Configurational Variation of a Natural Compound within Its Source Species. The Unprecedented Case of Areolal in Piptothrix areolare

Scalemic natural products

Covering: up to the end of 2022The area of scalemic natural products is often enigmatic from a mechanistic standpoint, since low optical purity is observed in compounds having multiple contiguous stereogenic centers resulting from mechanistically distinct biogenetic steps. A scalemic state is rarely the result of a sloppy enzymatic activity, rather resulting from the expression of antipodal enzymes/directing proteins or from the erosion of optical purity by enzymatic or spontaneous reactions. Evidence for these processes is critically reviewed, identifying the mechanisms most often associated to the enzymatic generation of scalemic natural products and also discussing analytical exploitations of natural products' scalemicity.

Chiral NMR analysis reveals the environmental dependence of areolal scalemization in Piptothrix areolare

The occurrence of racemic and enantiomerically enriched (scalemic) mixtures of secondary metabolites in their natural sources is a rare phenomenon. The unprecedent case of enantiomeric variations from levorotatory to dextrorotatory, and back to levorotatory, passing through an almost racemic mixture, was recently documented for areolal, the major epoxythymol of Piptothrix areolare. In an attempt to shed some light to understand the reasons for such an unusual behavior, herein, we evaluated this phenomenon by correlating the areolal enantiomeric purity with several environmental variables, including temperature, humidity, rain precipitation, wind speed, and radiation during over 1 year of the plant life cycle. The specific rotation and enantiomeric excess determined by ¹ H-NMR-BINOL measurements provided the scalemic variations of areolal samples isolated from the roots collected from the same location along a 427-day period. The ¹ H-NMR-BINOL methodology provided better sensitivity to enantiomeric variations than specific rotation measurements. Statistical data, including matrix correlation analysis, exploratory analysis by heatmap plotting, and the principal component analysis (PCA), suggested direct correlation of the scalemic variation with humidity, rain precipitation, and radiation variables with the best PCA explanation (78.4%) and noncritical or poor correlations in PCA explained in 60.2% and 48.4%, respectively. When variations in the optical activity parameter of any metabolite are observed, the search for scalemic mixtures along their host plant life cycle should be undertaken. Herein, this phenomenon could be associated with interactions with soil microorganisms and with evolutionary aspects of Piptothrix areolare which belongs to Asteraceae, one of the most successfully adaptable plant families.

Methodology for the Absolute Configuration Determination of Epoxythymols Using the Constituents of Piptothrix areolare

Since epoxythymols occur in Nature either as scalemic mixtures or as pure enantiomers, the knowledge of their chiral composition and of the absolute configuration (AC) of the dominant enantiomer turns out to be mandatory. This task has already been faced using 1,1-bis-2-naphthol (BINOL), as a chiral solvating agent in accurate ¹H NMR quantifications to determine the enantiomeric ratio, and vibrational circular dichroism (VCD) to evidence the AC of the dominant enantiomer. We now explore the use of electronic circular dichroism (ECD) to determine the AC of an epoxythymol for which time-expensive DFT calculations would be required unless the AC of a related molecule is already known, from either VCD studies or single-crystal X-ray diffraction analysis, since one could correlate the ECD Cotton effect with the AC because in ECD only chromophores and their neighborhoods are evidenced. This method is now applied by using the epoxythymols from Piptothrix areolare. Known areolal (1) and 10-cinnamoyloxy-8,9-epoxythymol isobutyrate (2) were isolated from the roots, while known 7-acetoxy-10-cinnamoyloxy-8,9-epoxythymol isobutyrate (3) and 10-cinnamoyloxy-7-hydroxy-8,9-epoxythymol isobutyrate (4), as well as the new enantiopure 7-acetoxy-10-cinnamoyloxy-6-hydroxy-8,9-epoxythymol isobutyrate (5) and 10-cinnamoyloxy-8,9-epoxy-6-hydroxy-7-northymol isobutyrate (6), were obtained from the extract of the flowers. Chemical correlation of epoxythymols 1 and 3 was achieved. Compounds 1-4 were obtained as scalemic mixtures, and 5 and 6 as the pure (8S) enantiomers. In addition, the new 10-cinnamoyloxy-7-oxo-8,9-dehydrothymol isobutyrate (7) was isolated from the roots. The structures of 5-7 followed from NMR and HRMS data, while enantiomeric compositions of 1-6 were determined by ¹H NMR-BINOL measurements. The AC determination for 2-6 was done by ECD using a sample of 1 to reference the ECD Cotton effect. In turn, the AC of 1 was determined by VCD and extensive DFT calculations. The ECD-BINOL methodology turned out to be some 500 times more sensitive than that combining VCD and ¹H NMR-BINOL.

