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To an ice bath cooled mixture of sodium hydride (60%, 0.59 g, 14.83 mmol) and trimethylsulfoxide iodide (2.76 g, 12.55 mmol) in dimethylsulfoxide (DMSO, 10 mL), the reaction mixture was kept at 10 °C. After stirring at 10 °C for 10 min, the mixture was brought to room temperature and stirring was continued for 1 hour. A solution of 1-benzyl-3-pyrrolidone (2 g, 11.41 mmol) in DMSO (10 mL) was slowly added via syringe over 10 min. The resulting reaction mixture was stirred at room temperature for 1.5 h, diluted with ether (Et2O) and the reaction was quenched by the addition of saturated aqueous ammonium chloride (NH4Cl). The organic and aqueous phases were separated and the aqueous phase was back-extracted with ether. The organic phases were combined, dried, filtered and concentrated under reduced pressure to give 5-benzyl-1-oxa-5-azaspiro[2.4]heptane (product, 2.10 g, 97% yield), which was used directly in the next step of the reaction. Mass spectrum (electrospray ionization, m/z): 190.1 [M+H]+.

[1] Patent: WO2016/42452, 2016, A1. Location in patent: Page/Page column 186
[2] Patent: WO2016/42453, 2016, A1. Location in patent: Page/Page column 91
[3] Patent: WO2018/178041, 2018, A1. Location in patent: Page/Page column 120
[4] Patent: WO2016/42451, 2016, A1. Location in patent: Page/Page column 97
[5] Synthesis (Germany), 2018, vol. 50, # 18, p. 3696 - 3707