Synthesis of guaia-4(5)-en-11-ol, guaia-5(6)-en-11-ol, aciphyllene, 1-epi-melicodenones C and E, and other guaiane-type sesquiterpenoids via the diastereoselective epoxidation of guaiol

Exploring the synthetic potential of epoxide ring opening reactions toward the synthesis of alkaloids and terpenoids: a review

Epoxides are oxygen containing heterocycles which are significantly employed as crucial intermediates in various organic transformations. They are considered highly reactive three-membered heterocycles due to ring strain and they undergo epoxide ring opening reactions with diverse range of nucleophiles. Epoxide ring-opening reactions have gained prominence as flexible and effective means to obtain various functionalized molecules. These reactions have garnered substantial attention in organic synthesis, driven by the need to comprehend the synthesis of biologically and structurally important organic compounds. They have also found applications in the synthesis of complex natural products. In this review article, we have summarized the implementation of epoxide ring opening reactions in the synthesis of alkaloids and terpenoids based natural products reported within the last decade (2014-2023).

Structurally diverse rearranged sesquiterpenoids, including a pair of rare tautomers, from the aerial parts of Daphne penicillata

Eight structurally diverse rearranged sesquiterpenoids, including seven undescribed sesquiterpenoids (1a/1b and 3-8) were obtained from the aerial parts of Daphne penicillata. 1a/1b, 3, 5 and 6 possess rare rearranged guaiane skeletons and 4 represents the first example of rearranged carotene sesquiterpenoids. Their structures and absolute configurations were determined by extensive spectroscopic analyses, NMR and ECD calculations. Interestingly, 1a and 1b were a pair of magical interconverting epimers that may interconvert by retro-aldol condensation. The mechanism of interconversion has been demonstrated indirectly by 9-OH derivatization of 1a/1b and a hypothetical biogenetic pathway was proposed. All compounds were evaluated for anti-inflammatory and cytotoxic activities. Among them, 1a/1b and 2 exhibited potential inhibitory activities on the production of NO against LPS-induced BV2 microglial cells.

A New Highly Oxygenated Polyketide Derivative from Trichoderma sp. and Its Antifungal Activity

A new highly oxygenated polyketide derivative, trichodersine (1), together with fourteen known compounds (2-15) were isolated from Trichoderma sp. MWTGP-04. The structure of trichodersine (1) was established based on comprehensive spectroscopic data analysis, and biogenesis argument. The results of double culture experiments indicated that the strain exhibited potential antifungal activity. The antifungal activities of all isolated compounds were evaluated, among them compound 1 exhibited remarkable antifungal activities against Fusarium solani, Plectosphaerella cucumerina, Alternaria panax, and Aspergillus niger, with minimum inhibitory concentrations (MICs) of 4, 4, 16, and 32 μg/mL, respectively. In addition, the antifungal experiments of polyketide derivatives (1-3) disclosed that their degree of oxidation was a key factor affecting the antifungal activity.

Hedycaryol – Central Intermediates in Sesquiterpene Biosynthesis, Part II

The known sesquiterpenes that arise biosynthetically from hedycaryol are summarised. Reasonings for the assignments of their absolute configurations are discussed. The analysis provided here suggests that reprotonations at the C1=C10 double bond of hedycaryol are directed toward C1 and generally lead to 6–6 bicyclic compounds, while reprotonations at the C4=C5 double bond occur at C4 and result in 5–7 bicyclic compounds. Read more in the Review by H. Xu and J. S. Dickschat (DOI: 10.1002/chem.202200405).

Kinetic Resolution in Transannular Morita-Baylis-Hillman Reaction: An Approximation to the Synthesis of Sesquiterpenes from Guaiane Family

An approximation to the synthesis of several sesquiterpenes from the Guaiane family is described in which the core structure was obtained through a transannular Morita-Baylis-Hillman reaction performed under kinetic resolution. Several manipulations of the obtained MBH adduct have been carried out directed towards the total synthesis of γ-Gurjunene, to the formal synthesis of Clavukerin A, to the synthesis of a non-natural isomer of isoguaiane and to the synthesis of an advanced intermediate in the total synthesis of Palustrol.

