Total Synthesis of (+)-Machaeriol D with a Key Regio- and Stereoselective SN2′ Reaction

Saturated Cannabinoids: Update on Synthesis Strategies and Biological Studies of These Emerging Cannabinoid Analogs

Natural and non-natural hexahydrocannabinols (HHC) were first described in 1940 by Adam and in late 2021 arose on the drug market in the United States and in some European countries. A background on the discovery, synthesis, and pharmacology studies of hydrogenated and saturated cannabinoids is described. This is harmonized with a summary and comparison of the cannabinoid receptor affinities of various classical, hybrid, and non-classical saturated cannabinoids. A discussion of structure-activity relationships with the four different pharmacophores found in the cannabinoid scaffold is added to this review. According to laboratory studies in vitro, and in several animal species in vivo, HHC is reported to have broadly similar effects to Δ9-tetrahydrocannabinol (Δ9-THC), the main psychoactive substance in cannabis, as demonstrated both in vitro and in several animal species in vivo. However, the effects of HHC treatment have not been studied in humans, and thus a biological profile has not been established.

Substitution of a triazole for the central olefin in biologically active stilbenes

The stilbene moiety is commonly found in natural products and these compounds display an extraordinary range of biological activity. Efforts to derive useful drugs from stilbenes must address the potential liabilities of this structure, including a propensity for cis/trans isomerization. To identify olefin replacements that address this limitation while preserving biological activity we have prepared analogues of two bioactive stilbenes, a pawhuskin and a schweinfurthin, where a 1,2,3-triazole ring formally replaces the stilbene double bond. The new schweinfurthin analogue (23) has been tested for anti-proliferative activity against 60 cell lines, and shows a strong correlation of bioactivity when compared to the compound that inspired its synthesis (22).

Synthesis and Inhibitory Activity of Machaeridiol-Based Novel Anti-MRSA and Anti-VRE Compounds and Their Profiling for Cancer-Related Signaling Pathways

Three unique 5,6-seco-hexahydrodibenzopyrans (seco-HHDBP) machaeridiols A−C, reported previously from Machaerium Pers., have displayed potent activities against methicillin-resistant Staphylococcus aureus (MRSA), vancomycin-resistant Enterococcus faecium, and E. faecalis (VRE). In order to enrich the pipeline of natural product-derived antimicrobial compounds, a series of novel machaeridiol-based analogs (1−17) were prepared by coupling stemofuran, pinosylvin, and resveratrol legends with monoterpene units R-(−)-α-phellandrene, (−)-p-mentha-2,8-diene-1-ol, and geraniol, and their inhibitory activities were profiled against MRSA ATCC 1708, VRE ATCC 700221, and cancer signaling pathways. Compounds 5 and 11 showed strong in vitro activities with MIC values of 2.5 μg/mL and 1.25 μg/mL against MRSA, respectively, and 2.50 μg/mL against VRE, while geranyl analog 14 was found to be moderately active (MIC 5 μg/mL). The reduction of the double bonds of the monoterpene unit of compound 5 resulted in 17, which had the same antibacterial potency (MIC 1.25 μg/mL and 2.50 μg/mL) as its parent, 5. Furthermore, a combination study between seco-HHDBP 17 and HHDBP machaeriol C displayed a synergistic effect with a fractional inhibitory concentrations (FIC) value of 0.5 against MRSA, showing a four-fold decrease in the MIC values of both 17 and machaeriol C, while no such effect was observed between vancomycin and 17. Compounds 11 and 17 were further tested in vivo against nosocomial MRSA at a single intranasal dose of 30 mg/kg in a murine model, and both compounds were not efficacious under these conditions. Finally, compounds 1−17 were profiled against a panel of luciferase genes that assessed the activity of complex cancer-related signaling pathways (i.e., transcription factors) using T98G glioblastoma multiforme cells. Among the compounds tested, the geranyl-substituted analog 14 exhibited strong inhibition against several signaling pathways, notably Smad, Myc, and Notch, with IC50 values of 2.17 μM, 1.86 μM, and 2.15 μM, respectively. In contrast, the anti-MRSA actives 5 and 17 were found to be inactive (IC50 > 20 μM) across the panel of these cancer-signaling pathways.

