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Step 1- Synthesis of ethyl 4-oxotetrahydrofuran-3-carboxylate (ge). NaH (60%, 4.6 g, 116 mmol) was suspended in anhydrous ether (200 mL) and cooled in an ice bath to 0°C. Ethyl ethanoate (10 mL, 100 mmol) was added slowly and dropwise to this suspension under nitrogen protection. The resulting white slurry was stirred at room temperature for 45 minutes. Subsequently, the ethyl ether was removed under reduced pressure to give a white solid. The solid was resuspended in DMSO (130 mL), cooled to 0°C in an ice bath, and then ethyl acrylate (13.7 mL, 127 mmol) was added slowly and dropwise. The resulting yellow colored mixture was stirred at room temperature overnight (~12 hours). After the reaction was completed, the reaction solution was slowly poured into 10% aqueous HCl solution (500 mL). Extraction was carried out with ether (3 x 200 mL). The ether extracts were combined, washed with saturated saline, dried over anhydrous MgSO4, filtered and concentrated under reduced pressure to afford 14 g (80% yield) of the target product ethyl 4-oxotetrahydrofuran-3-carboxylate (ge) as a clarified yellow liquid. The product was used directly in the next reaction without further purification.

[1] Patent: US2010/331305, 2010, A1. Location in patent: Page/Page column 80-81
[2] Patent: WO2004/26864, 2004, A1. Location in patent: Page 129; 130
[3] Patent: WO2014/15291, 2014, A1. Location in patent: Page/Page column 181