A. General procedure for the synthesis of ethyl 4-methylenecyclohexanecarboxylate: n-butyllithium (9.07 mL, 14.51 mmol) was slowly added to a solution of methyltriphenylphosphonium bromide (5.18 g, 14.51 mmol) in tetrahydrofuran (THF, 50 mL) at 0 °C. The reaction mixture was stirred at 0 °C for 30 min. Subsequently, a solution of ethyl 4-oxocyclohexanecarboxylate (1.9 g, 11.16 mmol) in THF (8 mL) was added dropwise at the same temperature. The reaction system was gradually warmed up to room temperature and stirring was continued for 2 hours. Upon completion of the reaction, the reaction was quenched with saturated aqueous ammonium chloride (NH4Cl) and diluted with ethyl acetate (EtOAc). The organic layer was separated, washed sequentially with water and brine, dried over anhydrous magnesium sulfate (MgSO4), filtered and concentrated under reduced pressure. The residue was purified by fast column chromatography on silica gel (SiO2) with an eluent gradient of 0 to 50% ethyl acetate/hexane to afford the title compound, ethyl 4-methylenecyclohexanecarboxylate (1.58 g, 84% yield), as a colorless oil.1H NMR (500 MHz, CDCl3) δ 4.65 (s, 2H), 4.17-4.11 (m, 2H), and 2.44 (tt, J = 11.1, 3.8 Hz, 1H), 2.35 (dt, J = 13.7, 4.0 Hz, 2H), 2.11-1.96 (m, 4H), 1.64-1.54 (m, 2H), 1.28-1.23 (m, 3H).