Synthesis of a Coumarin-Based Analogue of Schweinfurthin F

The natural schweinfurthins are stilbenes with significant antiproliferative activity and an uncertain mechanism of action. To obtain a fluorescent analogue with minimal deviation from the natural structure, a coumarin ring system was annulated to the D-ring, creating a new analogue of schweinfurthin F. This stilbene was prepared through a convergent synthesis, with a Horner-Wadsworth-Emmons condensation employed to form the central stilbene olefin. After preparation of a tricyclic phosphonate via a recent and more efficient modification of the classic Arbuzov reaction, condensation was attempted with an appropriately substituted bicyclic aldehyde but the coumarin system did not survive the reaction conditions. When olefin formation preceded generation of the coumarin, the stilbene formation proceeded smoothly and ultimately allowed access to the targeted coumarin-based schweinfurthin analogue. This analogue displayed the desired fluorescence properties along with significant biological activity in the National Cancer Institute's 60-cell line bioassay, and the pattern of this biological activity mirrored that of the natural product schweinfurthin F. This approach gives facile access to new fluorescent analogues of the natural schweinfurthins and should be applicable to other natural stilbenes as well.

One-flow synthesis of tetrahydrocannabinol and cannabidiol using homo- and heterogeneous Lewis acids

Continuous flow chemistry holds great potential for the production of biologically relevant molecules. Herein, we present an approach for the continuous synthesis of cannabidiol and tetrahydrocannabinol in a one-flow system. The designed route consists of a reaction cascade involving Friedel-Crafts alkylation, subsequent ring opening and cyclisation in up to 45% yield. The reactions were successfully performed using both hetero- and homogeneous Lewis acids in continuous flow and provide yields that are similar to comparable batch processes.

Graphical abstract

Synthetic pathways to tetrahydrocannabinol (THC): an overview

The therapeutic effects of molecules produced by the plant species Cannabis sativa have since their discovery captured the interest of scientists and society, and have spurred the development of a multidisciplinary scientific field with contributions from biologists, medical specialists and chemists. Decades after the first isolation of some of the most bioactive tetrahydrocannabinols, current research is mostly dedicated to exploiting the chemical versatility of this relevant compound class with regard to its therapeutic potential. This review will primarily focus on synthetic pathways utilised for the synthesis of tetrahydrocannabinols and derivatives thereof, including chiral pool-based and asymmetric chemo- and biocatalytic approaches.

Antimicrobial Constituents from Machaerium Pers.: Inhibitory Activities and Synergism of Machaeriols and Machaeridiols against Methicillin-Resistant Staphylococcus aureus, Vancomycin-Resistant Enterococcus faecium, and Permeabilized Gram-Negative Pathogens

Two new epimeric bibenzylated monoterpenes machaerifurogerol (1a) and 5-epi-machaerifurogerol (1b), and four known isoflavonoids (+)-vestitol (2), 7-O-methylvestitol (3), (+)-medicarpin (4), and 3,8-dihydroxy-9-methoxypterocarpan (5) were isolated from Machaerium Pers. This plant was previously assigned as Machaerium multiflorum Spruce, from which machaeriols A-D (6–9) and machaeridiols A-C (10–12) were reported, and all were then re-isolated, except the minor compound 9, for a comprehensive antimicrobial activity evaluation. Structures of the isolated compounds were determined by full NMR and mass spectroscopic data. Among the isolated compounds, the mixture 10 + 11 was the most active with an MIC value of 1.25 μg/mL against methicillin-resistant Staphylococcus aureus (MRSA) strains BAA 1696, −1708, −1717, −33591, and vancomycin-resistant Enterococcus faecium (VRE 700221) and E. faecalis (VRE 51299) and vancomycin-sensitive E. faecalis (VSE 29212). Compounds 6–8 and 10–12 were found to be more potent against MRSA 1708, and 6, 11, and 12 against VRE 700221, than the drug control ciprofloxacin and vancomycin. A combination study using an in vitro Checkerboard method was carried out for machaeriols (7 or 8) and machaeridiols (11 or 12), which exhibited a strong synergistic activity of 12 + 8 (MIC 0.156 and 0.625 µg/mL), with >32- and >8-fold reduction of MIC’s, compared to 12, against MRSA 1708 and −1717, respectively. In the presence of sub-inhibitory concentrations on polymyxin B nonapeptide (PMBN), compounds 10 + 11, 11, 12, and 8 showed activity in the range of 0.5–8 µg/mL for two strains of Acinetobacter baumannii, 2–16 µg/mL against Pseudomonas aeruginosa PAO1, and 2 µg/mL against Escherichia coli NCTC 12923, but were inactive (MIC > 64 µg/mL) against the two isolates of Klebsiella pneumoniae.

