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Step C: Preparation of S-oxo-4-thiopiperidine hydrochloride; N-tert-butoxycarbonyl-5-oxo-4-thiopiperidine (20.0 g, 91.8 mmol) was dissolved in an ether solution (1.3 L) of 2N HCl and mechanically stirred for 18 hours at room temperature. After completion of the reaction, the suspension was filtered and the solid was collected. The solids were ground with hot isopropanol (300 mL) and subsequently cooled to room temperature and filtered again. The solids were washed sequentially with isopropanol (2 x 50 mL) and ether (2 x 50 mL). Finally, the product was dried under high vacuum to afford S-oxo-4-thiopiperidine hydrochloride (10.0 g, 70% yield) in beige powder form. The product was characterized by 1H NMR (400 MHz, DMSO-d6): δ 9.64 (br s, 2H), 3.50 (m, 2H), 3.33 (m, 2H), 3.18 (m, 2H), 3.06 (m, 2H).