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Step 2: Preparation of thietane-3-carboxylic acid Thietane-3-carbonitrile (7.2 g, 72.7 mmol) was mixed with 10% sodium hydroxide solution (120 mL) and the reaction was stirred at 100 °C for 16 hours. After completion of the reaction, the mixture was acidified with 3N hydrochloric acid to pH=1 and subsequently extracted with ethyl acetate (200 mL). The organic layer was washed with saturated saline (50 mL), dried over anhydrous sodium sulfate, and concentrated under reduced pressure to afford thietane-3-carboxylic acid (3.2 g) as a black solid, which could be used in subsequent steps without further purification.

[1] Patent: WO2013/7582, 2013, A2. Location in patent: Page/Page column 34
[2] Patent: WO2016/177655, 2016, A1. Location in patent: Page/Page column 170