Synthesis and Crystal Structures of O ‐2′,3′‐Cyclic Cyclopentanone and Cyclohexanone Ketals of the Cytostatic 5‐Fluorouridine

The synthesis of two O‐ 2′,3′‐cyclic ketals, i.e. , 5 and 6 , of the cytostatic 5‐fluorouridine ( 2 ), carrying a cyclopentane and/or a cyclohexane ring, respectively, is described. The novel compounds were characterized by 1 H‐, 19 F‐, and 13 C‐NMR, and UV spectroscopy, as well as by elemental analyses. Their crystal structures were determined by X‐ray analysis. Both compounds 5 and 6 show an anti ‐conformation at the N ‐glycosidic bond which is biased from + ac to + ap compared to the parent nucleoside 2 . The sugar puckering is changed from 2′ E to 3′ E going along with a reduction of the puckering amplitude τ m by ca. 10–13° due to the ketalization. The conformation about the sugar exocyclic bond C(4′)C(5′) of 5 and 6 remains unchanged, i.e., g + , compared with compound 2 .