Uncommon Polyoxygenated Sesquiterpenoids from South China Sea Soft Coral Lemnalia flava

A detailed
chemical investigation of the Chinese soft coral Lemnalia
flava yielded four new nardosinane-type
sesquiterpenoids (1–4), one new neolemnane-type
sesquiterpenoid (5), and one new sesquiterpenoid with
an uncommon 6/9 fused bicyclic skeleton (6), together
with two known related compounds (7 and 8). The structures and absolute configurations of 1–8 were determined on the basis of extensive spectroscopic
data analyses, X-ray diffraction analysis, chemical reactions, and
computer-assisted structural elucidation including 13C
NMR data calculation, residual dipolar coupling based NMR analysis,
and time-dependent density functional theoryelectronic circular dichroism
calculation. Plausible biogenetic pathways of two uncommon sesquiterpenoids
(4 and 6) were proposed.