Enantiomeric natural products: occurrence and biogenesis

Metal-free oxidative cross-dehydrogenative coupling of quinones with benzylic C(sp3)–H bonds

A metal-free cross-dehydrogenative coupling of quinones with toluene derivatives has been established. A series of quinones were subjected to reaction with toluene derivatives in the presence of di-tertbutyl peroxide (DTBP) for direct synthesis of benzylquinones. The method exhibits good functional group tolerance, and desired products were obtained in moderate to good yields. Meanwhile, a radical pathway was proposed to describe the cross-dehydrogenative coupling of quinones with toluene derivatives.

A metal-free cross-dehydrogenative coupling of quinones with toluene derivatives has been established. A series of quinones were subjected to reaction with toluene derivatives in the presence of DTBP for direct synthesis of benzylquinones.

Phytochemistry of Three Ecuadorian Lamiaceae: Lepechinia heteromorpha (Briq.) Epling, Lepechinia radula (Benth.) Epling and Lepechinia paniculata (Kunth) Epling

In this research, the leaves of Lepechinia heteromorpha (Briq.) Epling, Lepechinia radula (Benth.) Epling and Lepechinia paniculata (Kunth) Epling have been collected in order to perform a phytochemical study. The first species was distilled to obtain a novel essential oil (EO), while the others were submitted to ethyl acetate extraction and secondary metabolite isolation. The chemical composition of the EO from L. heteromorpha has been investigated by Gas Chromatography-Mass Spectrometry (GC-MS) and Gas Chromatography with Retention Indices (GC(RI)), identifying 25 constituents. A major compound, (-)-ledol (21.2%), and a minor compound, (-)-caryophyllene oxide (1.0%), were isolated from the EO and their structures confirmed by Nuclear Magnetic Resonance (NMR) spectroscopy. Other major constituents of the EO were viridiflorene (27.3%), (E,E)-α-farnesene (1.4%), spirolepechinene and (E)-β-caryophyllene (7.1% each), allo-aromadendrene (6.1%), camphor (1.7%), limonene (1.3%) and β-phellandrene (4.6%). The enantiomeric composition of the EO monoterpene fraction was also studied, determining the enantiomeric excess and distribution of α-pinene, limonene, β-phellandrene and camphor. The ethyl acetate extract of L. radula and L. paniculata were fractionated by column chromatography. Spathulenol, angustanoic acid E and 5-hydroxy-4',7-dimethoxy flavone were isolated from L. radula extract; ledol, guaiol and (-)-carnosol were found in L. paniculata.

Enantiomeric ratios: Why so many notations?

The correct quantification of enantiomers is pivotal in a variety of fields, such as pharmacokinetic studies, enantioselective syntheses, chemical characterization of natural products, authentication of fragrance and food, biodegradation behavior, accurate evaluation of environmental risk, and it can also provide information for sentencing guidance in forensic field. Enantioselective chromatography is the first choice to assess the composition of an enantiomeric mixture. Different notations have been used to express the measured enantiomeric ratios, which compromise the results and represent a challenge for data comparison. This manuscript critically discusses the currently used notations and exemplifies with applications in different fields indicating the advantages and disadvantages of one of the adopted systems. In order to simplify the notations, the use of enantiomeric ratio (e.r.%) as standardization for nonchiroptical methods is proposed.

Diverse Engineered Heme Proteins Enable Stereodivergent Cyclopropanation of Unactivated Alkenes

Developing catalysts that produce each stereoisomer of a desired product selectively is a longstanding synthetic challenge. Biochemists have addressed this challenge by screening nature's diversity to discover enzymes that catalyze the formation of complementary stereoisomers. We show here that the same approach can be applied to a new-to-nature enzymatic reaction, alkene cyclopropanation via carbene transfer. By screening diverse native and engineered heme proteins, we identified globins and serine-ligated "P411" variants of cytochromes P450 with promiscuous activity for cyclopropanation of unactivated alkene substrates. We then enhanced their activities and stereoselectivities by directed evolution: just 1-3 rounds of site-saturation mutagenesis and screening generated enzymes that transform unactivated alkenes and electron-deficient alkenes into each of the four stereoisomeric cyclopropanes with up to 5,400 total turnovers and 98% enantiomeric excess. These fully genetically encoded biocatalysts function in whole Escherichia coli cells in mild, aqueous conditions and provide the first example of enantioselective, intermolecular iron-catalyzed cyclopropanation of unactivated alkenes.

