Synthetic Approaches for the Preparation of Phosphoramidate Prodrugs of 2′‐Deoxypseudoisocytidine

A synthetic procedure for the preparation of phosphoramidate prodrugs of C‐nucleosides is reported. Different phosphorochloridates were reacted with 3′‐O‐protected N‐acetyl‐2′‐deoxypseudoisocytidine or 3′‐O‐protected 2′‐deoxypseudoisocytidine, followed by acidic hydrolysis of the protecting group. In the presence of the N‐acetyl moiety, the enolisable keto group of the nucleobase was able to react (like the 5′‐OH) with the phosphorochloridates to give bisphosphorylated derivatives. Epimerisation (β to α) occurred if the amino group of the nucleobase was unprotected. These side reactions demonstrate the peculiar behaviour of C‐nucleosides compared to their nucleoside analogues. It was demonstrated that the first enzymatic activation step for this new class of prodrugs can be mediated by carboxypeptidase and that it follows the same pathway and rate reported for ProTides of more conventional nucleoside analogues. These new phosphoramidate derivatives deserve further investigation for their therapeutic potential as anti‐cancer agents. C the difference: 2′‐Deoxypseudoisocytidine is a C‐nucleoside with antileukemic activity. A series of phosphoramidate prodrugs of this C‐nucleoside were synthesized. Their synthesis proved challenging due to unexpected side reactions (double phosphorylation and epimerization). Their enzymatic activation was found to be similar to that of N‐linked nucleoside ProTides.