Amino acid nucleoside phosphoramidates

A more complex pathway of activation is seen in N-amino acid derivative of phosphoramidic acid diesters of antiviral nucleosides, as exemplified by prodrugs of stavudine (9.79, Fig. 9.14) [153 -155], The activation begins with a carboxylesterase-mediated hydrolysis of the terminal carboxylate. This is followed by a spontaneous nucleophilic cyclization-elimination, which forms a mixed-anhydride pentacycle (9.80, Fig. 9.14). The latter hydrolyzes spontaneously and rapidly to the corresponding phosphoramidic acid monoester (9.81, Fig. 9.14), which can then be processed by phosphodiesterase to the nucleoside 5 -monophosphate, and by possible further hydrolysis to the nucleoside. 

Phosphoramidate prodrugs bearing an amino acid ester residue In one of the most widely-used pro-nucleotide strategies a nucleoside monophosphate is masked as a phosphoramidate or phosphordiamidate. Westwell, McGuigan and co-workers prepared a series of such derivatives 43 of an interesting fluorinated nucleoside, namely S -deojy-S -fluorothymidine. ... 

Similar 0-atyl-phosphoramidate derivatives of nucleosides, bearing an amino acid residue, were prepared in several laboratories. Synthesis of such a prodrug form of l, 2 -cyclopentyl nucleosides 44 was reported from the already mentioned Qun Dang s group. Conformationally-restricted 2 -spironucleosides (bearing a 2 -oxetane motif) and their prodrugs 45 were synthesised and tested. It should be emphasised that P-diastereomerically pure compounds were evaluated. The prodrugs of a 2 -oxetane 2-amino-6-0-methylpurine nucleoside demonstrated potent anti-HCV activity in vitro. 

Nucleoside phosphoramidates are the oxidised form of nucleoside phosphoramidites, but they are designed for different purposes. Nucleoside phosphoramidites are very reactive and used as the monomers for chemical synthesis of oligomers while nucleoside phosphoramidates are relatively stable and used primarily as prodrugs since the amidate moiety is often constructed by linking with a natural amino acid to enhance the cellular uptake of a phosphoramidate which is then hydrolysed to generate the desired nucleoside monophosphate. Cho et al. developed an efficient method for the synthesis of exo-N-carbamoyl nucleosides and their phosphoramidate prodrugs (51) in excellent yields. 

The reaction of 5 -amino-5 -deoxyadenosine with trimetaphosphate affords the 5 -Af-triphosphate (23). When (23) is employed as substrate with glucose in the hexokinase-catalysed reaction, the 5 -AT-diphosphate (24) is obtained the latter is cleaved by snake venom phosphodiesterase to the 5 -phosphoramidate, and hydrolyses in acid to the amino-nucleoside. It does not appear to be polymerized by polynucleotide phosphorylase. In this context it is noteworthy that uridine 5 -5-thiopyrophosphate (25) is a competitive inhibitor for polynucleotide phosphorylase from E. coli, but not a substrate, and that the 5 -S-thiotriphosphates (26) and (27) show neither substrate nor inhibitory properties for RNA polymerase or DNA polymerase I, respectively. However, (23) can be polymerized using the latter enzyme, showing that the introduction of a 5 -heteroatom does not completely exclude these modified nucleotides as substrates for the polymerizing enzymes. 

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