic deacetylation or acetyl migration from the sulfur atom to the adjacent hydrated oxo group followed by chemical cyclization produces 7. The S–S‐bond‐mediated dimerization (8) competes as a side reaction. Prolonged treatment, however, resulted in the conversion of the S–S dimer 8 into 7 that undergoes slow alanine methyl ester hydrolysis to form 10.