Synthesis and Stability of Nucleoside 3′,5′-Cyclic Phosphate Triesters Masked with Enzymatically and Thermally Labile Phosphate Protecting Groups

Appropriately protected structurally modified nucleoside 3′,5′‐cyclic monophosphates are known to show antiviral activity. For this reason, a straightforward synthesis of nucleoside 3′,5′‐cyclic phosphates protected with three different enzymatically removable groups, viz. 3‐acetyloxy‐2,2‐bis(ethoxycarbonyl)propyl (in 1 and 4), 4‐acetylthio‐2,2‐dimethyl‐3‐oxobutyl (in 2), and 4‐(tert‐butyldisulfanyl)‐2,2‐dimethyl‐3‐oxobutyl (in 3) groups, is described. Removal of these protecting groups at pH 7.5 and 37 °C was monitored by reverse‐phase HPLC. Nucleoside 3′,5′‐cyclic phosphotriesters that bear 3‐acetyloxy‐2,2‐bis(ethoxycarbonyl)propyl, 4‐acetylthio‐2,2‐dimethyl‐3‐oxobutyl, and 4‐(tert‐butyldisulfanyl)‐2,2‐dimethyl‐3‐oxobutyl groups were prepared. Their conversion into cyclic phosphodiesters at pH 7.5 and 37 °C in the presence and absence of enzyme was monitored by reverse‐phase HPLC.