Dimethyl Sulfoxide → tert-Butyl 3,4-dioxopiperidine-1-carboxylate

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In a dried flask, a solution of 8.8 mL (9.7 g, 125 mmol) of anhydrous dimethyl sulfoxide in 400 mL of anhydrous dichloromethane was stirred under nitrogen at −60° C. as 15.9 mL (23.6 g, 113 mmol) of trifluoroacetic anhydride was added dropwise. The resultant solution was stirred at −60° C. for 20 min, at which time a solution of 8.5 g (39.1 mmol) of tert-butyl 3,4-dihydroxypiperidine-1-carboxylate from Step B in 100 mL of anhydrous dichloromethane was added via cannula. The solution was stirred at −60° C. for 1.5 h, during which time some precipitation occurred. Then 36.2 mL (26.3 g, 260 mmol) of triethylamine was added, and stirring was continued at −60° C. for an additional 30 min. The reaction mixture was concentrated in vacuo, and the residue was purified by flash chromatography (silica gel, 0-30% ethyl acetate/hexanes) to yield the title compound, which may exist as a mixture of tautomers. 1H NMR (500 MHz, CDCl3) δ 7.8-7.6 (br m, <1H), 5.3 (br apparent s, ≦1H), 4.00 (m, 2H), 2.69 (t, J=7.5 Hz, 2H), 1.57 (s, 9H). LC/MS: 214 (M+1).

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