Synthetic studies towards complex diterpenoids. Part 18. Total synthesis of (±)-isopisiferin and the related compounds
A simple convergent and general method has been developed for the synthesis of (±)-isopisiferin (1c), a rearranged abietane diterpene, having a hexahydrodibenzo [a,d] cycloheptene ring system, and the related model systems (1a) and (1b), through the respective enolisable tricyclic ketone mixtures (12c) and (13c), (12a) and (13a), and (12b) and (13b), obtained from the corresponding easily accessible 2-arylethyl-3,3-dimethylcyclohexanones (7c), (7a), and (7b). Demethylation of the styrenoid ethers (15b) and (15c) under acid conditions gave the tetracyclic dienones (17b) and (17c) through Ar1-5 cyclisation.