Versatile Asymmetric Synthesis of the Kavalactones: First Synthesis of (+)-Kavain

Combining total synthesis and genetic engineering to probe dihydropyran formation in ambruticin biosynthesis

The ambruticins are a family of potent antifungal polyketide derived natural products isolated from the myxobacterium Sorangium cellulosum. Their unusual structures include a trisubstituted cyclopropyl group and two oxygen heterocycles, a tetrahydropyran (THP) and dihydropyran (DHP). Herein we report a flexible modular approach for the total synthesis of ambruticins which is used to prepare ambruticins F and S as well as in the first total synthesis of 20,21-dihydroambruticin F. The flexible strategy unites 3 fragments via Julia-Kocienski olefinations and provides important standards for investigation of dihydropyran formation in ambruticin biosynthesis. Cultures of wild-type S. cellulosum So ce10 produce mainly ambruticin S and the VS series of metabolites. An efficient electroporation method enabled gene knockout experiments which revealed that the ΔambP-S mutant of S. cellulosum accumulated the bisTHP polyketide 20,21-dihydroambruticin F. In contrast, the ΔambN-S mutant gave ambruticin F with the 20,21-alkene as the major metabolite confirming that AmbP and AmbO (a Rieske enzyme and flavin-dependent monooxygenase respectively) are implicated in 20,21-alkene formation. The results of feeding studies to a Sorangium strain containing only ambP and ambO are in accord with formation of the 20,21-alkene occurring prior to generation of the C3 to C7 dihydroxylated tetrahydropyran in ambruticin biosynthesis.

Reshaping the 2-Pyrone Synthase Active Site for Chemoselective Biosynthesis of Polyketides

Engineering enzymes with novel reactivity and applying them in metabolic pathways to produce valuable products are quite challenging due to the intrinsic complexity of metabolic networks and the need for high in vivo catalytic efficiency. Triacetic acid lactone (TAL), naturally generated by 2-pyrone synthase (2PS), is a platform molecule that can be produced via microbial fermentation and further converted into value-added products. However, these conversions require extra synthetic steps under harsh conditions. We herein report a biocatalytic system for direct generation of TAL derivatives under mild conditions with controlled chemoselectivity by rationally engineering the 2PS active site and then rewiring the biocatalytic pathway in the metabolic network of E. coli to produce high-value products, such as kavalactone precursors, with yields up to 17 mg/L culture. Computer modeling indicates sterics and hydrogen-bond interactions play key roles in tuning the selectivity, efficiency and yield.

Kava (Piper methysticum) in the United States: the quiet rise of a substance with often subtle effects

Background: Piper methysticum, commonly called kava, has long been consumed in beverage form in the Pacific Islands. Kava use in the US has slowly increased since the 1990s, but is not assessed in major epidemiological surveys.Objectives: To analyze social-media posts about kava from current, past, and prospective users, for motivations, patterns of co-use, and effects.Methods: Text from Reddit posts, and accompanying metadata, were collected and thematically coded by two independent raters.Results: 423 posts were collected, spanning January 2006 through December 2021. Of the 1,211 thematic codes applied, 1,098 (90. 7%) were concordant. Motivations for use bifurcated into self-treatment (for psychiatric or physical health conditions) and recreation; these were not mutually exclusive. Kava was rarely considered strongly euphoriant, but was valued as an anxiolytic. Kava was frequently used with other substances, most commonly kratom. Kava was used at lower doses for self-treatment than for other purposes (pseudo-R² = 0.11). Undesirable effects (gastrointestinal upset, fatigue) were mentioned, though less often than benefits. Hepatotoxicity, reported elsewhere as a rare, non-dose-related risk, was disputed on the basis of its not having been experienced by those posting.Conclusion: Kava appears to be conceptualized among Reddit posters as an anxiolytic with few risks or adverse effects. As it grows in popularity, especially among people who use other drugs that are more liable to misuse or addiction, it should be assessed in probability samples (i.e. in the major national drug surveys) and clinical practice for its risks, potential benefits, and possible drug-drug interactions.