Cannabis through the looking glass: chemo- and enantio-selective separation of phytocannabinoids by enantioselective ultra high performance supercritical fluid chromatography

By using the Inverted Chirality Columns Approach (ICCA) we have developed an enantioselective UHPSFC method to determine the enantiomeric excess (ee) of (-)-Δ⁹-THC in medicinal marijuana (Bedrocan®). The ee was high (99.73%), but the concentration of the (+)-enantiomer (0.13₅%) was not negligible, and it is worth a systematic evaluation of bioactivity.

Enantiomeric Mixtures in Natural Product Chemistry: Separation and Absolute Configuration Assignment

Chiral natural product molecules are generally assumed to be biosynthesized in an enantiomerically pure or enriched fashion. Nevertheless, a significant amount of racemates or enantiomerically enriched mixtures has been reported from natural sources. This number is estimated to be even larger since the enantiomeric purity of secondary metabolites is rarely checked in the natural product isolation pipeline. This latter fact may have drastic effects on the evaluation of the biological activity of chiral natural products. A second bottleneck is the determination of their absolute configurations. Despite the widespread use of optical rotation and electronic circular dichroism, most of the stereochemical assignments are based on empirical correlations with similar compounds reported in the literature. As an alternative, the combination of vibrational circular dichroism and quantum chemical calculations has emerged as a powerful and reliable tool for both conformational and configurational analysis of natural products, even for those lacking UV-Vis chromophores. In this review, we aim to provide the reader with a critical overview of the occurrence of enantiomeric mixtures of secondary metabolites in nature as well the best practices for their detection, enantioselective separation using liquid chromatography, and determination of absolute configuration by means of vibrational circular dichroism and density functional theory calculations.

Free Monoterpene Isomer Profiles of Vitis Vinifera L. cv. White Wines

Monoterpene compounds contribute floral and fruity characters to wine and are desired by grape growers and winemakers for many white wines. However, monoterpene isomers, especially monoterpene enantiomers, have been little explored. It is possible to identify and quantitate 17 monoterpene isomers in 148 varietal wines from eight grape varieties; Chardonnay, Gewürztraminer, Muscat, Pinot gris, Riesling, Sauvignon blanc, Torrontes, and Viognier in two vintages by Headspace solidphase microextraction multidimensional gas chromatography mass spectrometry (HS-SPME-MDGC-MS). Results obtained from general linear models and discriminant analysis showed significant differences for the isomer profiles and enantiomer fractions among the eight grape varieties and four wine styles. The high R² values from the fitted line show low variation in enantiomeric differences based on variety. These results provide an overview of the monoterpene isomers of wide varietal wines, and support that isomer profiles and enantiomer fractions could differentiate our wines by varietal and wine style.

Synthesis of Highly Enantio-Enriched Heliespirones A and C by a Diastereoselective Aromatic Claisen Rearrangement

Computational methods were used to investigate the stereochemical course of the extra-annular Claisen rearrangement. The stereochemical fidelity of the synthetic strategy and comparison of the optical properties support the hypothesis that the heliespirones are scalemic natural products.

