On the Erosion of Enantiopurity of Rhodonoids via Their Asymmetric Total Synthesis

Phytocannabinoid-like meroterpenoids from twigs and leaves of Rhododendron spinuliferum

Six pairs of previously undescribed enantiomeric phytocannabinoid-like meroterpenoids, (±)-spinulinoids A‒F, and two naturally occurring compounds, (+)-rhododaurichromanic acid A and (E)-4-((3,7-dimethylocta-2,6-dien-1-yl)oxy)benzoic acid, together with one known congener, (-)-rhododaurichromanic acid A, were obtained from the twigs and leaves of Rhododendron spinuliferum. Their structures were established by their extensive spectral data (NMR and HRESIMS), ECD calculations, and single-crystal X-ray diffraction data. Spinulinoids A and B are unprecedented phytocannabinoid-like meroterpenoids constructed by the resorcinol moiety and a β-bisabolene unit, whereas spinulinoid C represents a rare adduct of quinone and β-bisabolene with a tricyclic 6/6/6 ring system.

Dauresorcinols A and B, two pairs of merosesquiterpenoid enantiomers with new carbon skeletons from Rhododendron dauricum

Five pairs of new merosesquiterpenoid enantiomers, named dauresorcinols A-E (1-5), were isolated from the leaves of Rhododendron dauricum. Their structures were elucidated by comprehensive spectroscopic data analysis, quantum chemical calculations, Rh2(OCOCF3)4-induced ECD, and single-crystal X-ray diffraction analysis. Dauresorcinols A (1) and B (2) possess two new merosesquiterpene skeletons bearing an unprecedented 2,6,7,10,14-pentamethyl-11-oxatetracyclo[8.8.0.02,7.012,17]octadecane and a caged 15-isohexyl-1,5,15-trimethyl-2,10-dioxatetracyclo[7.4.1.111,14.03,8]pentadecane motif, respectively. Plausible biosynthetic pathways of 1-5 are proposed involving key oxa-electrocyclization and Wagner-Meerwein rearrangement reactions. (+)/(-)-1 and 3-5 showed potent α-glucosidase inhibitory activity, 3 to 22 times stronger than acarbose, an antidiabetic drug targeting α-glucosidase. Docking results provide a basis to design and develop merosesquiterpenoids as potent α-glycosidase inhibitors.

Asymmetric Total Synthesis of (+)-Verrubenzospirolactone and (+)-Capillobenzopyranol

The first asymmetric total synthesis of (+)-verrubenzospirolactone (1), a distinctive highly fused benzosesquiterpenoid, characterized by a pentacyclic skeletal structure, is realized through a concise 10-step synthetic pathway with an impressive 22.8% overall yield. Notable highlights of this synthetic endeavor include (i) the introduction of a Ru-catalyzed ortho C-H activation step, (ii) the application of Pd-catalyzed asymmetric allylic alkylation to establish a pivotal stereocenter at C-3 with an excellent enantiomeric excess, (iii) B-alkyl Suzuki-Miyaura coupling to construct a Diels-Alder precursor, and, ultimately, (iv) the successful deployment of an intramolecular Diels-Alder reaction to complete the synthesis of (+)-verrubenzospirolactone without erosion of the enantiomeric excess.

Total Synthesis of Ganoderma Meroterpenoids Cochlearol B and Its Congeners Driven by Structural Similarity and Biological Homology

Secondary metabolites that have the same biological origin must share some relationship in their biosynthesis. Exploring this relationship has always been a significant task for synthetic biologists. However, from the perspective of synthetic chemists, it is equally important to propose, prove, or refute potential biosynthetic pathways in order to elucidate and understand the biosynthesis of homologous secondary metabolites. In this study, driven by the high structural similarity between the homologous Ganoderma meroterpenoids cochlearol B and ganocin B, two chemically synthetic strategies were designed and investigated sequentially for the synthesis of cochlearol B from ganocin B. These strategies include intramolecular metal-catalyzed hydrogen atom transfer (MHAT) and intramolecular photochemical [2+2] cycloaddition. The aim was to reveal their potential biosynthetic conversion relationship using chemical synthesis methods. As a result, a highly efficient total synthesis of cochlearol B, cochlearol T, cochlearol F, as well as the formal total synthesis of ganocins A-B, and ganocochlearins C-D, has been achieved. Additionally, a novel synthetic approach for the synthesis of 6,6-disubstituted 6H-dibenzo[b,d]pyran and its analogues has been developed through palladium(II)-catalyzed Wacker-type/cross-coupling cascade reactions.

