Kalparinol, a Salvialane (Isodaucane) Sesquiterpenoid Derived from Native Australian Dysphania Species That Suggests a Putative Biogenetic Link to Zerumbone

Research Progress on Sesquiterpene Compounds from Artabotrys Plants of Annonaceae

Artabotrys, a pivotal genus within the Annonaceae family, is renowned for its extensive biological significance and medicinal potential. The genus's sesquiterpene compounds have attracted considerable interest from the scientific community due to their structural complexity and diverse biological activities. These compounds exhibit a range of biological activities, including antimalarial, antibacterial, anti-inflammatory analgesic, and anti-tumor properties, positioning them as promising candidates for medical applications. This review aims to summarize the current knowledge on the variety, species, and structural characteristics of sesquiterpene compounds isolated from Artabotrys plants. Furthermore, it delves into their pharmacological activities and underlying mechanisms, offering a comprehensive foundation for future research.

seco-Pregnane Glycosides from Australian Caustic Vine (Cynanchum viminale subsp. australe)

Cynanchum viminale subsp. australe, more commonly known as caustic vine, is a leafless succulent that grows in the northern arid zone of Australia. Toxicity toward livestock has been reported for this species, along with use in traditional medicine and its potential anticancer activity. Disclosed herein are novel seco-pregnane aglycones cynavimigenin A (5) and cynaviminoside A (6), together with new pregnane glycosides cynaviminoside B (7) and cynavimigenin B (8). Cynavimigenin B (8) contains an unprecedented 7-oxobicyclo[2.2.1]heptane moiety in the seco-pregnane series, likely arising from a pinacol-type rearrangement. Interestingly, these isolates displayed only limited cytotoxicity in cancer and normal human cell lines, in addition to low activity against acetylcholinesterase and Sarcoptes scabiei bioassays, suggesting that 5-8 are not associated with the reported toxicity of this plant species.

Total Synthesis and Structural Revision of the 6,11-Epoxyisodaucane Natural Sesquiterpene Using an Anionic 8π Electrocyclic Reaction

A new synthetic strategy that forms a seven-membered carbocycle using an anionic 8π electrocyclic reaction facilitated the first total synthesis of the 6,11-epoxyisodaucane natural sesquiterpene in 9.0% yield over 10 steps in the longest linear sequence. The misassigned proposed stereochemistry was corrected by the synthesis of both the proposed structure and its C6 epimer. In addition, the 5-7-fused ring system was concisely constructed by tandem decyanation/five-membered-ring formation from an epoxynitrile.

Brønsted acid-induced transannulation of the phytochemical zerumbone

The sesquiterpene zerumbone was treated with HCl in ethyl acetate under the light-protected condition, and the time-dependent conversions were analyzed by gas chromatography. Nine products were isolated, and their structures were revealed by several NMR measurements such as ¹H NMR, ¹³C{¹H} NMR, distortionless enhancement by polarization transfer (DEPT)-135, ¹H-¹H correlation spectroscopy (COSY), ¹H-¹³C heteronuclear multiple quantum coherence (HMQC), and ¹H-¹³C heteronuclear multiple bond coherence (HMBC). The X-ray crystallography determined the stereochemistries of the three products and the two derivatives. After all, this acidic reaction was found to provide the (2Z,6E,10E)-isomer, the two HCl adducts, the two 7,6-bicyclic compounds, the valence isomers cycloheptatriene and norcaradiene, and the two dihydronaphthalenes. Based on the product analyses of the reactions from the isolated intermediates as well as the mechanistic considerations, these products were arranged into two paths: one of the paths ended in the two dihydronaphthalenes the same as previously reported under the Lewis acid condition; the other ended in the 7,6-bicyclic compound, the epimer of which was known. In addition, density functional theory (DFT) calculations indicated that the (2Z,6E,10E)-isomer was more stable than the (2E,6E,10Z)-isomer as well as that the activation energy for the isomerization at the C2-C3 double bond decreased to half by protonation. The closely examined reaction mechanisms under the simple acidic condition were established upon the intensive characterization of the intermediates and products, and these findings would add to the attractive value of zerumbone and would help understand the unknown biosynthetic pathway around sesquiterpenoids.

Transition Metal/Lewis Acid Catalyzed Reactions of Zerumbone for Diverse Molecular Motifs

Zerumbone is a naturally occurring humulene type sesquiterpene, isolated from the rhizomes of Zingiber zerumbet (L.) Smith with excellent therapeutic potential and is recognized as a valuable synthon for the construction of diverse array of natural product motifs. In this review, we intended to highlight our achievements in utilizing abundant natural product zerumbone and its derivatives for the development of pharmacologically relevant molecular scaffolds. We provided an account of the transition-metal catalyzed 1,4-conjugate addition reactions of zerumbone and its derivatives along with palladium-catalyzed cross-couplings, transition metal-based Lewis acid promoted interrupted Nazarov cyclisation reaction with substituted indoles and transannular cyclizations, photo-induced transformations of zerumbone and its epoxide.

Tandem Photoisomerization and Transannular Cyclizations of Zerumbone Epoxide: A Model for Diversity-Oriented Synthesis Using Abundant Natural Products

Photoirradiation of (6E,9E)-zerumbone-2,3-epoxide afforded a diverse range of transannular cyclized products in the presence of a catalytic amount of Sc(OTf)₃. At the behest of the geometrical isomers produced by photoirradiation, the diversity encompasses an unprecedented eudesmane core and oxo-bridged hydroxy-olefin skeletons. Structure elucidation and the stereochemical outcome of the products are described via extensive NMR analysis. The present study serves as a model for tandem photoisomerization and transannular cyclization of natural products with enone/dienone functionality.