The structure-stability relationships of 1-6 were investigated to show that 1 converted to 5 via a kinetic, solution-mediated autoxidation. In addition, alterbrassicene A (7), a fusicoccane-derived diterpenoid bearing an unprecedented 5/9/4-fused carbocyclic skeleton with a rare fused 2-cyclobuten-1-one motif, was characterized from Alternaria brassicicola. Its absolute structure was elucidated by spectroscopic analyses and quantum chemical calculations. Compound 7 was the first fusicoccane derivative acting as a potent IKKβ inhibitor in the NF-κB signaling pathway.