Dichloromethane (400 mL) was added to the crude product (compound-II-a) under ice bath conditions. A solution of titanium tetrachloride (11.3 g) in dichloromethane (50 mL) was slowly added dropwise, the internal temperature of the reaction system was controlled to be less than 10°C, followed by a slow warming up to room temperature with continuous stirring. The progress of the reaction was monitored by gas chromatography until the amount of raw material remaining was less than 3% (about 8 hours). Upon completion of the reaction, the reaction solution was poured into ice water (~2 L) and extracted with dichloromethane to combine the organic phases. The organic phase was dried over anhydrous sodium sulfate, filtered and concentrated to give the crude product. Purification by fast column chromatography (eluent ratio: ethyl acetate/petroleum ether = 1:10) gave a light yellow solid target product (compound-III-a, 21.66 g) in 65% yield.
