Sodium borohydride → Tert-butyl 3-hydroxycycloheptylcarbamate

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Reaction Steps

To a solution of tert-butyl 3-oxocycloheptylcarbamate (31.09 g, 137 mmol) in methanol (454 mL) was added sodium borohydride (15.52 g, 410 mmol) portion-wise over ˜10 minutes with mixing. The resulting solution was stirred for 2 h at room temperature and then diluted with water (200 mL). The methanol was removed in-vacuo and the resulting aqueous mixture was diluted further with 500 mL ethyl acetate and 100 mL saturated aqueous sodium bicarbonate. The layers were separated and the aqueous layer back-extracted with 2×1000 mL ethyl acetate. The combined organic layers were dried over anhydrous magnesium sulfate, filtered and concentrated to an oil that was purified by silica gel chromatography (0-50% ethyl acetate in hexanes). Product containing fractions were concentrated to afford tert-butyl 3-hydroxycycloheptylcarbamate (21.1 g, 92 mmol, 67.3% yield). MS (ESI) m/z 230.3 [M+H]+. The above two reactions were repeated starting with 9.52 g cyclohept-2-enone and yielding 9.27 g of tert-butyl 3-hydroxycycloheptylcarbamate. The two batches of tert-butyl 3-hydroxycycloheptylcarbamate were then combined. 29.2 g of the combined material was separated into the 4 constituent stereoisomers (two diastereomers and their corresponding enantiomers) by preparative chiral SFC utilizing multiple injections over a series of 3 separate columns. First column: ChiralPak IC-H, 250×30 mm I.D., isocratic 15% ethanol in CO2, 38° C. Second column: ChiralCel OJ-H, 250×30 mm I.D., isocratic 10% isopropanol in CO2, 38° C. Third column: ChiralPak AD-H, 250×50 mm I.D., isocratic 20% isopropanol in CO2, 38° C. The separated isomers were characterized on an analytical scale Phenomenex Lux Amylose-2 column, 250×4.6 mm I.D., isocratic 15% ethanol in CO, (10 min run time) and labeled as Intermediate 1 to Intermediate 4.

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