Step 1. Synthesis of 3,6,6-trimethyl-6,7-dihydro-1H-indol-4(5H)-one: (E)-acetone aldehyde-1-oxime (0.50 g, 5.74 mmol) and 5,5-dimethyl-1,3-cyclohexanedione (0.80 g, 5.74 mmol) were dissolved in a mixed solvent of acetic acid (35 mL) and water (15 mL). Zinc powder (0.75 g, 11.5 mmol) was slowly added to the reaction system at room temperature. The reaction mixture was refluxed under stirring conditions. After completion of the reaction, the reaction solution was concentrated under reduced pressure and subsequently extracted with dichloromethane and saturated brine. The pH of the aqueous phase was adjusted with saturated sodium bicarbonate solution to 6. The aqueous phase was again extracted with dichloromethane and the organic layers were combined and dried over anhydrous magnesium sulfate. After filtration to remove the desiccant, the filtrate was concentrated under reduced pressure to give the crude product. Purification of the crude product by silica gel column chromatography (eluent: hexane/ethyl acetate, 1/3) afforded 3,6,6-trimethyl-6,7-dihydro-1H-indol-4(5H)-one as a yellow solid (4.9 g, 52% yield).
