(Dimethyl sulfide)trihydroboron → Ethyl 5-oxa-6-azaspiro[3.4]octane-7-carboxylate

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To a solution of crude 5-oxa-6-aza-spiro[3.4]oct-6-ene-7-carboxylic acid ethyl ester (7.5 g, <40.9 mmol) in tetrahydrofuran (270 mL) at 0° C. was slowly added a solution of borane-dimethyl sulfide complex (10 M in THF, 16.4 mL, 164 mmol). The reaction was allowed to warm to room temperature overnight then recooled to 0° C., and quenched by the careful addition of water. The mixture was diluted with ethyl acetate, washed with water and brine, dried (MgSO4), and concentrated to yield a large amount of white solids. These solids were thoroughly triturated three times with dichloromethane (150 mL). The combined dichloromethane washings were concentrated and the resulting oil was purified by flash chromatography to yield 5-oxa-6-aza-spiro[3.4]octane-7-carboxylic acid ethyl ester (1.08 g, 29% over 2 steps). 1H-NMR: 400 MHz, (CDCl3) δ: 8.01-7.95 (br, 1H), 4.39-4.28 (m, 2H), 4.18-4.10 (m, 1H), 2.80-2.75 (m, 1H), 2.62-2.49 (m, 2H), 2.37-2.29 (m, 1H), 2.25-2.17 (m, 1H), 2.13-1.95 (m, 1H), 1.88-1.79 (m, 1H), 1.68-1.56 (m, 1H), 1.34 (t, J=7.1 Hz, 3H) ppm.

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