Ambruticins: tetrahydropyran ring formation and total synthesis

The ambruticins are a family of polyketide natural products which exhibit potent antifungal activity. Gene knockout experiments are in accord with the proposal that the tetrahydropyran ring of the ambruticins is formed via the AmbJ catalysed epoxidation of the unsaturated 3,5-dihydroxy acid, ambruticin J, followed by regioselective cyclisation to ambruticin F. Herein, a convergent approach to the total synthesis of ambruticin J is described as well as model studies involving epoxidation and cyclisations of unsaturated hydroxy esters to give tetrahydropyrans and tetrahydrofurans. The total synthesis involves preparation of three key fragments which were united via a Suzuki-Miyaura cross-coupling and Julia-Kocienski olefination to generate the required carbon framework. Global deprotection to a triol and selective oxidation of the primary alcohol gave, after hydrolysis of the lactone, ambruticin J.

A Review on Synthetic Approaches towards Kavalactones

Kavalactones are classes of α-pyrone and 5,6-dihydropyrone derivatives showing various biological activities, and numerous approaches have been reported for the preparation of these molecules. In this review, we discuss the different synthetic approaches towards these naturally occurring lactones, in both racemic and enantiomerically pure forms, that have been reported in the literature to date. It is hoped that this review will assist researchers in the development of additional and efficient synthetic routes towards kavalactones.

1 Introduction

2 Synthetic Approaches for the Preparation of Kavalactones

3 Conclusion

Copper(I)-Catalyzed Asymmetric Conjugate 1,6-, 1,8-, and 1,10-Borylation

Catalytic asymmetric remote conjugate borylation is challenging as the control of regioselectivity is not trivial, the electrophilicity of remote sites is extenuated, and the remote asymmetric induction away from the carbonyl group is difficult. Herein, catalytic asymmetric conjugate 1,6-, 1,8- and 1,10-borylation was developed with excellent regioselectivity, which delivered α-chiral boronates in moderate to high yields with high enantioselectivity. The produced chiral boronate smoothly underwent oxidation, cross-coupling, and one-carbon homologation to give synthetically versatile chiral compounds in moderate yields with excellent stereoretention. Furthermore, a stereomechanistic analysis was conducted using DFT calculations, which provides insights into the origins of the regioselectivity. Finally, the present 1,6-borylation was successfully applied in an efficient one-pot asymmetric synthesis of (-)-7,8-dihydrokavain.

DARK Classics in Chemical Neuroscience: Kava

Kava (kava kava, Piper methysticum) is a common drug-containing plant in the Pacific islands. Kavalactones, its psychoactive compounds, exert potent central nervous system (CNS) action clinically and in animal models. However, the exact pharmacological profiles and mechanisms of action of kava on the brain and behavior remain poorly understood. Here, we discuss clinical and experimental data on kava psychopharmacology and summarize chemistry and synthesis of kavalactones. We also review its societal impact, drug use and abuse potential, and future perspectives on translational kava research.

Direct Catalytic Asymmetric Synthesis of Oxindole-Derived δ-Hydroxy-β-ketoesters by Aldol Reactions

Direct asymmetric synthesis of δ-hydroxy-β-ketoesters was accomplished via regio- and enantioselective aldol reactions of β-ketoesters with isatins catalyzed by cinchona alkaloid thiourea derivatives. The C-C bond formation of the reactions occurred only at the γ-position of the β-ketoesters. Reaction progress monitoring and product stability analyses under the conditions that included the catalyst indicated that the γ-position reaction products were formed kinetically. Various δ-hydroxy-β-ketoesters bearing 3-alkyl-3-hydroxyoxindole cores relevant to the development of bioactive molecules were synthesized.

