Classical/nonclassical hybrid cannabinoids: southern aliphatic chain-functionalized C-6beta methyl, ethyl, and propyl analogues

Enantioselective Divergent Syntheses of Diterpenoid Pyrones

Capitalizing a synergy between late-stage C(sp3)-H alkynylation and a series of transition metal-catalyzed alkyne functionalization reactions, we reported herein enantioselective divergent synthesis of 10 diterpenoid pyrones within 14-16 steps starting from chiral pool enoxolone, including the first enantioselective synthesis of higginsianins A, B, D, E, and metarhizin C. Our synthesis also highlights an unprecedented biomimetic oxidative rearrangement of α-pyrone into 3(2H)-furanone, as well as applications of Echavarren C(sp3)-H alkynylation reaction and Toste chiral counterion-mediated Au-catalyzed intramolecular allene hydroalkoxylation in natural product synthesis.

Cannabinoids as multifaceted compounds

Since ancient times, Cannabis and its preparations have found various applications such as for medical, recreational and industrial purposes. Subsequently the 1930s, legislation in many countries has restricted its use due to its psychotropic properties. More recently, the discovery of endocannabinoid system, including new receptors, ligands, and mediators, its role in maintaining the homeostasis of the human body and the possible implication in various physiological and pathophysiological processes has also been understood. Based on this evidence, researchers were able to develop new therapeutic targets for the treatment of various pathological disorders. For this purpose, Cannabis and cannabinoids were subjected for the evaluation of their pharmacological activities. The renewed interest in the medical use of cannabis for its potential therapeutic application has prompted legislators to take action to regulate the safe use of cannabis and products containing cannabinoids. However, each country has an enormous heterogeneity in the regulation of laws. Here, we are pleased to show a general and prevailing overview of the findings regarding cannabinoids and the multiple research fields such as chemistry, phytochemistry, pharmacology and analytics in which they are involved.

Novel Functionalized Cannabinoid Receptor Probes: Development of Exceptionally Potent Agonists

We report the development of novel cannabinergic probes that can stabilize the cannabinoid receptors (CBRs) through tight binding interactions. Ligand design involves the introduction of select groups at a judiciously chosen position within the classical hexahydrocannabinol template (monofunctionalized probes). Such groups include the electrophilic isothiocyanato, the photoactivatable azido, and the polar cyano moieties. These groups can also be combined to produce bifunctionalized probes potentially capable of interacting at two distinct sites within the CBR-binding domains. These novel compounds display remarkably high binding affinities for CBRs and are exceptionally potent agonists. A key ligand (27a, AM11245) exhibits exceptionally high potency in both in vitro and in vivo assays and was designated as "megagonist," a property attributed to its tight binding profile. By acting both centrally and peripherally, 27a distinguishes itself from our previously reported "megagonist" AM841, whose functions are restricted to the periphery.

Prediction of the Binding Affinities and Selectivity for CB1 and CB2 Ligands Using Homology Modeling, Molecular Docking, Molecular Dynamics Simulations, and MM-PBSA Binding Free Energy Calculations

Cannabinoids are a group of chemical compounds that have been used for thousands of years due to their psychoactive function and systemic physiological effects. There are at least two types of cannabinoid receptors, CB1 and CB2, which belong to the G protein-coupled receptor superfamily and can trigger different signaling pathways to exert their physiological functions. In this study, several representative agonists and antagonists of both CB1 and CB2 were systematically studied to predict their binding affinities and selectivity against both cannabinoid receptors using a set of hierarchical molecular modeling and simulation techniques, including homology modeling, molecular docking, molecular dynamics (MD) simulations and end point binding free energy calculations using the molecular mechanics/Poisson-Boltzmann surface area-WSAS (MM-PBSA-WSAS) method, and molecular mechanics/generalized Born surface area (MM-GBSA) free energy decomposition. Encouragingly, the calculated binding free energies correlated very well with the experimental values and the correlation coefficient square (R²), 0.60, was much higher than that of an efficient but less accurate docking scoring function (R² = 0.37). The hotspot residues for CB1 and CB2 in both active and inactive conformations were identified via MM-GBSA free energy decomposition analysis. The comparisons of binding free energies, ligand-receptor interaction patterns, and hotspot residues among the four systems, namely, agonist-bound CB1, agonist-bound CB2, antagonist-bound CB1, and antagonist-bound CB2, enabled us to investigate and identify distinct binding features of these four systems, with which one can rationally design potent, selective, and function-specific modulators for the cannabinoid receptors.

Tetrahydrobenzochromene Synthesis Enabled by a Deconjugative Alkylation/Tsuji-Saegusa-Ito Oxidation on Knoevenagel Adducts

A modular and practical route to versatile cyano-1,3-dienes by a sequence involving deconjugative alkylation and "Tsuji-Saegusa-Ito oxidation" is reported. In this letter, the versatility of the products is also explored, including a route to benzochromene scaffolds common to many natural products.