Bidimensional heart-cut achiral-chiral liquid chromatography coupled to high-resolution mass spectrometry for the separation of the main chiral phytocannabinoids and enantiomerization studies of cannabichromene and cannabichromenic acid

In this work, a heart-cut bidimensional achiral-chiral liquid chromatography method coupled to high-resolution mass spectrometry was developed for the separation of the main carboxylated phytocannabinoids, namely cannabidiolic acid (CBDA), tetrahydrocannabinolic acid (THCA), cannabichromenic acid (CBCA), and cannabicyclolic acid (CBLA), and decarboxylated derivatives, namely cannabidiol (CBD), Δ9-tetrahydrocannabinol (Δ9-THC), cannabichromene (CBC), and cannabicyclol (CBL), and the evaluation of their enantiomeric composition in extracts of different Cannabis sativa L. varieties. Optimal conditions for the chiral analysis of CBC- and CBL-type compounds were found with methanol and water (95:5, v/v, with 0.1% formic acid, 1.5 mL/min) on an amylose-based chiral stationary phase. These settings also allowed to evaluate the parameters responsible for CBC and CBCA racemization.

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.

Natural Cannabichromene (CBC) Shows Distinct Scalemicity Grades and Enantiomeric Dominance in Cannabis sativa Strains

Cannabichromene (CBC, 1a) occurs in Cannabis (Cannabis sativa) as a scalemate having a composition that is strain-dependent in terms of both enantiomeric excess and enantiomeric dominance. In the present work, the chirality of CBC (1a), a noncrystalline compound, was shown not to be significantly affected by standard conditions of isolation and purification, and enantiomeric self-disproportionation effects were minimized by carrying out the chiral analysis on crude fractions rather than on purified products. A genetic basis for the different enantiomeric state of CBC in Cannabis therefore seems to exist, implying that the chirality status of natural CBC (1a) in the plant is associated with the differential expression of CBCA-synthase isoforms and/or of associated directing proteins with antipodal enantiospecificity. The biological profile of both enantiomers of CBC should therefore be investigated independently to assess the contribution of this compound to the activity of Cannabis preparations.

Gambogic acid: Multi-gram scale isolation, stereochemical erosion toward epi-gambogic acid and biological profile

Introduction

Extracted from gamboge resin, gambogic acid (GBA) is a natural product that displays a complex caged xanthone structure and exhibits promising antitumor properties. However, efforts to advance this compound to clinical applications have been thwarted by its limited availability that in turn, restricts its pharmacological optimization.

Methods

We report here an efficient method that allows multigram scale isolation of GBA in greater than 97% diastereomeric purity from various sources of commercially available gamboge. The overall process includes: (a) isolation of organic components from the resin; (b) separation of GBA from the organic components via crystallization as its pyridinium salt; and (c) acidification of the salt to isolate the free GBA.

Results and Discussion

We found that GBA is susceptible to epimerization at the C2 center that produces epi-gambogic acid ( epi-GBA), a common contaminant of all commercial sources of this compound. Mechanistic studies indicate that this epimerization proceeds via an ortho-quinone methide intermediate. Although the observed stereochemical erosion accounts for the chemical fragility of GBA, it does not significantly affect its biological activity especially as it relates to cancer cell cytotoxicity. Specifically, we measured similar levels of cytotoxicity for either pure GBA or an equilibrated mixture of GBA/ epi-GBA in MBA-MB-231 cells with IC50 values at submicromolar concentration and induction of apoptosis after 12 hours of incubation. The results validate the pharmacological promise of gambogic acid and, combined with the multigram-scale isolation, should enable drug design and development studies.