Selected α-pyrones from the plants Cryptocarya novoguineensis (Lauraceae) and Piper methysticum (Piperaceae) with activity against Haemonchus contortus in vitro

Due to the widespread occurrence and spread of anthelmintic resistance, there is a need to develop new drugs against resistant parasitic nematodes of livestock animals. The Nobel Prize-winning discovery and development of the anti-parasitic drugs avermectin and artemisinin has renewed the interest in exploring natural products as anthelmintics. In the present study, we screened 7500 plant extracts for in vitro-activity against the barber's pole worm, Haemonchus contortus, a highly significant pathogen of ruminants. The anthelmintic extracts from two plants, Cryptocarya novoguineensis and Piper methysticum, were fractionated by high-performance liquid chromatography (HPLC). Subsequently, compounds were purified from fractions with significant biological activity. Four α-pyrones, namely goniothalamin (GNT), dihydrokavain (DHK), desmethoxyyangonin (DMY) and yangonin (YGN), were purified from fractions from the two plants, GNT from C. novoguineensis, and DHK, DMY and YGN (= kavalactones) from P. methysticum. The three kavalactones induced a lethal, eviscerated (Evi) phenotype in treated exsheathed third-stage larvae (xL3s), and DMY and YGN had moderate potencies (IC₅₀ values of 31.7 ± 0.23 μM and 23.7 ± 2.05 μM, respectively) at inhibiting the development of xL3s to fourth-stage larvae (L4s). Although GNT had limited potency (IC₅₀ of 200–300 μM) at inhibiting L4 development, it was the only compound that reduced L4 motility (IC₅₀ of 6.25–12.50 μM). The compounds purified from each plant affected H. contortus in an irreversible manner. These findings suggest that structure-activity relationship studies of α-pyrones should be pursued to assess their potential as anthelmintics.

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7500 plant extracts were screened against Haemonchus for anthelmintic activity.

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Three of these extracts were potent inhibitors of larval motility and/or development.

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Pure α-pyrones isolated from active fractions exhibited significant nematocidal activity.

SmCpR2-mediated cross-coupling of allyl and propargyl ethers with ketoesters and a telescoped approach to complex cycloheptanols

A selective coupling of allyl/propargyl ethers and δ-ketoesters, mediated by SmCp^R₂, delivers δ-lactones, or complex cycloheptanols via a telescoped approach.

Dihydro-5,6-dehydrokavain (DDK) from Alpinia zerumbet: Its Isolation, Synthesis, and Characterization

Dihydro-5,6-dehydrokavain (DDK) is the major and most promising component of the tropical plant Alpinia zerumbet (shell ginger), a species of the ginger family Zingiberaceae. Alpinia zerumbet is known for its human use as a traditional herbal medicine, food, and dietary supplement. With its α-lactone ring, DDK belongs to the large chemical group of kavalactones, which are also found in kava (Piper methysticum), another herbal medicine; DDK is characterized by a double-bond linkage at positions 5,6 and the absence of a double-bond linkage at positions 7,8. This dissociates DDK from other kavalactones with their linkages at positions 7,8 and 5,6 that are both either completely saturated or unsaturated, or may have an unsaturated bond at the position 7,8 as well as a saturated bond at the position 5,6. DDK is easily identified and quantified by HPLC and GC. DDK contents in fresh leaves, stems and rhizomes range from 80 to 410 mg/g, requiring solvent extraction procedures to ensure high DDK yield. This is best achieved by hexane extraction from fresh rhizomes that were previously boiled in water, allowing DDK yields of up to 424 mg/g. Successful synthesis of DDK can be achieved by asymmetric pathways, whereas its simple chemical structure facilitates the synthesis of DDK derivatives by HCl hydrolysis. Thus, all synthesized products may be used for various commercial purposes, including the potential development of promising antiobesity pharmaceutical drugs, preparation of specific and safe dietary supplements, and use as effective natural herbicides or fungicides.

The taumycin A macrocycle: asymmetric total synthesis and revision of relative stereochemistry

The first asymmetric total synthesis and revision of the relative configuration of the 12-membered taumycin A macrocycle is described. Key to the success of this work was a novel α-keto ketene macrocyclization that provided an efficient means by which to access two diastereomers of the desired macrolide without the need to employ additional coupling agents or unnecessary oxidation state adjustments.

