Diisopropyl Ether → (3aR,5R,7aS)-5-[(Benzyloxy)methyl]-7a-(2,4-difluorophenyl)hexahydro-1H-pyrano[3,4-c][1,2]oxazole

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Boron trifluoride diethyl etherate (60.1 mL, 474 mmol) was added to a solution of C4 (50.0 g, 202 mmol) in a 1:1 mixture of toluene and diisopropyl ether (2 L) at an internal temperature of −76° C. The reaction was stirred at this temperature for 30 minutes, then treated with 2,4-difluoro-1-iodobenzene (27.1 mL, 226 mmol). While the reaction temperature was maintained at −76 to −71° C., n-butyllithium (2.5 M in hexanes, 85.7 mL, 214 mmol) was slowly added. The reaction mixture was stirred at −76° C. for 1.5 hours, then quenched with saturated aqueous ammonium chloride solution (1 L) and partitioned between water (1 L) and ethyl acetate (750 mL). After the mixture warmed to room temperature, the aqueous layer was extracted with ethyl acetate (3×250 mL), and the combined organic layers were washed with saturated aqueous sodium chloride solution (550 mL), dried over sodium sulfate, filtered, and concentrated in vacuo. Silica gel chromatography (Gradient: 0% to 70% ethyl acetate in heptane) gave the product as a yellow oil. Yield: 48.14 g, 133.2 mmol, 66%. 1H NMR (400 MHz, CDCl3) δ 7.94 (ddd, J=9, 9, 7 Hz, 1H), 7.28-7.40 (m, 5H), 6.87-6.93 (m, 1H), 6.80 (ddd, J=12.0, 8.6, 2.4 Hz, 1H), 4.60 (AB quartet, JAB=12.1 Hz, ΔνAB=21.4 Hz, 2H), 4.14 (br dd, J=12.8, 1.3 Hz, 1H), 3.82-3.90 (m, 2H), 3.72 (d, J=7.2 Hz, 1H), 3.54-3.60 (m, 2H), 3.50 (dd, half of ABX pattern, J=10.3, 4.1 Hz, 1H), 3.04-3.13 (m, 1H), 1.86 (ddd, J=14.0, 7.0, 2.0 Hz, 1H), 1.49-1.61 (m, 1H).

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