Vibrational Circular Dichroism Absolute Configuration of Natural Products From 2015 to 2019

Although demonstrated in 1975, vibrational circular dichroism (VCD) finally started to popularize during this century as a reliable tool to determine the absolute configuration (AC) of organic molecules. This research field continues to be a very dynamic one, in particular for the study of natural products which are a unlimited source of chiral molecules. It therefore turns of interest to summarize the accomplishments published in recent years and to comment on some eventual difficulties that emerged in rare cases to complete the AC determination task. Therefore the aim of this review is to update VCD results for the AC assignment of natural products published from 2015 to 2019, a period in which VCD was reported in some 126 publications involving almost 300 molecules. They are organized according the type of studied metabolite allowing an easily search. The molecules correspond to 28 monoterpenes concerning 17 papers, to 42 sesquiterpenes in 14 papers, to 51 diterpenes in 19 publications, to 5 other terpenoids in three papers, to 48 aromatic molecules in 15 reports, to 20 polyketides in 10 publications, to 27 miscellaneous formulas also in 10 papers, and to 76 nitrogen containing compounds, which include alkaloids and their synthetic analogs, in 38 articles. The landscape of reviewed molecules is quite wide as it goes from simple monoterpenes, like borneol or camphor, to very relevant biological molecules like the alkaloid cocaine or tadalafil samples to distinguish genuine and counterfeit Cialis®. In addition, 5 natural products and a simple derivative published outside the reviewed period, were used to illustrate some aspects of density functional theory calculations.

Asaroidoxazines from the Roots of Asarum asaroides Induce Apoptosis in Human Neuroblastoma Cells

Plants in the family Aristolochiaceae contain phenanthrene skeleton-containing chemical constituents that exhibit nephrotoxic, carcinogenic, mutagenic, anti-inflammatory, and cytotoxic effects. Two new phenanthrene-containing 1,2-oxazin-6-ones, designated as asaroidoxazine A (1) and asaroidoxazine B (2), and a known aristolactam, 5-methoxyaristololactam I (3), were isolated from the roots of Asarum asaroides. The structures of compounds 1 and 2 were determined using spectroscopic methods and X-ray crystallography. Treatment of SH-SY5Y human neuroblastoma cells with 1 μM of asaroidoxazine A (1) induced nuclear condensation as well as caspase-3/7 activation, indicating that this compound is a strong apoptosis inducer in neuronal cells. This is the first report of apoptosis induction by phenanthrene-containing oxazines.

The Chemistry of Kratom [Mitragyna speciosa]: Updated Characterization Data and Methods to Elucidate Indole and Oxindole Alkaloids

Two separate commercial products of kratom [Mitragyna speciosa (Korth.) Havil. Rubiaceae] were used to generate reference standards of its indole and oxindole alkaloids. While kratom has been studied for over a century, the characterization data in the literature for many of the alkaloids are either incomplete or inconsistent with modern standards. As such, full 1H and 13C NMR spectra, along with HRESIMS and ECD data, are reported for alkaloids 1-19. Of these, four new alkaloids (7, 11, 17, and 18) were characterized using 2D NMR data, and the absolute configurations of 7, 17, and 18 were established by comparison of experimental and calculated ECD spectra. The absolute configuration for the N(4)-oxide (11) was established by comparison of NMR and ECD spectra of its reduced product with those for compound 7. In total, 19 alkaloids were characterized, including the indole alkaloid mitragynine (1) and its diastereoisomers speciociliatine (2), speciogynine (3), and mitraciliatine (4); the indole alkaloid paynantheine (5) and its diastereoisomers isopaynantheine (6) and epiallo-isopaynantheine (7); the N(4)-oxides mitragynine-N(4)-oxide (8), speciociliatine-N(4)-oxide (9), isopaynantheine-N(4)-oxide (10), and epiallo-isopaynantheine-N(4)-oxide (11); the 9-hydroxylated oxindole alkaloids speciofoline (12), isorotundifoleine (13), and isospeciofoleine (14); and the 9-unsubstituted oxindoles corynoxine A (15), corynoxine B (16), 3-epirhynchophylline (17), 3-epicorynoxine B (18), and corynoxeine (19). With the ability to analyze the spectroscopic data of all of these compounds concomitantly, a decision tree was developed to differentiate these kratom alkaloids based on a few key chemical shifts in the 1H and/or 13C NMR spectra.