Scalemic natural products

BBL enzymes exhibit enantiospecific preferences in the biosynthesis of pyridine alkaloids in Nicotiana tabacum L

Plant species can accumulate secondary metabolites in optically pure form or, occasionally, as enantiomeric mixtures. Interestingly, enantiomers of the same molecule can confer different biological activities. In tobacco (Nicotiana tabacum L.), the pyridine alkaloids nicotine, nornicotine, anatabine, and anabasine naturally exist as scalemic mixtures of (R)- or (S)-enantiomers, with the (S)-isoforms predominating. The mechanisms by which tobacco alkaloid enantiomers accumulate remain largely unknown. Experiments were carried out involving tobacco genotypes possessing induced deleterious mutations in three genes coding for nicotine demethylase (NND) enzymes and three genes coding for Berberine Bridge Like (BBL) enzymes that act near the end of the nicotine, anatabine, and anabasine biosynthetic pathways. Data indicate that (R)-nicotine is naturally produced at appreciable levels but is observed in only small amounts due to preferential demethylation by NND enzymes. Data further suggest that BBL-a and BBL-b are preferentially involved in the biosynthesis of (S)-alkaloid enantiomers, while BBL-c is preferentially involved in the biosynthesis of (R)-enantiomers. Gene duplication followed by genetic divergence thus played a role in the evolution of scalemic alkaloid accumulation in tobacco. Through a combination of mutation breeding and transgene overexpression, tobacco genotypes were generated in which the predominant alkaloid enantiomers were reversed from the (S)- to the (R)-isoforms. These results shed light on the genetic control of alkaloid accumulation in N. tabacum and on mechanisms of scalemic mixture formation of secondary metabolites in plants.

Cannabichromene (CBC) Shows Matrix-Dependent Thermal Configurational Stability

The optical purity of cannabichromene (CBC, 1a) is affected by the matrix in which it is generated by thermolysis from its native carboxylated form (cannabichromenic acid, CBCA, 1b). Thus, thermolysis at 130 °C in planta caused a marked decrease of the enantiomeric excess (ee), while, under the same conditions, only a modest decrease of optical purity was observed when thermolysis was carried out in extracto. To rationalize these puzzling observations, the kinetics of thermal (100 °C) racemization of enantiopure cannabichromene (1a) was evaluated by enantioselective ultrahigh performance liquid chromatography in solvents (decalin and isopropyl alcohol, neat and acidified with TFA) and surfaces (native and silanized borosilicate glass) of complementary polarity. Optical stability was more than halved in isopropanol compared to decalin (t1/2 50 h vs 135 h), but acidification had no effect on racemization. However, contact with a solid surface dramatically accelerated the process, with a t1/2 of only 6 h on both glass surfaces. The overall extent of racemization of enantiopure CBC (1a) was compared under conditions commonly used for decarboxylation (heating at 130 °C) between a decalin solution and a thin film on three different surfaces (native and silanized borosilicate glass and powdered blank cannabis biomass). In line with the kinetic data, a significant erosion of enantiopurity was observed on all solid surfaces compared to the solution. These observations suggest that discrepancies in the reported enantiomeric purity of natural CBC could be not only of biogenetic derivation but also be associated with the decarboxylation protocol of cannabichromenic acid (1b). These findings, while relevant for the exploitation of the bioactivity of natural CBC for human health, should also prompt the adoption of a standardized decarboxylation protocol for the studies on the configurational status of CBC (1a) in cannabis and, in general, of cannabinochromanoids in nature.

Resolution and Absolute Configuration of Fargesin Enantiomers

Fargesin is an important bioactive furofuran lignan isolated from different plant species. Despite presenting potent biological activities, its stereochemical characterization has relied mostly on empirical correlations of optical rotation, an approach considered risky that commonly leads to misassignments and error propagation. Additionally, the enantiomeric purity of fargesin isolates used for biological assays has not been previously investigated. Herein, we report the enantioresolution of a scalemic mixture of fargesin isolated from Aristolochia warmingii along with the first unambiguous determination of the absolute configuration of each enantiomer by means of optical rotatory dispersion, as well as electronic and vibrational circular dichroism aided by quantum-chemical calculations.

Xanthone Inhibitors of Unfolded Protein Response Isolated from Calophyllum caledonicum

The unfolded protein response (UPR) is a key component of fungal virulence. The prenylated xanthone γ-mangostin isolated from Garcinia mangostana (Clusiaceae) fruit pericarp, has recently been described to inhibit this fungal adaptative pathway. Considering that Calophyllum caledonicum (Calophyllaceae) is known for its high prenylated xanthone content, its stem bark extract was fractionated using a bioassay-guided procedure based on the cell-based anti-UPR assay. Four previously undescribed xanthone derivatives were isolated, caledonixanthones N-Q (3, 4, 8, and 12), among which compounds 3 and 8 showed promising anti-UPR activities with IC50 values of 11.7 ± 0.9 and 7.9 ± 0.3 μM, respectively.

Altering the mobile phase composition to enhance self-disproportionation of enantiomers in achiral chromatography

The influence of mobile phase composition on the efficiency of enantiomer separation by achiral chromatography (ACh) was investigated. The separation was induced by the phenomenon of self-disproportionation of enantiomers (SDE) triggered by their homo and hetero-chiral interactions in an achiral environment. Typically, SDE occurs in apolar mobile phases of weak elution strength, which causes the separation time to extend and the process productivity to deteriorate. To mitigate that effect, we altered the content of a strong solvent (modifier) in the mobile phase by use of a solvent gradient in which the target enantiomer was separated in the presence of the weak solvent, whereas the unresolved mixture of enantiomers was eluted by increasing the modifier content in the mobile phase. This enabled accelerating the solute elution while preserving the separation selectivity. The approach was examined for the separation of nonracemic mixtures of two structurally different compounds that exhibited the SDE effect in ACh, i.e., metalaxyl (MX) and methyl p-tolyl sulfoxide (MTSO). The target compound of the separation was the more abundant enantiomer in the enantiomeric mixture. The process realization was preceded by the determination of the effect of the modifier content on the separation yield for enantiomeric mixtures of MX and MTSO of different enantiomeric excess (ee). In the case of MX, yield of the pure target enantiomer varied from 2 %, for the maximum concentration of the modifier, to 45 % for the minimum modifier concentration and the largest ee used in the experiments. In the case of MTSO, the yield varied from minimum 40 % to maximum 66 %. To predict the process, we employed a dynamic model, in which underlying thermodynamic dependencies were implemented.