Biosynthetic Enantiodivergence in the Eburnane Alkaloids from Kopsia

Reexamination of the absolute configuration of recently isolated eburnane alkaloids from Malaysian Kopsia and Leuconotis species by X-ray diffraction analysis and ECD/TDDFT has revealed the existence of biosynthetic enantiodivergence. Three different scenarios are discerned with respect to the composition of the enantiomeric eburnane alkaloids in these plants: first, where the new eburnane congeners possess the same C-20, C-21 absolute configurations as the common eburnane alkaloids (eburnamonine, eburnamine, isoeburnamine, eburnamenine) occurring in the same plant; second, where the new eburnane congeners possess opposite or enantiomeric C-20, C-21 absolute configurations compared to the common eburnane alkaloids found in the same plant; and, third, where the four common eburnane alkaloids were isolated as racemic or scalemic mixtures, while the new eburnane congeners were isolated as pure enantiomers with a common C-20, C-21 configuration (20α, 21α). Additionally, the same Kopsia species (K. pauciflora) found in two different geographical locations (Peninsular Malaysia and Malaysian Borneo) showed different patterns in the composition of the enantiomeric eburnane alkaloids. Revision of the absolute configurations of a number of new eburnane congeners (previously assigned based on the assumption of a common biogenetic origin to that of the known eburnane alkaloids co-occurring in the same plant) is required based on the present results.

Green Routes for the Production of Enantiopure Benzylisoquinoline Alkaloids

Benzylisoquinoline alkaloids (BIAs) are among the most important plant secondary metabolites, in that they include a number of biologically active substances widely employed as pharmaceuticals. Isolation of BIAs from their natural sources is an expensive and time-consuming procedure as they accumulate in very low levels in plant. Moreover, total synthesis is challenging due to the presence of stereogenic centers. In view of these considerations, green and scalable methods for BIA synthesis using fully enzymatic approaches are getting more and more attention. The aim of this paper is to review fully enzymatic strategies for producing the benzylisoquinoline central precursor, (S)-norcoclaurine and its derivatives. Specifically, we will detail the current status of synthesis of BIAs in microbial hosts as well as using isolated and recombinant enzymes.

Cephalotaxus Alkaloids

Cephalotaxus alkaloids represent a family of plant secondary metabolites known for 60 years. Significant activity against leukemia in mice was demonstrated for extracts of Cephalotaxus. Cephalotaxine (CET) (1), the major alkaloid of this series was isolated from Cephalotaxus drupacea species by Paudler in 1963. The subsequent discovery of promising antitumor activity among new Cephalotaxus derivatives reported by Chinese, Japanese, and American teams triggered extensive structure elucidation and biological studies in this family. The structural feature of this cephalotaxane family relies mainly on its tetracyclic alkaloid backbone, which comprises an azaspiranic 1-azaspiro[4.4]nonane unit (rings C and D) and a benzazepine ring system (rings A and B), which is linked by its C3 alcohol function to a chiral oxygenated side chain by a carboxylic function alpha to a tetrasubstituted carbon center. The botanical distribution of these alkaloids is limited to the Cephalotaxus genus (Cephalotaxaceae). The scope of biological activities of the Cephalotaxus alkaloids is mainly centered on the antileukemic activity of homoharringtonine (HHT) (2), which in particular demonstrated marked benefits in the treatment of orphan myeloid leukemia and was approved as soon as 2009 by European Medicine Agency and by US Food and Drug Administration in 2012. Its exact mechanism of action was partly elucidated and it was early recognized that HHT (2) inhibited protein synthesis at the level of the ribosome machinery. Interestingly, after a latency period of two decades, the topic of Cephalotaxus alkaloids reemerged as a prolific source of new natural structures. To date, more than 70 compounds have been identified and characterized. Synthetic studies also regained attention during the past two decades, and numerous methodologies were developed to access the first semisynthetic HHT (2) of high purity suitable for clinical studies, and then high grade enantiomerically pure CET (1), HHT (2), and analogs.

Scent from Jasminum grandiflorum flowers: Investigation of the change in linalool enantiomers at various developmental stages using chemical and molecular methods

