Irregular sesquiterpenoids from Ligusticum grayi roots

Area of Science:

• Natural Product Chemistry
• Organic Chemistry
• Phytochemistry

Background:

• Sesquiterpenoids are a diverse class of natural products derived from isoprene units.
• The plant Ligusticum grayi (Apiaceae) is known to produce various secondary metabolites in its root oil.
• Understanding the chemical diversity of plant secondary metabolites can provide insights into plant evolution and potential applications.

Purpose of the Study:

• To investigate the chemical composition of Ligusticum grayi root oil.
• To identify and characterize novel sesquiterpenoids and their carbon skeletons.
• To propose a biosynthetic pathway for the observed sesquiterpenoids.

Main Methods:

• Gas chromatography-mass spectrometry (GC-MS) for separation and identification of volatile compounds.
• Nuclear magnetic resonance (NMR) spectroscopy for detailed structural elucidation of isolated compounds.
• Chemical analysis of root oil from Ligusticum grayi specimens.

Main Results:

• Identification of numerous irregular sesquiterpenoids in L. grayi root oil.
• Discovery of eight previously unknown sesquiterpenoid carbon skeletons, including prethapsane, isothapsane, ligustigrane, isoligustigrane, preisothapsane, isoprethapsane, allothapsane, and oshalagrane.
• Characterization of known compounds such as sesquilavandulol and a sesquilavandulyl aldehyde, alongside two thapsanes and one epithapsane.

Conclusions:

• Ligusticum grayi root oil is a rich source of structurally diverse and novel sesquiterpenoids.
• The identified sesquiterpenoids are likely biosynthesized via a pathway involving the head-to-head coupling of geranyl diphosphate and dimethylallyl diphosphate, followed by cyclization and rearrangements.
• The remarkable structural variety suggests complex enzymatic processes and potential methyl migrations during biosynthesis.