Spirocyclic iridoid alkaloids from Plumeria rubra

Area of Science:

• Natural Product Chemistry
• Organic Chemistry
• Pharmacognosy

Background:

• Plumeria rubra is a plant source of bioactive alkaloids.
• Structural elucidation of complex natural products presents significant challenges.
• Epimeric compounds often exhibit similar spectroscopic properties, complicating differentiation.

Purpose of the Study:

• To isolate and characterize new alkaloids from Plumeria rubra.
• To investigate the structural features of iridoid alkaloids, including new spirocyclic skeletons.
• To address the challenges in distinguishing epimeric alkaloids with distant chiral centers.

Main Methods:

• Isolation and purification of alkaloids using chromatographic techniques.
• Structure determination through comprehensive spectroscopic analyses (NMR, MS).
• Revision of known alkaloid structures and characterization of new compounds.

Main Results:

• Isolation of Plumerianoids A-D (1-4) with a novel spirocyclic iridoid alkaloid skeleton.
• Identification of a new degraded alkaloid, 8-epi-plumerianine (5), and revision of known alkaloid 6 to (8R,13S)-plumerianine.
• Observation of identical NMR spectra and mirror-like ECD curves for epimeric pairs (1/2, 3/4, 5/6).
• Compound 3 demonstrated moderate antiplasmodial activity.

Conclusions:

• The study highlights the difficulties in structural elucidation of alkaloids with chiral centers far from chromophores.
• Standard spectroscopic methods may be insufficient for differentiating certain epimeric alkaloid pairs.
• Plumeria rubra is a valuable source of structurally diverse alkaloids with potential bioactivity.