Jasminum species are among the most preferred fresh cut flowers in India since ancient times. The plant produces small and fragrant flowers, which are of great demand in the preparation of fragrant garlands and also in perfume industries. Floral volatile of Jasminum grandiflorum L. (Family: Oleaceae) was extracted using solid-phase microextraction and analyzed in enantioselective gas chromatography. Chemical classes of identified volatiles revealed the presence of terpenoids, phenylpropanoids, and fatty acid derivatives. Marker constituent of flower volatiles, linalool was selected for analytical characterization on ethyl- and acetyl-β-cyclodextrin stationary phase. (R)-(-)-Linalool was found as major enantiomer in volatiles of floral buds whereas (S)-(+)-linalool predominated in the volatiles of matured flowers. Simultaneously, a quantitative real-time PCR was performed to find the gene expression of linalool synthase to investigate the mechanism of enantiomeric inversion. The emission pattern of (R)-(-)-linalool at different flower developmental stages was well correlated (P = 0.01) with the gene expression of the cloned linalool synthase from J. grandiflorum. We observed that the successive change in (R)- to (S)-linalool ratio from bud to mature flower was mainly due to the enantio- specific transformation and temporal decline of (R)-linalool producing gene in J. grandiflorum. This enantiomeric change also leads to the difference in flower aroma. Furthermore, this is probably the reason behind consumer's acceptance for jasmine buds rather than bloomed flowers in cut flower segments.

The Effects of Selected Sesquiterpenes from Myrica rubra Essential Oil on the Efficacy of Doxorubicin in Sensitive and Resistant Cancer Cell Lines

β-caryophyllene oxide (CAO), α-humulene (HUM), trans-nerolidol (NER) and valencene (VAL) are constituents of the essential oil of Myrica rubra (MEO), which has significant antiproliferative effect in various cancer cell lines. In the present study, we compared the antiproliferative effect of these sesquiterpenes alone and in combination with the cytostatic drug doxorubicin (DOX) in cancer cell lines with different sensitivity to DOX. Two ovarian cancer cell lines (sensitive A2780 and partly resistant SKOV3) and two lymphoblast cancer cell lines (sensitive CCRF/CEM and completely resistant CEM/ADR) were used. The observed effects varied among sesquiterpenes and also differed in individual cell lines, with only VAL being effective in all the cell lines. A strong synergism of DOX with NER was found in the A2780 cells, while DOX acted synergistically with HUM and CAO in the SKOV3 cells. In the CCRF/CEM cells, a synergism of DOX with CAO and NER was observed. In resistant CEM/ADR cells, sesquiterpenes did not increase DOX efficacy, although they significantly increased accumulation of DOX (up to 10-times) and rhodamine-123 (substrate of efflux transporter ABCB1) within cancer cells. In conclusion, the tested sesquiterpenes were able to improve DOX efficacy in the sensitive and partly resistant cancer cells, but not in cells completely resistant to DOX.

Monoterpenes Support Systemic Acquired Resistance within and between Plants

This study investigates the role of volatile organic compounds in systemic acquired resistance (SAR), a salicylic acid (SA)-associated, broad-spectrum immune response in systemic, healthy tissues of locally infected plants. Gas chromatography coupled to mass spectrometry analyses of SAR-related emissions of wild-type and non-SAR-signal-producing mutant plants associated SAR with monoterpene emissions. Headspace exposure of Arabidopsis thaliana to a mixture of the bicyclic monoterpenes α-pinene and β-pinene induced defense, accumulation of reactive oxygen species, and expression of SA- and SAR-related genes, including the SAR regulatory AZELAIC ACID INDUCED1 (AZI1) gene and three of its paralogs. Pinene-induced resistance was dependent on SA biosynthesis and signaling and on AZI1 Arabidopsis geranylgeranyl reductase1 mutants with reduced monoterpene biosynthesis were SAR-defective but mounted normal local resistance and methyl salicylate-induced defense responses, suggesting that monoterpenes act in parallel with SA The volatile emissions from SAR signal-emitting plants induced defense in neighboring plants, and this was associated with the presence of α-pinene, β-pinene, and camphene in the emissions of the "sender" plants. Our data suggest that monoterpenes, particularly pinenes, promote SAR, acting through ROS and AZI1, and likely function as infochemicals in plant-to-plant signaling, thus allowing defense signal propagation between neighboring plants.

