Absolute configuration and complete assignment of 13C NMR data for new sesquiterpenes from Maytenus chiapensis

Intramolecular Oxyalkylation of Unactivated Alkenes

The synthesis of cyclopropanes by the cyclization of allylic diazoesters is well-known. In prior studies toward the sesquiterpenoid euonyminol, we attempted to carry out an intramolecular cyclopropanation of an allylic diazoester containing an electronically-unbiased alkene embedded in a 6-oxa-bicyclo[3.2.1]-oct-3-ene skeleton. We obtained exclusively a product arising from 1,2-addition of oxygen and carbon (oxyalkylation) to the alkene. While oxyalkylation products have been reported when electron-rich alkenes (e.g. enol ethers) are employed, examples derived from electronically-unbiased alkenes are rare. Here, we establish that the oxyalkylation is general for a range of 6-oxa-bicyclo[3.2.1]-oct-3-ene substrates and show that these products form competitively in the cyclization of simpler α-diazo β-ketoesters. Our data suggest increasing charge separation in the transition state for the addition promotes the oxyalkylation pathway.

Dihydro-β-agarofuran-type sesquiterpenoids from the seeds of Celastrus virens with lifespan-extending effect on the nematode Caenorhabditis elegans

Twelve dihydro-β-agarofuran-type sesquiterpenoids, including five new ones (1-5), were purified from the seeds of Celastrus virens (Wang et Tang) C. Y. Chent et T. C. Kao. Their chemical structures were characterized via comprehensive spectroscopic analysis, single-crystal X-ray diffraction analysis, and computational prediction of ECD, as well as comparison of observed and reported NMR spectral data. Among the isolates, nine abundant dihydro-β-agarofuran-type sesquiterpenoids were evaluated for their lifespan-extending activity using the nematode Caenorhabditis elegans model. As a result, compounds 1, 2, 5, 6, 8, and 9 (50 μM) significantly extended the mean survival time of C. elegans, respectively, compared with the blank control group (p < 0.05). Further Quantitative RT-PCR showed that the prolonging of lifespan mediated by compounds 1, 6, 8, and 9 were dependent on the transcription factors skn-1 and hsf-1.

Total Synthesis of Euonymine and Euonyminol Octaacetate

Euonymine (1) and euonyminol octaacetate (2) share the core structure of euonyminol (3), the most hydroxylated member of the dihydro-β-agarofuran family. In 2, eight of the nine hydroxy groups of 3 are acetylated, and 1 has six acetyl groups and a 14-membered bislactone comprising a pyridine dicarboxylic acid with two methyl groups. The different acylation patterns provide distinct biological activities: 1 and 2 display anti-HIV and P-glycoprotein inhibitory effects, respectively. The 11 contiguous stereocenters and 9 oxygen functionalities of the ABC-ring system of 1 and 2 represent a formidable challenge, which is further heightened by the macrocyclic structure of 1. Here we disclose an efficient synthetic strategy for enantioselective total synthesis of 1 and 2. Starting from (R)-glycerol acetonide, we constructed the B-ring by an Et₃N-accelerated Diels-Alder reaction, the C-ring by intramolecular iodoetherification, and the A-ring by ring-closing olefin metathesis. The 10 stereocenters were installed through a series of substrate-controlled stereoselective C-C and C-O bond formations by exploiting the three-dimensional structures of judiciously designed substrates. These newly developed reaction sequences led to protected euonyminol 5, which served as a common intermediate for assembling 1 and 2. Global deprotection of 5 and subsequent acetylation produced 2. Alternatively, the discriminative protective groups of 5 allowed for site-selective bis-esterification to generate bislactone. Combining [3 + 2]-cycloaddition and reductive desulfurization introduced the last remaining stereocenters of the two methyl groups on the macrocycle. Finally, deprotection and acetylation gave rise to fully synthetic 1 for the first time.

