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

Thirty new, highly oxygenated and stereogenic 14-membered macrocyclic diterpenoids, papyrifuranols A-Z (1-26) and AA-AD (27-30), and eight known analogs have been 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 cembranoid (CB) representation in the past seven decades by analyzing of 25 X-ray structures, lending a hand for the innately challenging 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 are proposed, and wound healing bioassays reveal that papyrifuranols N-P could significantly stimulate the proliferation and differentiation of umbilical cord mesenchymal stem cells.