Highly Rigid Labdane-Type Diterpenoids from a Chinese Liverwort and Light-Driven Structure Diversification

Area of Science:

• Natural Product Chemistry
• Organic Chemistry
• Photochemistry

Background:

• The Chinese liverwort *Haplomitrium mnioides* is a source of complex natural products.
• Diterpenoids are a diverse class of organic compounds with various biological activities.
• Understanding the biosynthesis of natural products can reveal novel chemical transformations.

Purpose of the Study:

• To isolate and characterize novel labdane-type diterpenoids from *Haplomitrium mnioides*.
• To investigate the photochemical reactions of related compounds and their implications in biosynthesis.
• To explore the potential use of these compounds as molecular markers and assess their allelopathic effects.

Main Methods:

• Isolation and structure elucidation of diterpenoids using spectroscopic techniques (e.g., NMR, MS).
• Photochemical reactions of haplomitrenonolides under light irradiation.
• Intramolecular cyclization studies.
• Preliminary allelopathic effect testing.

Main Results:

• Isolation of two unprecedented six-ring labdane-type diterpenoids, haplomintrins A (1) and B (2).
• Photochemical reactions of haplomitrenonolides C (6), A (4), and D (3) yielded haplomintrins A-C (1, 2, and 7).
• Compound 4 underwent intramolecular cyclization to form more complex congeners, haplomintrins D-G (8-11).
• The formation of haplomintrins A and B from compounds 6 and 4, respectively, suggests a photochemical role in their biosynthesis.
• The isolated compounds show potential as molecular markers for *H. mnioides*.

Conclusions:

• The study reports the discovery of novel labdane-type diterpenoids with unique six-ring systems.
• Photochemical reactions are proposed to be involved in the biosynthetic pathway of these compounds.
• The structural features of these diterpenoids may serve as valuable molecular markers for *H. mnioides*.
• Initial tests indicate potential allelopathic effects of these compounds.