Catalytic Asymmetric Synthesis of 3,4-Disubstituted Cyclohexadiene Carbaldehydes: Formal Total Synthesis of Cyclobakuchiols A and C

The first catalytic approach for the asymmetric synthesis of 3,4-disubstituted cyclohexadiene carbaldehydes through an inverse-electron-demand Diels-Alder reaction is described. A variety of arylacetaldehydes and α,β,γ,δ-unsaturated aldehydes are tested under the mild reaction conditions catalyzed by l-proline to obtain the trans diastereomeric products with good yields and high enantioselectivities. The scope of this methodology is further extended to the asymmetric synthesis 3,4-disubstituted cyclohexane carbaldehydes and their derivatives. The practicality of this method is demonstrated by the gram-scale synthesis. This methodology is successfully applied for the formal total synthesis of cyclobakuchiol A, an antipyretic and anti-inflammatory agent, and cyclobakuchiol C.

Novel C-Ring-Hydroxy-Substituted Controlled Deactivation Cannabinergic Analogues

In pursuit of safer controlled-deactivation cannabinoids with high potency and short duration of action, we report the design, synthesis, and pharmacological evaluation of novel C9- and C11-hydroxy-substituted hexahydrocannabinol (HHC) and tetrahydrocannabinol (THC) analogues in which a seven atom long side chain, with or without 1'-substituents, carries a metabolically labile 2',3'-ester group. Importantly, in vivo studies validated our controlled deactivation approach in rodents and non-human primates. The lead molecule identified here, namely, butyl-2-[(6aR,9R,10aR)-1-hydroxy-9-(hydroxymethyl)-6,6-dimethyl-6a,7,8,9,10,10a-hexahydro-6H-benzo[c]chromen-3-yl]-2-methylpropanoate (AM7499), was found to exhibit remarkably high in vitro and in vivo potency with shorter duration of action than the currently existing classical cannabinoid agonists.

Novel adamantyl cannabinoids as CB1 receptor probes

In previous studies, compound 1 (AM411), a 3-(1-adamantyl) analogue of the phytocannabinoid (-)-Δ(8)-tetrahydrocannabinol (Δ(8)-THC), was shown to have improved affinity and selectivity for the CB1 receptor. In this work, we further explored the role of the 1-adamantyl group at the C-3 position in a series of tricyclic cannabinoid analogues modified at the 9-northern aliphatic hydroxyl (NAH) position. Of these, 9-hydroxymethyl hexahydrocannabinol 11 (AM4054) exhibited high CB1 affinity and full agonist profile. In the cAMP assay, the 9-hydroxymethyl cannabinol analogue 24 (AM4089) had a partial agonist profile, with high affinity and moderate selectivity for rCB1 over hCB2. In vivo results in rat models of hypothermia and analgesia were congruent with in vitro data. Our in vivo data indicate that 3-(1-adamantyl) substitution, within NAH cannabinergics, imparts improved pharmacological profiles when compared to the corresponding, traditionally used 3-dimethylheptyl analogues and identifies 11 and 24 as potentially useful in vivo CB1 cannabinergic probes.

C3-heteroaroyl cannabinoids as photolabeling ligands for the CB2 cannabinoid receptor

A series of tricyclic cannabinoids incorporating a heteroaroyl group at C3 were prepared as probes to explore the binding site(s) of the CB1 and CB2 receptors. This relatively unexplored structural motif is shown to be CB2 selective with K(i) values at low nanomolar concentrations when the heteroaromatic group is 3-benzothiophenyl (41) or 3-indolyl (50). When photoactivated, the lead compound 41 was shown to successfully label the CB2 receptor through covalent attachment at the active site while 50 failed to label. The benzothiophenone moiety may be a photoactivatable moiety suitable for selective labeling.

Latest advances in cannabinoid receptor agonists

BACKGROUND

Since the discovery of cannabinoid receptors and their endogenous ligands in early 1990s, the endocannabinoid system has been shown to play a vital role in several pathophysiological processes. It has been targeted for the treatment of several diseases including neurodegenerative diseases (Parkinson's disease, Alzheimer's disease, Huntington's disease and MS), cancer, obesity, inflammatory bowel disease, neuropathic and inflammatory pain. The last decade has witnessed remarkable advances in the development of cannabinergic ligands displaying high selectivity and potency towards two subtypes of cannabinoid receptors, namely CB1 and CB2.

OBJECTIVE

In this review, we highlight the latest advances made in the development of cannabinoid agonists and summarize recently disclosed, novel chemical scaffolds as CB-selective agonists in patents that appeared during January 2008 - June 2009.

METHODS

Data presented here are obtained through the search of PubMed for research articles and reviews, and the website of European patents (http://ep.espacenet.com), SciFinder Scholar and US patents (www.uspto.gov).

