The synthesis of (+)-4,4-dinor-9BH-pimara-7,15-diene, 4, a model compound for momilactones, has been achieved via stereospecific Diels-Alder reaction and alkylation. The fungitoxic momilactone diterpenes (eg. momilactone A, 2) form a class of rather unusual natural products, possessing a trans-syn hydrophenanthrene skeleton I**. Cther diterpenes with similar structures are annonalide3, icacine and analoga and humirianthenolides A-F5. The only total synthesis of a trans-syn tricyclic diterpene reported in the literature, is the synthesis of the 9BH-pimara-8(14),15-dienes by Church and Ireland' in 1963. More recently, Orsini and Pelizzoni described some synthetic approaches toward annonalide 7,839 . As a result of our research into the synthesis of these compounds, we now wish to report the synthesis of model compound 4 10 , which possesses all main features of the BC-ringsystem of the momilactones, in particular the trans-syn ring arrangement, the 47y8 -double bond and the geminally disposed u-methyl and B-vinyl groups at C-13. A Diels-Alder approach, starting from formyldecalone 1, seemed particularly attractive, since it might be expected to furnish the correct stereochemistry at C-911y12. When 2-silyloxybutadienes are used as dienes, an adventitious benefit is found in the regiospecific construction of a silylenolether, which is directly suitable for alkylation at C-13.