Sulfuric Acid → 4H,5H,6H-cyclopenta(b)thiophen-6-one

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To a solution of 1-(thiophen-2-yl)-prop-2-en-1-one (8.5 g; 62 mmoles) of Example 1b in 1,2-dichloroethane (47 mL) a solution of sulphuric acid (47 mL) is added dropwise at room temperature. The mixture is then heated to 80° C. for 75 minutes. Then cooling to room temperature is carried out and the cooled liquid mass is poured in an ice bath. An extraction with dichloromethane (2×20 mL) is then carried out and the organic phase washed with a 5% NaHCO3 solution, anhydrified with sodium sulphate, filtered and concentrated under vacuum. The obtained residue is purified by chromatography on silica gel with a ligroin/ethyl acetate mixture (elution gradient from 10% to 80% ethyl acetate). After solvent removal 2.4 g of 4H-cyclopenta[b]thiophen-6(5H)-one are isolated (yellow solid; yield 28%). Rf=0.54 (ligroin/ethyl acetate 9/7 volume/volume); 1H NMR (CDCl3) δ (ppm): 7.89 (d, 1H, ArH), 7.05 (d, 1H, ArH), 3.01 (m, 4H, CH2); 13C NMR (CDCl3) δ (ppm): 197.3, 169.0, 141.2, 140.5, 124.0, 41.2, 24.0 FT-IR (film) νmax: 3063.8, 1668.5, 1419.1, 1248.1, 960.7, 749.8 cm−1.

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