Dimeric guaianes from leaves of Xylopia vielana as snail inhibitors identified by high content screening

The transcriptional repressor Snail trriggers epithelial-mesenchymal transition (EMT), the process allowing cancer cells with invasive and metastasis properties. In this study, we screened medicinal plants for the Snail inhibitory active components by high content screen (HCS) and found that the crude extract of Xylopia vielana leaves showed potential activity. Subsequently, bioassay-guided isolation of the extract of Xylopia vielana was performed to obtain twenty-four dimeric guaianes (1-24), including 16 new analogues (1-5, 8-11, 13-15, 17, 18, 21, and 22). Their structures were elucidated by the comprehensive application of multiple spectroscopic methods. Compounds 1, 11, 12, and 16 were initially identified as the active compounds. Wound healing assay, transwell migration assay and western blot experiments verified that compounds 1 and 12 inhibited the expression of Snail in a concentration-dependent manner, and compound 12 was verified as a potent tumor migration inhibitory agent. This work showed a practical strategy for the discovery of new Snail inhibitors from natural products and provided potential insights for dimeric guaianes as anticancer lead compounds specifically targeting Snail protein.

Direct formation of the sesquiterpeonid ether liguloxide by a terpene synthase in Senecio scandens

SsLOS directly catalyzed formation of the sesquiterpenoid ether liguloxide in the medicinal plant Senecio scandens. Terpene synthases determine the diversity of terpene skeletons and corresponding terpenoid natural products. Oxygenated groups introduced in catalysis of terpene synthases are important for solubility, potential bioactivity and further elaboration of terpenoids. Here we identified one terpene synthase, SsLOS, in the Chinese medicinal plant Senecio scandens. SsLOS acted as the sesquiterpene synthase and utilized (E,E)-farnesyl diphosphate as the substrate to produce a blend of sesquiterpenoids. GC-MS analysis and NMR structure identification demonstrated that SsLOS directly produced the sesquiterpenoid ether, liguloxide, as well as its alcoholic isomer, 6-epi-guaia-2(3)-en-11-ol. Homology modeling and site-directed mutagenesis were combined to explore the catalytic mechanism of SsLOS. A few key residues were identified in the active site and hedycaryol was identified as the neutral intermediate of SsLOS catalysis. The plausible catalytic mechanism was proposed as well. Altogether, SsLOS was identified and characterized as the sesquiterpenoid ether synthase, which is the second terpenoid ether synthase after 1,8-cineol synthase, suggesting some insights for the universal mechanism of terpene synthases using the water molecule in the catalytic cavity.

Germacrene A-A Central Intermediate in Sesquiterpene Biosynthesis

This review summarises known sesquiterpenes whose biosyntheses proceed through the intermediate germacrene A. First, the occurrence and biosynthesis of germacrene A in Nature and its peculiar chemistry will be highlighted, followed by a discussion of 6-6 and 5-7 bicyclic compounds and their more complex derivatives. For each compound the absolute configuration, if it is known, and the reasoning for its assignment is presented.

Mn(i)-Catalyzed nucleophilic addition/ring expansion via C–H activation and C–C cleavage

The Mn(i)-Catalyzed C–H alkenylation/carbonyl addition/retro-Aldol cascade was realized leading to the convenient synthesis of seven- or eight-membered carbocycles.