Development of an Enantioselective Synthesis of (-)-Euonyminol

We detail the development of the first enantioselective synthetic route to euonyminol (1), the most heavily oxidized member of the dihydro-β-agarofuran sesquiterpenes and the nucleus of the macrocyclic alkaloids known as the cathedulins. Key steps in the synthetic sequence include a novel, formal oxyalkylation reaction of an allylic alcohol by [3 + 2] cycloaddition; a tandem lactonization-epoxide opening reaction to form the trans-C2-C3 vicinal diol residue; and a late-stage diastereoselective trimethylaluminum-mediated α-ketol rearrangement. We report an improved synthesis of the advanced unsaturated ketone intermediate 64 by means of a 6-endo-dig radical cyclization of the enyne 42. This strategy nearly doubled the yield through the intermediate steps in the synthesis and avoided a problematic inversion of stereochemistry required in the first-generation approach. Computational studies suggest that the mechanism of this transformation proceeds via a direct 6-endo-trig cyclization, although a competing 5-exo-trig cyclization, followed by a rearrangement, is also energetically viable. We also detail the challenges associated with manipulating the oxidation state of late-stage intermediates, which may inform efforts to access other derivatives such as 9-epi-euonyminol or 8-epi-euonyminol. Our successful synthetic strategy provides a foundation to synthesize the more complex cathedulins.

A Review on Phytochemicals of the Genus Maytenus and Their Bioactive Studies

The genus Maytenus is a member of the Celastraceae family, of which several species have long been used in traditional medicine. Between 1976 and 2021, nearly 270 new compounds have been isolated and elucidated from the genus Maytenus. Among these, maytansine and its homologues are extremely rare in nature. Owing to its unique skeleton and remarkable bioactivities, maytansine has attracted many synthetic endeavors in order to construct its core structure. In this paper, the current status of the past 45 years of research on Maytenus, with respect to its chemical and biological activities are discussed. The chemical research includes its structural classification into triterpenoids, sesquiterpenes and alkaloids, along with several chemical synthesis methods of maytansine or maytansine fragments. The biological activity research includes activities, such as anti-tumor, anti-bacterial and anti-inflammatory activities, as well as HIV inhibition, which can provide a theoretical basis for the better development and utilization of the Maytenus.

Enantioselective Synthesis of Euonyminol

We describe an enantioselective total synthesis of the nonahydroxylated sesquiterpenoid euonyminol, the dihydro-β-agarofuran nucleus of the macrocyclic terpenoid alkaloids known as the cathedulins. Key features of the synthetic sequence include a highly diastereoselective intramolecular alkene oxyalkylation to establish the C10 quaternary center, an intramolecular aldol-dehydration to access the tricyclic scaffold of the target, a tandem lactonization-epoxide opening to form the trans-C2-C3 vicinal diol residue, and a late-stage diastereoselective α-ketol rearrangement. The synthesis provides the first synthetic access to enantioenriched euonyminol and establishes a platform to synthesize the cathedulins.

Development and Validation of an HPLC-PDA Method for Biologically Active Quinonemethide Triterpenoids Isolated from Maytenus chiapensis

Background: Quinonemethide triterpenoids, known as celastroloids, constitute a relatively small group of biologically active compounds restricted to the Celastraceae family and, therefore, they are chemotaxonomic markers for this family. Among this particular type of metabolite, pristimerin and tingenone are considered traditional medicines in Latin America. The aim of this study was the isolation of the most abundant celastroloids from the root bark of Maytenus chiapensis, and thereafter, to develop an analytical method to identify pristimerin and tingenone in the Celastraceae species. Methods: Pristimerin and tingenone were isolated from the n-hexane-Et₂O extract of the root bark of M. chiapensis through chromatographic techniques, and were used as internal standards. Application of a validated RP HPLC-PDA method was developed for the simultaneous quantification of these two metabolites in three different extracts, n-hexane-Et₂O, methanol, and water, to determine the best extractor solvent. Results: Concentration values showed great variation between the solvents used for extraction, with the n-hexane⁻Et₂O extract being the richest in pristimerin and tingenone. Conclusions: M. chiapensis is a source of two biologically active quinonemethide triterpenoids. An analytical method was developed for the qualification and quantification of these two celastroloids in the root bark extracts of M. chiapensis. The validated method reported herein could be extended and be useful in analyzing Celastraceae species and real commercial samples.

Construction of the septahydroxylated ABC-ring system of dihydro-β-agarofurans: application of 6-exo-dig radical cyclization

A synthetic route to the septahydroxylated ABC-ring system of dihydro-β-agarofurans was established. The B-ring was formed by a base-promoted diastereoselective Diels-Alder reaction between 3-hydroxy-2-pyrone and a d-glyceraldehyde-derived dienophile, while the C-ring was cyclized by PhSeCl-mediated etherification. The remaining A-ring was constructed via a 6-exo-dig radical reaction. Selective transformations gave rise to the ABC-ring system 1 with nine contiguous stereocenters. The thus obtained 1 corresponded to the enantiomer of the densely oxygenated core structure of dihydro-β-agarofurans.

