Diastereoselective Synthesis of (Aryloxy)phosphoramidate Prodrugs

The first diastereoselective synthesis of aryloxyphosphoramidate prodrugs of 3'-deoxy-2',3'-didehydrothymidine monophosphate (d4TMP) was recently reported. The synthetic approach utilized the chiral auxiliary (S)-4-isopropylthiazolidine-2-thione (2). For this strategy, a stereochemically pure phosphorodiamidate intermediate was needed. The diastereoselective formation of this key compound was investigated by using different phenols and L-alanine methyl or benzyl ester. Generally, the reaction with 3- or 4-substituted phenols led to significantly better diastereoselectivities compared to their 2-substituted counterparts. Moreover, variation of the ester group in the amino acid residue resulted in no significant differences with regard to the obtained diastereoselectivity. From the reported results, a model for the transition state was elaborated. Finally, eight new (S-P)-aryl-phosphoramidates were synthesized with very high diastereoselectivities (up to >= 95% de) and tested for their anti-HIV potency, showing a tendency for higher antiviral activity from the (S-P) diastereomers.