Absolute Configurations and Bioactivities of Guaiane-Type Sesquiterpenoids Isolated from Pogostemon cablin

Seven novel guaiane sesquiterpenoids (1-7) and three known seco-guaianes were isolated from the volatile oil of Pogostemon cablin. Their structures including absolute configurations were determined by spectroscopic analyses, a modified Mosher's method, and X-ray diffraction and ECD data. The results indicated that the ECD Cotton effects arising from one or two nonconjugated olefinic chromophores could be applied to define the absolute configurations of guaiane sesquiterpenoids. Compounds 3 and 6 exhibited significant vasorelaxant activity against phenylephrine-induced and KCl-induced contractions of rat aorta rings [half-maximal effective concentration (EC₅₀) of 3 against PHE-induced contraction, 5.4 μM; EC₅₀ of 6 against PHE- and KCl-induced contractions, 1.6 and 24.2 μM, respectively]. They also showed antifungal activity against Candida albicans (minimum inhibitory concentrations, 500 and 300 μM, respectively). In addition, 2 and 7-9 displayed a neuroprotective effect against glutamate-induced injury in PC12 cells.

Towards a complete characterisation of guaiacwood oil

Guaiacwood oil is a common perfume ingredient used in modern compositions for its suave woody-rosy scent. This essential oil is a byproduct of the timber industry obtained by hydrodistillation of the heartwood of Bulnesia sarmientoi, a tree native from Latin America. Despite being widely used in perfumery, guaiacwood oil has been poorly described in the past. This study aims at giving an in-depth characterisation of its chemical composition as well as disclosing the odorant compounds responsible for its characteristic fragrance. Our methodology was based on a combination of fractionation and analytical techniques, including comprehensive two-dimensional gas chromatography coupled to mass spectrometry and preparative capillary-gas chromatography. The entire analytical work led to the isolation of 20 constituents among which 14 have never been reported so far in natural extracts. Each isolated compound was fully characterised by spectroscopic methods. Finally, the accurate knowledge of the chemical composition permitted the identification of the odour-active constituents by gas chromatography-olfactometry.

Isolation and structure elucidation of natural products of three soft corals and a sponge from the coast of Madagascar

We investigated the three soft corals Sarcophyton stellatum, Capnella fungiformis and Lobophytum crassum and the sponge Pseudoceratina arabica, which have been collected at the coast of Madagascar. In addition to previously known marine natural products, S. stellatum provided the new (+)-enantiomer of the cembranoid (1E,3E,11E)-7,8-epoxycembra-1,3,11,15-tetraene (2). Capnella fungiformis afforded three new natural products, ethyl 5-[(1E,5Z)-2,6-dimethylocta-1,5,7-trienyl]furan-3-carboxylate (6), ethyl 5-[(1E,5E)-2,6-dimethylocta-1,5,7-trienyl]furan-3-carboxylate (7) and the diepoxyguaiane sesquiterpene oxyfungiformin (9a). The extracts of all three soft corals exhibited moderate activities against the malarial parasite Plasmodium falciparum. Extracts of the sponge Pseudoceratina arabica proved to be very active against a series of Gram-positive and Gram-negative bacteria.

Intramolecular cycloaddition/rearrangement cascade from gold(iii)-catalysed reactions of propargyl aryldiazoesters with cinnamyl imines

Surprising rearrangement of dihydroazepinyl aryldiazoacetates from a gold-catalysed [4+3]-cycloaddition to conjugated cycloheptene-1,4-dione-enamines.

Revisiting the Chemistry of Guaiacwood Oil: Identification and Formation Pathways of 5,11- and 10,11-Epoxyguaianes

Guaiacwood oil from Bulnesia sarmientoi Lorentz ex. Griseb is a common natural ingredient of the perfume industry used in both domestic and luxury fragrances for its highly appreciated woody-rosy odor, as well as its excellent fixative properties. Despite its long and traditional use as a perfume ingredient, guaiacwood oil has not been extensively studied. Thus, the chemical characterization of its constituents by using a full array of GC-hyphenated techniques (GC-MS, GC × GC-MS, and pc-GC) combined with conventional chemical fractionation was undertaken. In the course of this work, 15 new sesquiterpenoids mostly belonging to the 5,11- and 10,11-epoxyguaiane families were identified. Each isolated compound was fully characterized by NMR and MS. Collectively, the specific chemical relationships observed between sesquiterpene oxides and alcohols permitted the formulation of probable formation pathways regarding their presence as natural constituents of guaiacwood extracts.

