Hydrochloric Acid → Benzyl 4-(5-chloro-2-methoxyphenyl)piperidine-1-carboxylate

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To a solution of tert-butyl 4-(5-chloro-2-methoxyphenyl)-4-hydroxypiperidine-1-carboxylate (Preparation 235, 90 g, 0.263 mol) in 1,4-dioxane (200 ml) was added hydrogen chloride (4 M in dioxane, 150 ml, 0.6 mol) under argon. The mixture was stirred for 24 hours and evaporated in vacuo. Diethyl ether was added before concentrating in vacuo. Water (300 ml) and diethyl ether (500 ml) were added to this residue before the addition of sodium carbonate (32 g, 0.3 mol) under vigorous stirring. Mixture cooled over an ice bath before the dropwise addition of benzyl chlorocarbonate (43 ml, 0.3 mol). The bath was removed and the mixture stirred for 1 hour. The aqueous layer was extracted with ether (2×200 ml). The combined organic layers were washed with water (200 ml), saturated aqueous sodium chloride solution (200 ml), dried over sodium sulfate, filtered through silica gel (100 g, 40/63 μm) and evaporated in vacuo. 1,4-Dioxane was added and concentrated in vacuo before diluting with dichloromethane (300 ml). Triethylsilane (132 ml, 0.828 mol) and trifluoroacetic acid (96 ml, 1.24 mol) were added under argon and the mixture was stirred for 20 hours before concentrating in vacuo. To this residue were added saturated aqueous potassium carbonate solution to basify the solution to pH 10. Water (200 ml) was added and the aqueous layer was extracted with diethyl ether. The combined organic fractions were washed with water (2×200 ml), saturated aqueous sodium chloride solution (200 ml), dried over sodium sulphate, filtered through silica gel (100 g, 40/63 μm) and evaporated in vacuo. 1,4-dioxane was added to this residue before concentrating in vacuo. To a solution of this residue in tetrahydrofuran (300 ml), cooled over an ice bath in an atmosphere of argon, was added borane (1 M in tetrahydrofuran, 260 ml). The mixture was stirred at room temperature for 2 hours before cooling over an ice bath in argon and adding acetic acid (260 ml). The mixture was stirred for 24 hours before evaporating in vacuo. To this residue were added saturated aqueous potassium carbonate solution to basify the solution to pH 10. Water (200 ml) was added and the aqueous layer was extracted with diethyl ether. The organic fractions were washed with water (2×200 ml), saturated aqueous sodium chloride solution (200 ml), dried over sodium sulfate and evaporated in vacuo. The residue was purified on silica gel (500 g, 60/100 μm) eluting with carbon tetrachloride:ethyl acetate (gradient 1:0 to 10:1, by volume) to give the title compound as a yellow oil, 73 g, 77% yield.

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