In a 50 mL three-neck flask with trans-3-(benzyloxyethyl)cyclobutyl-mesylate (0.34 g, 1.2 mmol), N3-PMB-5-fluoro-uracil (0.36 g, 1.44 mmol), K2CO3 (0.2 g, 1.44 mmol) and 18-crown-6 (0.38 g, 1.44 mmol) inside, dry DMF 10 mL was added under argon to give a light yellow solution with some white solids at the bottom of the flask. After 5 min, start to heat to 120° C. After 24 hr, add 30 mL EtOAc and 20 mL H2O. The organic phase was separated and washed with brine once. The organic phase was dried over MgSO4 and the solvents were evaporated to give the crude product, which was purified by silica gel flash chromatography (Hexane:EtOAc=3:1) to give an off-white solid (0.27 g, 50.9%) combined with some three-membered ring by-products. 1H NMR (CDCl3, 300 MHz): δ 1.72-1.79 (m, 4H), 2.16-2.24 (m, 1H), 2.51-2.60 (m, 2H), 3.43-3.48 (m, 2H), 3.77 (s, 3H), 4.48 (s, 2H), 4.64-4.75 (m, 1H), 5.05 (s, 2H), 6.81-6.84 (d, J=9.0, 2H), 7.29-7.36 (m, 6H), 7.44-7.47 (d, J=9.0, 2H). MS (FAB): expected for C25H27FN2O4 (M+H)+ 439.49. Found 439.20320. IR (neat) νmax 2926, 2854, 1712, 1458, 1377, 1265, 895, 740, 705.