Shiraz wines made from grape berries (Vitis vinifera) delayed in ripening by plant growth regulator treatment have elevated rotundone concentrations and "pepper" flavor and aroma

Preveraison treatment of Shiraz berries with either 1-naphthaleneacetic acid (NAA) or Ethrel delayed the onset of ripening and harvest. NAA was more effective than Ethrel, delaying harvest by 23 days, compared to 6 days for Ethrel. Sensory analysis of wines from NAA-treated fruit showed significant differences in 10 attributes, including higher "pepper" flavor and aroma compared to those of the control wines. A nontargeted analysis of headspace volatiles revealed modest differences between wines made from control and NAA- or Ethrel-treated berries. However, the concentration of rotundone, the metabolite responsible for the pepper character, was below the level of detection by solid phase microextraction-gas chromatography-mass spectrometry in control wines, low in Ethrel wines (2 ng/L), and much higher in NAA wines (29 ng/L). Thus, NAA, and to a lesser extent Ethrel, treatment of grapes during the preveraison period can delay ripening and enhance rotundone concentrations in Shiraz fruit, thereby enhancing wine "peppery" attributes.

Comparison of the formation of peppery and woody sesquiterpenes derived from α-guaiene and α-bulnesene under aerial oxidative conditions

Deuterium-labeled guaiane derivatives and their precursors, namely, d5-2R-rotundol (11a), d5-2S-rotundol (11b), d5-bulnesone (14), d5-2R-bulnesol (16), d7-α-guaiene (10), and d7-α-bulnesene (15), were synthesized in good yields as GC-MS internal standards for comparing the behavior of α-guaiene (1) and α-bulnesene (5) under autoxidative conditions. It was found that approximately 99% of α-guaiene coated onto filter paper and exposed to air at ambient temperature was autoxidized after 48 h and up to 7% of rotundone (3) and 0.6% of rotundols (2a/b) were formed during this period. Autoxidation of α-bulnesene (5) was considerably slower, with approximately 80% remaining after 2 days and yielding less than 1.5% of α-bulnesone (7) and 0.3% and 0.9% of bulnesols 6a and 6b, respectively, after 5 days. The results indicate the feasibility of rapid changes of aroma profiles of herbs and other plant materials over time when exposed to air.

Mechanistic studies on the autoxidation of α-guaiene: structural diversity of the sesquiterpenoid downstream products

Two unstable hydroperoxides, 6b and 10a, and 13 downstream sesquiterpenoids have been isolated from the autoxidation mixture of the bicyclic sesquiterpene α-guaiene (1) on cellulose filter paper. One of the significant natural products isolated was rotundone (2), which is the only known impact odorant displaying a peppery aroma. Other products included corymbolone (4a) and its C-6 epimer 4b, the (2R)- and (2S)-rotundols (7a/b), and several hitherto unknown epimers of natural chabrolidione A, namely, 7-epi-chabrolidione A (3a) and 1,7-epi-chabrolidione A (3b). Two 4-hydroxyrotundones (8a/b) and a range of epoxides (9a/b and 5a/b) were also formed in significant amounts after autoxidation. Their structures were elucidated on the basis of spectroscopic data and X-ray crystallography, and a number of them were confirmed through total synthesis. The mechanisms of formation of the majority of the products may be accounted for by initial formation of the 2- and 4-hydroperoxyguaienes (6a/b and 10a/b) followed by various fragmentation or degradation pathways. Given that α-guaiene (1) is well known to exist in the essential oils of numerous plants, coupled with the fact that aerial oxidation to form this myriad of downstream oxidation products occurs readily at ambient temperature, suggests that many of them have been overlooked during previous isolation studies from natural sources.