Example 1. Preparation of isopropyl ((((R,S)-(2R,3R,4R,5R)-5-(2-amino-6-(methylamino)-9H-purin-9-yl)-4-fluoro-3-hydroxy-4-methyltetrahydrofuran-2-yl)methoxy)-phenoxy-phosphoryl)-L-alaninate
Step 1. Preparation of ((2R,3R,4R,5R)-3-(benzoyloxy)-5-bromo-4-fluoro-4-methyltetrahydrofuran-2-yl)methyl benzoate (2)
To a solution of (2R)-3,5-di-O-benzoyl-2-fluoro-2-C-methyl-D-ribono-γ-lactone (24.8 g, 66.6 mmol) in dry THF (333 mL), under a nitrogen atmosphere and cooled to −30° C., was added lithium tri-tert-butoxyaluminum hydride (1.0 M in THF, 22.6 mL, 22.6 mmol) dropwise. After completion of the addition the reaction mixture was slowly warmed up to −15° C. over 90 min then EtOAc was added (300 mL) and the mixture was quenched with a saturated aq. NH 4Cl solution (200 mL). The resulting solution was filtered on Celite® and the filtrate was extracted twice with EtOAc. The combined organics were dried (Na 2SO 4), filtered and concentrated. The residue was taken up in dry DCM (225 mL) under a nitrogen atmosphere, cooled to −20° C., then PPh 3 (19.1 g, 72.8 mmol) was added. After 10 min of stirring at −20° C., CBr 4 (26.0 g, 78.4 mmol) was added and the reaction mixture was allowed to slowly warm up to 0° C. over 2 h. The resulting mixture was poured onto a silica gel column and eluted with PE/EtOAc (gradient 100:0 to 80:20). The fractions containing the α-bromofuranoside were collected and concentrated to afford the product 2 (18.1 g, 41.3 mmol, 62% over two steps) as a thick colorless oil.
Step 2. Preparation of (2R,3R,4R,5R)-5-(2-amino-6-chloro-9H-purin-9-yl)-2-(benzoyloxymethyl)-4-fluoro-4-methyltetrahydrofuran-3-yl benzoate (3)
2-Amino-6-chloropurine (2.63 g, 15.5 mmol) was suspended in t-BuOH (54 mL) under a nitrogen atmosphere. The reaction mixture was heated to 30° C. then potassium tert-butoxide (1.69 g, 15.1 mmol) was added. After 45 min a solution of bromofuranoside 2 (2.24 g, 5.12 mmol) dissolved in anhydrous MeCN (6 mL) was added, the reaction mixture was heated to 65° C. for 16 h then cooled down to room temperature. A saturated aq. NH 4Cl solution (70 mL) was added and the resulting solution was extracted with EtOAc (3×60 mL). The combined organics were dried (Na 2SO 4), filtered and concentrated. The residue was purified twice by column chromatography (gradient PE/EtOAc 80:20 to 0:100 then 60:40 to 20:80) to afford the product 3 (1.56 g, 2.96 mmol, 57%) as a white solid.
Step 3. Preparation of (2R,3R,4R,5R)-5-(2-amino-6-(methylamino)-9H-purin-9-yl)-4-fluoro-2-(hydroxymethyl)-4-methyltetrahydrofuran-3-ol (4)
To a solution of compound 3 (575 mg, 1.09 mmol) in MeOH (9 mL) was added methylamine (33% in absolute EtOH, 1.7 mL, 1.81 mmol). The reaction mixture was heated to 85° C. in a sealed tube for 16 h, cooled down to room temperature and concentrated. The residue was purified by column chromatography (gradient DCM/MeOH 100:0 to 85:15) then reverse phase column chromatography (gradient H 2O/MeOH 100:0 to 0:100) to afford the product 4 (286 mg, 0.91 mmol, 84%) as a white solid.
Step 4. Preparation of isopropyl ((((R,S)-(2R,3R,4R,5R)-5-(2-amino-6-(methylamino)-9H-purin-9-yl)-4-fluoro-3-hydroxy-4-methyltetrahydrofuran-2-yl)methoxy)-phenoxy-phosphoryl)-L-alaninate (5)
To a solution of compound 4 (114 mg, 365 μmol) in dry THF (4 mL), under a nitrogen atmosphere and cooled to 0° C. was added t-butyl magnesium chloride (1.0 M in THF, 0.66 mL, 660 μmol) dropwise over 10 min. The reaction mixture was stirred 15 min at 0° C. then another 15 min at room temperature. The reaction mixture was cooled down to 0° C. then a solution of isopropyl ((R,S)-(pentafluorophenoxy)-phenoxy-phosphoryl)-L-alaninate, Ross, B. S., Reddy, P. G., Zhang, H. R., Rachakonda, S., and Sofia, M. J., J. Org, Chem., (2011), (253 mg, 558 μmol) dissolved in dry THF (1 mL) was added dropwise over 10 min. The reaction mixture was stirred at 0° C. for 30 min followed by 18 h at room temperature then quenched with a saturated aq. NH 4Cl solution (4 mL) and extracted with EtOAc (3×5 mL). The combined organics were dried, filtered (Na 2SO 4) and concentrated. The residue was purified by column chromatography (gradient DCM/MeOH 100:0 to 90:10) then reverse phase column chromatography (gradient H 2O/MeOH 100:0 to 0:100) to afford product 5 (a mixture of diastereomers, 101 mg, 174 μmol, 48%) as a white solid.
1H NMR (300 MHz, CD 3OD) δ 7.83 (s, 0.55H), 7.82 (s, 0.45H), 7.38-7.16 (m, 5H), 6.15 (d, J=18.5 Hz, 0.45H), (d, J=18.8 Hz, 0.55H), 4.99-4.88 (overlapped with H 2O, m, 1H), 4.65-4.36 (m, 3H), 4.25-4.17 (m, 1H), 3.97-3.85 (m, 1H), 3.05 (br s, 3H), 1.32-1.28 (m, 3H), 1.25-1.15 (m, 9H). 19F NMR (282 MHz, CD 3OD) δ −162.8 (s), −163.3 (s). 31P NMR (121 MHz, CD 3OD) δ 4.10 (s), 3.99 (s). MS (ESI) m/z calcd. for C 24H 34FN 7O 7P [M+H] + 582.2; found 582.2.
