Enantioselective total syntheses of (+)-decursin and related natural compounds using catalytic asymmetric epoxidation of an enone

The enantioselective total syntheses of (+)-decursin (1) and related natural dihydropyranocoumarins (-)-prantschimgin (3), (+)-decursinol (4), and (+)-marmesin (5) were achieved for the first time using catalytic asymmetric epoxidation of an enone as the key step. Catalytic asymmetric epoxidation of the enone was effectively promoted by the novel multifunctional asymmetric catalyst generated from La(O-i-Pr)₃, BINOL, and Ph₃As=O in a 1:1:1 ratio to afford epoxide in 94% yield and 96% ee, which was recrystallized to give optically pure epoxide. After conversion to the common key intermediate (-)-peucedanol (7), all natural dihydropyranocoumarins were synthesized through palladium-catalyzed intramolecular C-O coupling reactions. A possible reaction mechanism of the catalytic asymmetric epoxidation of enones is also described based on X-ray analysis, laser desorption/ionization time-of-flight mass spectrometry, kinetic studies, and asymmetric amplification studies.