Boesenbergia rotunda: From Ethnomedicine to Drug Discovery

Boesenbergia rotunda is a herb from the Boesenbergia genera under the Zingiberaceae family. B. rotunda is widely found in Asian countries where it is commonly used as a food ingredient and in ethnomedicinal preparations. The popularity of its ethnomedicinal usage has drawn the attention of scientists worldwide to further investigate its medicinal properties. Advancement in drug design and discovery research has led to the development of synthetic drugs from B. rotunda metabolites via bioinformatics and medicinal chemistry studies. Furthermore, with the advent of genomics, transcriptomics, proteomics, and metabolomics, new insights on the biosynthetic pathways of B. rotunda metabolites can be elucidated, enabling researchers to predict the potential bioactive compounds responsible for the medicinal properties of the plant. The vast biological activities exhibited by the compounds obtained from B. rotunda warrant further investigation through studies such as drug discovery, polypharmacology, and drug delivery using nanotechnology.

Synthesis of β-methoxyacrylate natural products based on box-Pd(II)-catalyzed intermolecular methoxycarbonylation of alkynoles

Bis(oxazoline)-palladium(II) catalyzed carbonylation of homopropargyl alcohols afforded acyclic methoxyacrylate 2 and 6-membered lactone 3a-k in good combined yield. In the case of propargyl alcohols, 5-membered lactones 3p, 3q, 16 were obtained in moderate yields. The one-pot synthesis of kawa lactones 3a, 3r, 3s and formal synthesis of dihydroxycystothiazole A and dihydroxycystothiazole C are presented. To elucidate the stereochemistry of (+)-annularin G and (-)-annularin H, the first asymmetric syntheses of these natural products were achieved.

Identification of methysticin as a potent and non-toxic NF-kappaB inhibitor from kava, potentially responsible for kava's chemopreventive activity

Nuclear factor-kappaB (NF-kappaB) is a transcription factor that plays an essential role in cancer development. The results of our recent chemopreventive study demonstrate that kava, a beverage in the South Pacific Islands, suppresses NF-kappaB activation in lung adenoma tissues, potentially a mechanism responsible for kava's chemopreventive activity. Methysticin is identified as a potent NF-kappaB inhibitor in kava with minimum toxicity. Other kava constituents, including four kavalactones of similar structures to methysticin, demonstrate minimum activities in inhibiting NF-kappaB.

Synthesis and stereochemistry-activity relationship of small bacteriocin, an autoinducer of the symbiotic nitrogen-fixing bacterium Rhizobium leguminosarum

The four stereoisomers of small bacteriocin, an autoinducer of the symbiotic nitrogen-fixing bacterium Rhizobium leguminosarum, were synthesized via a versatile methodology for 3'-hydroxyacyl homoserine lactones based on the Nagao asymmetric aldol reaction. The synthetic isomers were much less effective at inhibiting the growth of R. leguminosarum RBL5523 than the natural isomer, showing the importance of stereochemistry for activity.

A Positional Scanning Approach to the Discovery of Dipeptide-Based Catalysts for the Enantioselective Addition of Vinylzinc Reagents to Aldehydes

[reaction: see text] A combinatorial library of dipeptide N-acylethylenediamine-based ligands was synthesized by parallel solid-phase methods. These ligands were screened in crude form as catalysts for the asymmetric addition of vinylzinc reagents to aldehydes to give chiral allylic alcohols. Three sites of diversity on the ligands were optimized using a positional scanning approach. The optimized structure from the library, ligand 54, was found to catalyze the formation of 10 different (E)-allylic alcohols with enantioselectivities ranging from 90% to 95% ee. This ligand was effective for both aromatic and alpha-branched aldehydes, and vinylzinc reagents derived from both bulky and straight chain terminal alkynes.