Total syntheses of tetracyolic diterpenes such as gibberellin and grayanotoxin are one of the big goals for the organic chemists. During the course of the investigations of the structure, total synthesis and some novel reaotions of Hibaene, the authors have found the possibilities to the syntheses of these biologically active diterpenes. In this paper, some approaches to the syntheses of C_<19>-gibberellins and staohenone are desoribed in two separated sections (A and B). A. Approach to the C_<19>-gibberellin A new synthetic route to the gibbane skeleton was established by the B-ring construction from compound 13 which was synthetized from compound 10 following the scheme III. Model experiments for the synthesis of ring A in gibberellin is also desoribed in scheme II. B. Approach to the staohenone Tricyclic intermediate 5a was synthetized by the application of Robinson-Mannich annelation reaction to compound 4, obtained from 4-methyl-cyclohexane-1,3-dione as following saheme IV. Oxidation of allyl group of compound 5a, followed by base catalyzed oyalization led to the potential tetracyclic intermediate 7a, which was converted to the compound 2 (scheme V). Conversion of 9 to stachenone 11 is now being actively investigated.