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.

中文翻译：

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Pterolobirins A 和 B，来自 Pterolobium macropterum 果实的氧桥连卡桑二萜二聚体。

从Pterolobium macropterum的果实中分离出两种具有前所未有的 6/6/6/6/6/5/6/6/6 非环骨架的二聚体卡桑二萜类化合物，即紫檀素 A 和 B（1和2）。它们的结构是通过解释光谱数据来分配的。单晶 X 射线衍射数据明确证实了1的绝对构型。基于规则的分子间 Diels-Alder 反应和分子内亲核加成，提出了一种假定的生物合成途径。