We have previously reported our SAR optimization of the anticancer agent thymectacin. Tuning of the parent ProTide structure initially involved the amino acid and, subsequently, the aromatic masking group on the phosphate moiety. Herein, derivatives bearing the combined modifications are reported and biological evaluation is described. Moreover, separation of the diastereoisomeric final product mixture shows a different cytostatic activity for the two diastereoisomers. Through computational and NMR studies, the absolute stereochemistry of the phosphorus center of the two diastereoisomers has been suggested.