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4-(3-Ethoxy-3-oxopropan-1-yl)-4-hydroxy-1-piperidinecarboxylic acid 1,1-dimethylethyl ester (27.63 g, 0.092 mol) was used as raw material and dissolved in toluene (250 mL). To this solution was added p-toluenesulfonic acid (1.75 g, 9.2 mmol) and the mixture was heated to reflux for 3 hours. After the reaction was complete, the mixture was cooled and the solvent was removed by evaporation under reduced pressure. Water (400 mL) and ethyl acetate (400 mL) were added to the residue to separate the organic and aqueous layers. The aqueous layer was further extracted with ethyl acetate (200 mL). All organic layers were combined, washed with brine, dried over anhydrous magnesium sulfate, and the solvent was evaporated under reduced pressure to afford 1-oxo-8-N-BOC-azaspiro[4.5]decan-2-one (20.86 g, 88% yield). The mass spectrum (electrospray positive ion mode) showed m/z 256 ([M+1]+).

[1] Patent: US2003/236250, 2003, A1. Location in patent: Page 29
[2] Patent: WO2004/78750, 2004, A1. Location in patent: Page 79-80
[3] Bioorganic and Medicinal Chemistry Letters, 2002, vol. 12, # 13, p. 1759 - 1762