Methyl chloroformate → Methyl 3-cyclopentylprop-2-ynoate

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To a stirred solution of cyclopentylacetylene (7, 17.49 mL, 150.0 mmol) in anhydrous tetrahydrofuran (THF, 200 mL, 2466 mmol) at −78° C. was added 2.50 M of n-butyllithium in hexane (66.0 mL, 165 mmol, 1.1 equiv). The resulting milky suspension was stirred at −78° C. for 30 min. Methyl chloroformate (17.6 mL, 225 mmol, 1.5 equiv) was then added. The reaction mixture became a clear solution. The cooling bath was then removed, and the reaction mixture was allowed to warm to room temperature and stirred at room temperature for 1 h. The reaction mixture became a suspension again. When TLC (5% EtOAc/hexane, KMnO4 stain) showed the reaction was deemed complete, the reaction mixture was quenched with saturated aqueous NH4Cl solution (150 mL) and extracted with diethyl ether (Et2O, 2×200 mL). The combined organic layers were washed with saturated aqueous NaCl solution, dried over magnesium sulfate (MgSO4), filtered and concentrated under the reduced pressure. The residue was distilled under vacuum (99-101° C./16 mbar) to afford methyl 3-cyclopentylpropiolate (18, 21.856 g, 22.83 g theoretical, 96% yield) as a colorless oil. For 18: 1H NMR (CDCl3, 400 MHz) δ ppm 3.74 (s, 3H), 2.73 (m, 1H), 1.95 (m, 2H), 1.72 (m, 4H), 1.57 (m, 2H).

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