Diastereocontrolled Electrophilic Fluorinations of 2-Deoxyribonolactone: Syntheses of All Corresponding 2-Deoxy-2-fluorolactones and 2′-Deoxy-2′-fluoro-NAD+s

Methods to construct 2'-deoxy-2'-fluoro nucleosides have undergone limited improvement in the last 20 years in spite of the substantially increased value of these compounds as pharmaceuticals and as tools for studying biological processes. We herein describe a consolidated approach to synthesize precursors to these commercially and scientifically valuable compounds via diastereocontrolled fluorination of the readily available precursor 2-deoxy-D-ribonolactone. With employment of appropriate sterically bulky silyl protecting groups at the 3 and 5 positions, controlled electrophilic fluorination of the Li-ribonolactone enolate by N-fluorodibenzenesulfonamide yielded the corresponding 2-deoxy-2-fluoroarabinolactone in high isolated yield (72%) The protected 2-deoxy-2,2-difluororibonolactone was obtained similarly in high yield from a second round of electrophilic fluorination (two steps, 51% from protected ribonolactone starting material). Accomplishment of the difficult ribofluorination of the lactone was achieved by the directive effects of a diastereoselectively installed (x-trimethylsilyl group. Electrophilic fluorination of a protected 2-deoxy-2-trimethylsilylarabinolactone via enolate generation provided the protected 2-deoxy-2-fluororibolactone as the exclusive fluorinated product. The reaction also yielded the starting material, the desilylated protected 2-deoxyribonolactone, which was recycled to provide a 38% chemical yield of the fluorinated product (versus initial protected ribonolactone),after consecutive silylation and fluorination cycles. Using our fluorinated sugar precursors, we prepared the 2'-fluoroarabino-, 2'-fluororibo-, and 2',2'-difluoronicotinamide adenine dinucleotides (NAD(+)) of potential biological interest. These syntheses provide the most consolidated and efficient methods for production of sugar precursors of 2'-deoxy-2'-fluoronucleosides and have the advantage of utilizing an air-stable electrophilic fluorinating agent. The fluorinated NAD(+)s are anticipated to be useful for studying a variety of cellular metabolic and signaling processes.