Two dimeric cassane diterpenoids with an unprecedented 6/6/6/6/6/5/6/6/6 nonacyclic framework, pterolobirins A and B (1 and 2), were isolated from the fruits of Pterolobium macropterum. Their structures were assigned by interpreting the spectroscopic data. The absolute configuration of 1 was unequivocally confirmed by single-crystal X-ray diffraction data. A putative biosynthetic pathway is proposed based on a regular intermolecular Diels-Alder reaction and an intramolecular nucleophilic addition.