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- Typical procedure: A CH3CN solution (1 mL) of thiophene-2-carbaldehyde (50 mg, 0.45 mmol), 2,3-dimethyl-1,3-butadiene (184 mg, 2.25 mmol), and HMDSS (20 mg, 0.09 mmol) was treated under inert atmosphere at room temperature with CoCl2· 6H2O (107 mg, 0.45 mmol) dissolved in 1.5 mL of CH3CN. The mixture was added portionwise every 30 min with 20 mg of HMDSS, and the progress of the reaction was monitored by TLC (petroleum ether/diethyl ether 3:1). After the addition of 1 equiv. of HMDSS, the mixture was stirred overnight. After quenching with water, the product was extracted with diethyl ether and washed with water and brine. The resulting organic phase was then filtered to remove the dark precipitate and dried over Na2SO4. Evaporation of the solvent under vacuum gave 83 mg of crude 4,5-dimethyl-2-thienyl-3,6-dihydro-2 H-selenopyran 3e. Chromatographic purification (petroleum ether/diethyl ether 3:1) afforded 63 mg (54%) of 3e. 1H NMR (200 MHz, CDCl3), δ (ppm): 1.75 (3H, s), 1.83 (3H, s), 2.68 (2H, bd, J = 6 Hz), 3.09 (1H, d, J = 15 Hz), 3.25 (1H, d, J = 15 Hz), 4.55 (1H, t, J = 6 Hz), 6.90–6.93 (3H, m). 13C NMR spectrum (50 MHz, CDCl3), δ (ppm): 18.6, 19.0, 33.2, 34.1, 41.9, 119.8, 121.64, 122.2, 125.2, 126.1, 128.3. MS, m/z (I rel, %): 258 (19) [M]+, 176 (100), 143 (26), 97 (53), 77 (39), 65 (69). The reaction performed with cyclohexadiene afforded 2-(2-selena-bicyclo[2.2.2]oct-5-en-3-yl)thiophene 5b (57%) (endo:exo 95:5). Endo- 5b: 1H NMR (400 MHz, CDCl3), δ (ppm): 1.45–1.59 (1H, m), 1.71–1.92 (2H, m), 2.14–2.22 (1H, m), 3.02–3.06 (1H, m), 3.73–3.76 (1H, m), 5.02 (1H, d, J = 3.2 Hz), 6.28 (1H, ap t), 6.45 (1H, ap t), 6.81–6.86 (2H, m), 7.05 (1H, dd, J = 5.3, 1.5 Hz). 13C NMR spectrum (50 MHz, CDCl3), δ (ppm): 25.4, 29.2, 38.1, 42.5, 46.1, 124.2, 124.5, 126.1, 126.6, 129.8, 136.8. MS, m/z (Irel, %): 256 (10) [M]+, 176 (50), 97 (100), 97 (53), 79 (57)