Lewis-Acid-Mediated Union of Epoxy-Carvone Diastereomers with Anisole Derivatives: Mechanistic Insight and Application to the Synthesis of Non-natural CBD Analogues

The use of trimethylsilyl trifluoromethanesulfonate as a mild means to unite epoxy-carvone silyl ethers with anisole derivatives to yield products that are structurally similar to the CBD scaffold is reported. Importantly, unlike related methods, this process can utilize both epoxy-carvone diastereomers and does not require the use of air/moisture-sensitive organometallic reagents. Several examples of aryl nucleophiles as well as mechanistic insight based on in silico computational analysis are presented.

Total synthesis of conosilane A via a site-selective C–H functionalization strategy

The strategy developed for the first total synthesis of highly oxygenated natural product conosilane A involving double manipulation of allylic C(sp³)–H functionalization renders the power of C–H functionalization in organic syntheses.

Divergent Strategy for the Diastereoselective Synthesis of the Tricyclic 6,7-Diaryltetrahydro-6H-benzo[c]chromene Core via Pt(IV)-Catalyzed Cycloaddition of o-Quinone Methides and Olefin Ring-Closing Metathesis

A divergent strategy for the synthesis of the tricyclic 6,7-diaryltetrahydro-6H-benzo[c]chromene core was successfully developed. The 2,3-trans, 2,4-cis trisubstituted chroman moiety was formed via highly efficient and stereoselective Pt(IV)-catalyzed cycloaddition reactions of the corresponding quinone methides with chalcones. Subsequent steps provided the common diene alcohol, which underwent BF₃·Et₂O-mediated Et₃SiH reduction and olefin ring-closing metathesis (RCM) using Ru(II) catalysts. The sequence of the final two steps provided a handle to diversify the stereochemical outcomes at C6 as well as C10a.

A Cu-mediated one-pot Michael addition/α-arylation strategy using a diaryliodonium salt: a direct and efficient approach to α-aryl-β-substituted cyclic ketone scaffolds

The direct and efficient construction of α-aryl-β-substituted cyclic ketone scaffolds has been achieved using a Cu-mediated one-pot Michael addition/α-arylation strategy. The reaction features easily available materials, broad substrate scope, moderate to good yield, and an excellent diastereoselectivity.

Phytocannabinoids: a unified critical inventory

Cannabis sativaL. is a prolific, but not exclusive, producer of a diverse group of isoprenylated resorcinyl polyketides collectively known as phytocannabinoids.

Denticulatains A and B: unique stilbene–diterpene heterodimers from Macaranga denticulata

Two novel heterodimers were isolated from the fronds of Macaranga denticulata. They possess an unprecedented stilbene–diterpene-type skeleton, which represents a unique class of prenylated stilbene.

VK. Protecting group free enantiospecific total syntheses of structurally diverse natural products of the tetrahydrocannabinoid family

A simple, highly diastereoselective, Lewis acid catalyzed Friedel–Crafts coupling of a cyclic allylic alcohol with resorcinol derivatives has been developed.

