Structure Revision of the Sesquiterpene Nordine Based on NMR Spectroscopic Analysis and X-ray Crystallography

Nordine was reported to be an unusual humulenetype macrocyclic sesquiterpenoid that contains an ether-bridged bicyclic ring between C-10 and C-6 with a hydroxy group at position 2. Here, we report the structure revision of nordine based on incongruities found for carbon chemical shifts in the originally proposed structure, in addition to formation of a diacetylated derivative. As expected, a single-crystal X-ray diffraction analysis unambiguously confirmed our proposal that the nordine (1) structure contains an ether-bridged bicyclic ring between C-10 and C-7 and hydroxy groups at C-2 and C-6. Furthermore, the absolute configuration was determined by ECD spectroscopic analysis.