The crystal structure of 7 was determined to analyze the effect on the molecular conformation of the 3'-substituent. None of the bond lengths and angles are exceptional. The most important feature of the structure is the C3'-endo conformation of the sugar ring (pseudorotational phase angle P= 16.0'). This conformation is commonly observed for other uridine analogues, but not for AZT or 3'-azido-2 ', 3'-dideoxyuridine ((3-87), 18 active anti- ...