X-Ray Study of Cembranoids with Flexible Rings from Boswellia papyrifera Resins Allowing Structural Revisions of Misleading Structures in the Past 70 Years.

30 new highly oxygenated and stereogenic 14-membered macrocyclic diterpenoids, papyrifuranols A-Z (1-26) and AA-AD (27-30), and 8 known analogs were isolated from Boswellia papyrifera resins. All the structures were characterized by detailed spectral analyses, quantum calculations, X-ray diffraction, and modified Mosher's methods. Notably, six previously reported structures were revised. Our study points out misleading factors of macrocyclic cembranoids (CBs) representation in the past 7 decades by analysis of 25 X-ray structures, lending a hand for innately challengeable structure identification of such flexible macrocyclic CBs and avoiding following the tracks of an overturned cart during future structure characterization and total synthesis. Biosynthetic conversions of all the isolates were proposed and wound healing bioassays reveal that papyrifuranols N-P could significantly stimulate the proliferation and differentiation of umbilical cord mesenchymal stem cells.