A solution of 5,6-dihydro-4H-cyclopenta[b]thiophene-2-carbonyl chloride (1.75 g, 9.36 mmol) in anhydrous dioxane (20 mL) was bubbled with argon for 10 minutes. Triethylamine (3.27 mL, 23.41 mmol) was added and the bubbling was maintained for 5 minutes. Bis(triphenylphosphine)palladium(II) dichloride (219 mg, 0.31 mmol), copper iodide (119 mg, 0.62 mmol) and propargylaldehyde diethyl acetal (1.12 mL, 7.80 mmol) were added, and the reaction mixture was stirred for 2 hours at room temperature, under argon atmosphere. The solution was filtered and water (20 mL) was added to the filtrate. Layers were separated, and the aqueous layer was extracted with dichloromethane (2×20 mL). The combined organic layers were washed with a saturated solution of sodium hydrogenocarbonate (20 mL), dried over sodium sulfate and concentrated in vacuo. The residue was added to a solution of methylhydrazine (0.82 mL, 15.6 mmol) in ethanol (80 mL). The reaction mixture was stirred overnight at room temperature, concentrated in vacuo and dissolved in ethyl acetate (20 mL). The organic layer was successively washed with water (20 mL) and brine (20 mL) before being dried over sodium sulfate and concentrated in vacuo. The residue was purified by flash chromatography on silica gel (cyclohexane/ethyl acetate 100/0 to 90/10) to provide 3-{4H,5H,6H-cyclopenta[b]thiophen-2-yl}-5-(diethoxymethyl)-1-methyl-1H-pyrazole (37a) (1.80 g, 5.87 mmol, 75%).
