Hydrochloric Acid → Ethyl (2R)-2-methyl-5-(propan-2-ylidene)cyclopentane-1-carboxylate

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Reaction Steps

(R)-(+)-Pulegone (76.12 g, 0.5 mmol), anhydrous NaHCO3 (12.5 g) and anhydrous ether (500 mL) were added to a 1 L round-bottom flask. The reaction mixture was cooled with an ice-bath under nitrogen. Bromine (25.62 mL, 0.5 mmol) was added dropwise over 30 minutes. The mixture was filtered and carefully added to NaOEt (21%, 412 mL, 1.11 mmol) in an ice-cooled bath. The mixture was stirred at room temperature overnight, and then 5% HCl (1 L) and ether (300 mL) were added. The aqueous phase was extracted with ether (2×300 mL). The combined organic phase was washed with water, dried and concentrated. The residue was added to a warmed solution of semicarbazide hydrochloride (37.5 g) and NaOAc (37.5 g) in water (300 mL), and then boiling ethanol (300 mL) was added to give a clear solution. The mixture was refluxed for 2.5 hours and then stirred at room temperature overnight. The mixture was treated with water (1 L) and ether (300 mL). The aqueous phase was extracted with ether (2×300 mL). The combined organic phase was washed with water, dried and concentrated. The residue was purified by vacuum distillation (73-76° C. at 0.8 mm Hg) to give (2R)-ethyl 2-methyl-5-(propan-2-ylidene)cyclopentanecarboxylate (63 g, 64%). 1H NMR (CDCl3, 400 MHz) δ 4.13 (m, 2H), 3.38 (d, J=16 Hz, 0.5H), 2.93 (m, 0.5H), 2.50-2.17 (m, 2H), 1.98 (m, 1H), 1.76 (m, 1H), 1.23 (m, 6H), 1.05 (m, 6H).

Provenance & Evidence