Novel synthesis of 12,13-seco norditerpenoid alkaloids via semipinacol rearrangement and reaction with Br2–HOAc

Treatment of 14-methylsulfonyl pseudaconine 3 with DMF–NaOH at 150°C for 10 h afforded the desired C-nor and 12,13-seco norditerpenoid alkaloids 16α-methoxyl ketone 7 (70%) and 16β-methoxyl ketone 8 (15%) as a pair of epimers. Reaction of 7 with Br2–HOAc at room temperature for 1.5–2 h produced the phenols 9 (40%), 10 (10%) and 13 (29%). Whereas, treatment of 8 with Br2–HOAc under same conditions as the case for 7 gave phenolic compound 9 (38%) besides the by-product α-bromoketone 14 (25%).