A Completely Diastereoselective Electrophilic Fluorination of a Chiral, Noncarbohydrate Sugar Ring Precursor: Application to the Synthesis of Several Novel 2′-Fluoronucleosides

A new and completely diastereoselective method for the introduction of fluorine into a noncarbohydrate sugar ring precursor has been developed. The use of N-fluorodibenzenesulfonimide (5) for the electrophilic fluorination of chiral lactone 4, which is derived from L-glutamic acid, yields the key intermediate 6. This is transformed into an anomeric acetate 8 and is used for the synthesis of a number of novel R-2′-fluoronucleosides. Since glutamic acid is used as the synthetic starting material, the L enantiomer may also be synthesized simply by using D-glutamic acid. The incorporation of fluorine into the 2′ position of the nucleoside provides several advantages including acid stability of the anomeric bond and general resistance to oxidative metabolism. Further, fluorine is a close mimic of hydroxyl groups in size and polarity and in its ability to act as a hydrogen bond acceptor. This may aid in the recognition of these nucleosides by the enzymes involved in nucleoside activation.