Hydrochloric Acid → 1-Cyclopentyl-2-(2-methyl-4-nitrophenylimino)-3-thia-1-azaspiro[4.4]nonane

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A solution of 2-(2-methyl-4-nitrophenylimino)-3-thia-1-azaspiro[4.4]nonane (Method C2a; 33.2 g, 114 mmol) in DMF (1 L) was treated with NaOH (690 g, 17.3 mol) and cyclopentyl bromide (865 mL, 6.3 mol) and the resulting mixture was stirred at 20-40° C. for 18 h, then cooled to 4° C., and treated with water (1.5 L). A conc. HCl solution was added to adjust the pH to 0, and the mixture was extracted with EtOAc (80 mL). The organic phase was washed with a 1N HCl solution (1 L), dried (MgSO4) and concentrated under reduced pressure. The residue was dissolved in CH2Cl2 (500 mL) and filtered through a pad of silica gel (9×4 cm). Hexane was added to the resulting solution and volatiles were slowly removed by partial vacuum until crystals formed. The solids were collected to yield 1-cyclopentyl-2-(2-methyl-4-nitrophenylimino)-3-thia-1-azaspiro[4.4]nonane as yellow crystals (10.9 g, 26%): mp 118-9° C.; TLC (5% EtOAc/hex) Rf0.34.

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