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

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 selectiv ity. Reduction and then alkylation of a tetracyclic adduct with 1,3-dithien ium tetrafluoroborate provided compound 56, which has the correct stereoche mistry at three key carbons for elaboration to the kempane diterpenes. Expl oratory reactions with tricyclic model compounds and with tetracyclic adduc ts have been used to assess the development of the desired stereochemistry about the decalin moiety. X-Ray structures for 52, 53 and 59 were determine d.