An enantioselective synthesis of the fullyelaborated tricyclic decahydrofluorene core (ABC-ring system) of GKK1032s, novel antimicrobial and antitumor agents, has been accomplished for the first time by employing a highly diastereoselective intramolecular Diels−Alder (IMDA) reaction. The key substrate for the IMDA reaction was efficiently prepared through (i) an intermolecular Diels−Alder reaction