Crystal structure of a 1:1 complex of natural diterpenoids: Absolute configurations and unambiguous NMR spectral assignments of neoangustifolin and epinodosinol

Diterp-Complex-RA (DCRA), a unique, inseparable, natural hydrogen-bonded 1:1 complex consisting of a novel diterpenoid neoangustifolin and a known diterpenoid epinodosinol, was isolated from Rabdosia angustifolia, along with daucosterol and the known diterpenes sodoponin, trichorabdal A, and cirsiliol. The binding mode of the complex and the absolute configuration of the two substrates were elucidated through X-ray crystallography. Unambiguous NMR data assignments were achieved by a combination of various 2D-NMR techniques. It was clearly demonstrated that DCRA consists of two different diterpene substrates that are bound together by an intermolecular hydrogen bond and the very good hydrophobic close approach of mutual matching surfaces. In addition, intramolecular hydrogen bonds also contribute to the stabilization of the complex.