Antiplasmodial Sesquiterpenoid Lactones from Trichospira verticillata: Structure Elucidation by Spectroscopic Methods and Comparison of Experimental and Calculated ECD Data

A dichloromethane extract of Trichospira verticillata from the Natural Products Discovery Institute was discovered to have good antiplasmodial activity (IC50 ∼5 μg/mL). After purification by liquid-liquid partition and C18 reversed-phase HPLC, four new germacranolide-type sesquiterpenoid lactones named trichospirolides A-D (1-4) were isolated. The structures of the new compounds were elucidated by analysis of their 1D and 2D NMR and MS data. The relative and absolute configurations were assigned based on a comparison of calculated and experimental ECD and UV spectra, specific rotations, internuclear distances, and coupling constants for all possible diastereomers for each compound. Among these four compounds, the conjugated dienone 1 displayed the most potent antiplasmodial activity, with an IC50 value of 1.5 μM.

Enantiodivergence in the Biosynthesis of Bromotyrosine Alkaloids from Sponges?

The isolation of bromotyrosine alkaloids, some of which are enantiomers of previously isolated compounds, has highlighted a possible enantiodivergence in their biosynthesis. Two new (1, 2) and six known bromotyrosine alkaloids (4-9), and the enantiomer (10) of a known compound, have been isolated from a Western Australian marine sponge, Pseudoceratina cf. verrucosa. The compounds inhibited the growth of multidrug-resistant and methicillin-resistant Staphylococcus aureus with comparable activity to vancomycin. In addition, one possible artifact of extraction (3) containing an ethoxy group was isolated. From analysis of the known bromotyrosine alkaloids, a biogenesis is proposed that explains the formation of antipodal natural products within this family of sponges.

The chemistry and biology of guanidine natural products

The chemistry and biology of natural guanidines isolated from microbial culture media, from marine invertebrates, as well as from terrestrial plants and animals, are reviewed.

Isolated alkali cation complexes of the antibiotic ionophore nonactin: correlation with crystalline structures

The stabilization of the nonactin–Na⁺ complex in a S₄ or C₂ conformation constitutes a challenging benchmark for experimental and modelling methods.

ent-Labdane Diterpenoids from Dodonaea viscosa

Seven new and two known ent-labdane diterpenoids have been isolated from a single plant specimen of Dodonaea viscosa ssp. spatulata, found in Tasmania, Australia. Prior to this study, only seven different labdane diterpenoids had been isolated from D. viscosa. The structures of the natural products were assigned via 1D and 2D NMR spectroscopy and other standard spectroscopic methods. The absolute configuration of three ent-labdane diterpenoids was determined by single-crystal X-ray crystallography of synthetic derivatives. Significantly, the results of this study suggest that the absolute configuration of some known labdane diterpenoids may have been misassigned.

Enantioselective Synthesis and Racemization of (-)-Sinoracutine

Sinoracutine is a recently isolated alkaloid with unusual stereochemical and biological properties. It features an unprecedented tetracyclic 6/6/5/5 skeleton that bears an N-methylpyrrolidine ring fused to a cyclopentenone. Interestingly, both enantiomers of sinoracutine have been independently isolated from the same plant, yet the molecule does not appear to occur as a racemate. Here, we present a short synthesis of (-)-sinoracutine that relies on a highly diastereoselective Pauson-Khand reaction and a Mandai-Claisen reaction to install the quaternary stereocenter. Our work establishes the absolute configuration of the levorotatory isomer and suggests that the optical purity of sinoracutine varies in nature due to its gradual racemization. Experimental evidence supports this proposal, and a plausible mechanism for the racemization is provided.

Garmultins A-G, Biogenetically Related Polycyclic Acylphloroglucinols from Garcinia multiflora

Garmultins A and B (1 and 2), two polycyclic polyprenylated acylphloroglucinols characterized by the coupling of two novel cages, 2,11-dioxatricyclo[4.4.1.0^3,9]undecane and tricyclo[4.3.1.0^3,7]decane, along with five biogenetically related analogues (3-7), were isolated from Garcinia multiflora. Their structures and absolute configurations were determined by extensive NMR analysis, X-ray crystallography, and electronic circular dichroism calculations. Three compounds were capable of inhibiting oncogene expression and inducing apoptosis in human erythroleukemia cells.