[BH4-].[Na+].[Br:3][C:4]1[C:5]2[CH2:11][CH:10]([CH3:12])[C:9](=O)[C:6]=2[S:7][CH:8]=1.Cl.C1(C)C=CC=CC=1>C1COCC1.CO.O>[Br:3][C:4]1[C:5]2[CH2:11][C:10]([CH3:12])=[CH:9][C:6]=2[S:7][CH:8]=1A total of 1.30 g (34.4 mmol) of NaBH4 were added to a solution of 5.0 g (21.5 mmol) of the resulting 3-bromo-5-methyl-4,5-dihydro-6H-cyclopenta[b]thiophen-6-one in 60 ml of THF-methanol (2:1, vol.) over 1 h at −5° C. (Caution: temperature must be lower 0° C.). The mixture was stirred for 10 h at ambient temperature, then diluted with 20 ml of water, and acidified with 10% HCl to pH=1. The crude product was extracted with 3×100 ml of CH2Cl2. The combined extracts were washed with the saturated aqueous Na2CO3, dried over K2CO3, and evaporated to dryness. To the yellow oil obtained, 150 ml of toluene were added. This toluene solution was treated with a catalytic amount of pTolSO3H (ca. 0.2 g) for 10 min at reflux, cooled to room temperature, and, then, passed through a short Silica Gel 60 column (40-63 μm, d 20 mm, 1 50 mm). This column was additionally eluted with 300 ml of hexanes-methyl-tert-butyl ether mixture (10:1, vol.). The chromatographed product was evaporated to dryness. This procedure gave 4.61 g (99%) of beige crystalline product of 3-bromo-5-methyl-4H-cyclopenta[b]thiophene (containing 3-5% of 3-bromo-5-methyl-6H-cyclopenta[b]thiophene).
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