Dynamic NMR and Computational Studies Inform the Conformational Description of Dendrillane Terpenes from the Nudibranch Goniobranchus coi.

Two new oxygenated terpenes (1 and 2) have been characterized from the Australian nudibranch Goniobranchus coi. Broadened 1H NMR signals, together with the absence of individual carbon NMR signals, complicated analysis of 5,9-epoxydendrillolide A (1); increasing the temperature to 323 K revealed the missing NMR signals. Low-temperature 1H NMR experiments provided an activation barrier of ∼15 kcal mol-1 and, together with DFT calculations, supported interconversion of a twist chair conformer with two different chair conformers. X-ray crystallographic analysis coupled with biosynthetic reasoning suggested a (5R, 8S, 9R, 13R, 14R, 15R, 16R) configuration. Ketone 2 demonstrated similar dynamic conformational processes to 1.