Synthetic studies toward the kempane diterpenes. Diels–Alder additions to bicyclic dienes

Abstract

Diels–Alder additions of 2,6-dimethyl-p-benzoquinone to the bicyclic dienes 24, 30 and 32 took place with very high regio-, stereo- and facial selectivity. Reduction and then alkylation of a tetracyclic adduct with 1,3-dithienium tetrafluoroborate provided compound 56, which has the correct stereochemistry at three key carbons for elaboration to the kempane diterpenes. Exploratory reactions with tricyclic model compounds and with tetracyclic adducts have been used to assess the development of the desired stereochemistry about the decalin moiety. X-Ray structures for 52, 53 and 59 were determined.