Sonogashira coupling in natural product synthesis

This review will focus on selected applications of Sonogashira coupling and subsequent transformations as key steps in the total synthesis of natural products.

The Pyridyldiisopropylsilyl Group: A Masked Functionality and Directing Group for Monoselective ortho-Acyloxylation and ortho-Halogenation Reactions of Arenes

A novel, easily removable and modifiable silicon-tethered pyridyldiisopropylsilyl directing group for C-H functionalizations of arenes has been developed. The installation of the pyridyldiisopropylsilyl group can efficiently be achieved via two complementary routes using easily available 2-(diisopropylsilyl)pyridine (5). The first strategy features a nucleophilic hydride substitution at the silicon atom in 5 with aryllithium reagents generated in situ from the corresponding aryl bromides or iodides. The second milder route exploits a highly efficient room-temperature rhodium(I)-catalyzed cross-coupling reaction between 5 and aryl iodides. The latter approach can be applied to the preparation of a wide range of pyridyldiisopropylsilyl-substituted arenes possessing a variety of functional groups, including those incompatible with organometallic reagents. The pyridyldiisopropylsilyl directing group allows for a highly efficient, regioselective palladium(II)-catalyzed mono-ortho-acyloxylation and ortho-halogenation of various aromatic compounds. Most impor-tantly, the silicon-tethered directing group in both acyloxylated and halogenated products can easily be removed or efficiently converted into an array of other valuable functionalities. These transformations include protio-, deuterio-, halo-, boro-, and alkynyldesilylations, as well as a conversion of the directing group into the hydroxy functionality. In addition, the construction of aryl-aryl bonds via the Hiyama-Denmark cross-coupling reaction is feasible for the acetoxylated products. Moreover, the ortho-halogenated pyridyldiisopropylsilylarenes, bearing both nucleophilic pyridyldiisopropylsilyl and electrophilic aryl halide moieties, represent synthetically attractive 1,2-ambiphiles. A unique reactivity of these ambiphiles has been demonstrated in efficient syntheses of arylenediyne and benzosilole derivatives, as well as in a facile generation of benzyne. In addition, preliminary mechanistic studies of the acyloxylation and halogenation reactions have been performed. A trinuclear palladacycle intermediate has been isolated from a stoichiometric reaction between diisopropyl-(phenyl)pyrid-2-ylsilane (3a) and palladium acetate. Furthermore, both C-H functionalization reactions exhibited equally high values of the intramolecular primary kinetic isotope effect (kH/kD = 6.7). Based on these observations, a general mechanism involving the formation of a palladacycle via a C-H activation process as the rate-determining step has been proposed.

Synthesis and biological activity of a fluorescent schweinfurthin analogue

Most of the natural schweinfurthins are potent and selective inhibitors of cell growth as measured by the National Cancer Institute's 60-cell line screen. Due to the limited supply of these natural products, we have initiated a program aimed at their synthesis. To date, this effort has led to the preparation of three natural schweinfurthins and more than 40 analogues, and assays on these compounds have afforded some understanding of structure-activity relationships in this family. Further development of schweinfurthins as chemotherapeutic agents would benefit from characterization of their mechanism(s) of action. This perspective led to development of a fluorescent schweinfurthin analogue that retains the differential activity of the natural products, and yet has properties that facilitate its visualization within cells.

Total synthesis of atrochamins F, H, I, and J through cascade reactions

A concise and efficient cascade-based total synthesis of artochamins F, H, I, and J is described. The potential biogenetic connection between artochamin F, or a derivative thereof, and artochamins H, I, and J, through an unusual formal [2+2] cycloaddition process, was shown to be feasible. An alternative mechanism for this transformation is also proposed.

A Rhodium(I)-Catalyzed Demethylation−Cyclization of o-Anisole-Substituted Ynamides in the Synthesis of Chiral 2-Amido Benzofurans

A Rh(I)-catalyzed demethylation-cyclization sequence for a direct transformation of o-anisole-substituted ynamides to benzofurans is described here. The Ag salt functions synergistically with Rh(I) for the key demethylation step.