[C:1]1([NH:7]N)[CH:6]=[CH:5][CH:4]=[CH:3][CH:2]=1.[C:9]1(=O)[CH2:13][CH2:12][CH2:11][CH2:10]1.O.S(=O)(=O)(O)O>CC(O)C>[CH2:11]1[C:10]2[C:2]3[CH:3]=[CH:4][CH:5]=[CH:6][C:1]=3[NH:7][C:9]=2[CH2:13][CH2:12]1A solution of phenylhydrazine (32.44 g, 300 mmol) in 2-propanol (300 mL) was treated with cyclopentanone (27 mL, 25.7 g, 305 mmol). The solution was stirred at 20° C. for 1 h and poured onto mixture of ice (900 g) and water (300 mL). The chilled mixture was stirred until the ice had melted then filtered. The filter-cake was washed with water (2×300 mL) an the resultant damp solid (85 g) was added to water (540 mL). The stirred suspension was treated with concentrated sulfuric acid (33 mL, 61 g, 600 mmol) then heated under reflux for 30 min, cooled to 0° C. and stirred for 15 min. The dark red solid was filtered off, washed with water (2×60 mL) and air-dried for 18 h. The crude product was added to dichloromethane (300 mL), stirred for 30 min then filtered, washing with dichloromethane (100 mL). The filtrate was treated with silica (48 g), stirred for 1 h, filtered and washed with dichloromethane (400 mL). The filtrate was concentrated to give a solid, which was triturated with hexane to give 1,2,3,4-tetrahydrocyclopent[b]indole (30 g, 65%) as a pink solid. Analytical data for 1,2,3,4-tetrahydrocyclopent[b]indole are included in Table 3 below.
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