Biological evaluation, structure-activity relationships, and three-dimensional quantitative structure-activity relationship studies of dihydro-beta-agarofuran sesquiterpenes as modulators of P-glycoprotein-dependent multidrug resistance

Multidrug resistance (MDR) is one of the main challenges in the chemotherapy of cancer, malaria, and other important diseases. Here, we report the inhibitory activity of a series of 76 dihydro-beta-agarofuran sesquiterpenes, tested on NIH-3T3 cells expressing the human P-glycoprotein (Pgp) multidrug transporter, to establish quantitative comparisons of their respective abilities to block the drug transport activity. The screening was performed on the basis of the ability of sesquiterpenes to modulate the intracellular accumulation of the classical Pgp substrate daunorubicin. To understand the structural basis for inhibitory activity and guide the design of more potent Pgp inhibitors, we have performed a three-dimensional quantitative structure-activity relationship model using the comparative molecular similarity indices analysis (CoMSIA). The most salient features of these requirements are in the region of the substituents at the C-2, C-3, and C-8 positions, which seem to be critical for determining the overall effectiveness of sesquiterpenes as Pgp inhibitors.

The dihydro-β-agarofuran sesquiterpenoids

Dihydro-Beta-agarofuran sesquiterpenoids are a structurally diverse class of natural products based on tricyclic 5,11-epoxy-5Beta,10alpha-eudesman-4-(14)-ene skeleton. Between January 1990 and June 2006, 462 new dihydro-Beta-agarofuran sesquiterpenoids of 74 structural types have been isolated from about 64 species of Celastraceae, 3 species of Hippocrateaceae and one species of Lamiaceae. The present review covers the chemical and biological activity research of dihydro-Beta-agarofuran sesquiterpenoids in the past 16 years. The chemical research includes structural classification into sesquiterpene polyesters and macrolide sesquiterpene pyridine alkaloids, synthesis of dihydro-Beta-agarofuran as well as extraction, isolation and purification methods. The biological activity research includes activities such as multidrug resistance (MDR) reversal activity, HIV inhibition, cytotoxicity, antitumor activity, antifeedant activity and insecticidal activity with some insights to their modes of actions.

Lupane triterpenoids from Maytenus species

Five new lupane triterpenes (1-5), in addition to 24 known ones, were isolated from Maytenus cuzcoina and M. chiapensis. Their structures were elucidated on the basis of spectroscopic analysis, including homonuclear and heteronuclear correlation NMR (COSY, ROESY, HSQC, and HMBC) experiments. The compounds were assayed for antimicrobial and cytotoxic activities, with 3-epi-betulinic acid and 28,30-dihydroxy-3-oxolup-20(29)-ene showing moderate cytotoxicity.

Reversion of Human Pgp-Dependent Multidrug Resistance by New Sesquiterpenes from Zinowiewia costaricensis

In an intensive study of South American medicinal plants, herein we report the isolation, structure elucidation and biological activity of fourteen new and five known dihydro-beta-agarofuran sesquiterpenes from the leaves of Zinowiewia costaricensis (1-19). Their structures were determined by means of (1)H and (13)C NMR spectroscopic studies, including homonuclear and heteronuclear correlation experiments. The absolute configurations of the new compounds were determined by CD studies, chemical correlations or biogenetic grounds. All the natural compounds and derivative 20 have been tested on human MDR1-transfected NIH-3T3 cells, to determine their ability to revert the multidrug resistance phenotype due to P-glycoprotein overexpression. Six compounds from this series (1, 8, 11, 12, 13 and 14) showed similar effectiveness to the classical P-glycoprotein modulator verapamil when reversing resistance to daunorubicin, but it is up to sixteen times greater than that of verapamil when reversing resistance to vinblastine. The structure-activity relationships are discussed.

Insecticidal sesquiterpene pyridine alkaloids from Maytenus chiapensis

The new sesquiterpene pyridine alkaloids chiapenines ES-I (1), ES-II (2), ES-III (3), and ES-IV (4), in addition to the known alkaloids wilfordine (5), alatamine (6), wilforidine (7), alatusinine (8), euonine (9), euonymine (10), ebenifoline E-I (11), forrestine (12), mayteine (13), and 4-hydroxy-7-epi-chuchuhuanine E-V (14), were isolated from the leaves of Maytenus chiapensis. Their structures were elucidated by 1D and 2D NMR spectroscopy, including homonuclear and heteronuclear correlation (COSY, ROESY, HSQC, and HMBC) experiments. Wilfordine, alatusinine, and euonine exhibited strong antifeedant activity against Spodoptera littoralis.