CONCLUSIONS

Our analysis reveals prolific patenting activity mainly in the CB2 selective agonist area. Limiting the BBB penetrability, thereby, leading to peripherally restricted CB1/CB2 agonists and enhancing CB2-selectivity emerge as likely prerequisites for avoidance of adverse central CB1 mediated side effects.

Oxaza adamantyl cannabinoids. A new class of cannabinoid receptor probes

The preparation of C3 oxaza adamantyl cannabinoids has been described starting from phloroglucinol. Straightforward manipulations of the aromatic ring lead to a bromononaflate that is a benzyne precursor and that serves as a common intermediate for the synthesis of diverse C3-substituted tricyclic cannabinoids. Generation of the benzyne in the presence of an oxaza adamantyl amide anion results in efficient and regiospecific addition to C3 of the aromatic ring. This represents an attractive strategy for the synthesis of classical tricyclic cannabinoids that bear a modified aromatic appendage. The oxaza adamantyl cannabinoids that have been prepared represent a new class of ligands for the CB1 and CB2 receptors.

Synthesis of (-)-Delta9-trans-tetrahydrocannabinol: stereocontrol via Mo-catalyzed asymmetric allylic alkylation reaction

[reaction: see text] Delta9-THC is synthesized in enantiomericaly pure form, where all of the stereochemistry is derived from the molybdenum-catalyzed asymmetric alkylation reaction of the extremely sterically congested bis-ortho-substituted cinnamyl carbonate in high regio- and enantioselectivity.

Synthesis of Cannabidiols via Alkenylation of Cyclohexenyl Monoacetate

[reaction: see text] Because of the lack of potency binding to the receptors responsible for psychoactivity, cannabidiol has received much attention as a lead compound to develop a nonpsychotropic drug. Herein, we establish a method to access not only cannabidiol but also its analogues. The key reaction is nickel-catalyzed allylation of 2-cyclohexene-1,4-diol monoacetate with a new reagent, (alkenyl)ZnCl/TMEDA, which gives a S(N)2-type product with 94% regioselectivity in good yield.

Natural cannabinoids: templates for drug discovery

Recent studies have elucidated the biosynthetic pathway of cannabinoids and have highlighted the preference for a C-3 n-pentyl side chain in the most prominently represented cannabinoids from Cannabis sativa and their medicinally important decarboxylation products. The corresponding C-3 n-propyl side chain containing cannabinoids are also found, although in lesser quantities. Structure-activity relationship (SAR) studies performed on Delta(9)-tetrahydrocannabinol (Delta(9)-THC), the key psychoactive ingredient of Cannabis, and its synthetic analogues have identified the C-3 side chain as the key pharmacophore for ligand affinity and selectivity for the known cannabinoid receptors and for pharmacological potency. Interestingly, the terminal n-pentyl saturated hydrocarbon side chain of endocannabinoids also plays a corresponding crucial role in conferring similar properties. This review briefly summarizes the biosynthesis of cannabinoids and endocannabinoids and focuses on their side chain SAR.

Structural requirements for cannabinoid receptor probes

The discovery and cloning of CB1 and CB2, the two known G(i/o) protein-coupled cannabinoid receptors, as well as the isolation and characterization of two families of endogenous cannabinergic ligands represented by arachidonoylethanolamide (anandamide) and 2-arachidonoylglycerol (2-AG), have opened new horizons in this newly discovered field of biology. Furthermore, a considerable number of cannabinoid analogs belonging to structurally diverse classes of compounds have been synthesized and tested, thus providing substantial information on the structural requirements for cannabinoid receptor recognition and activation. Experiments with site-directed mutated receptors and computer modeling studies have suggested that these diverse classes of ligands may interact with the receptors through different binding motifs. The information about the exact binding site may be obtained with the help of suitably designed molecular probes. These ligands either interact with the receptors in a reversible fashion (reversible probes) or alternatively attach at or near the receptor active site with the formation of covalent bonds (irreversible probes). This review focuses on structural requirements of cannabinoid receptor ligands and highlights their pharmacological and therapeutic potential.

Synthesis of covalent probes for the radiolabeling of the cannabinoid receptor

The main psychoactive constituent of marijuana, (-)-Delta(9)-tetrahydrocannabinol, produces most of its physiological effects by interacting with the CB1 cannabinoid receptor, a membrane protein belonging to the large superfamily of G-protein coupled receptors. The 3-D structure of the receptor binding site is of value in the design of novel medications for a variety of therapeutic indications. To obtain information on the amino acid residues associated with this binding site, we have designed and synthesized a cannabinergic CB1 ligand prototype carrying an electrophilic isothiocyanato group capable of reacting covalently with amino acid residues bearing thiol or unprotonated amino groups. The ligand also incorporates an iodide atom, which can serve as a high-activity radiolabel. The key step in our synthesis involves a rapid intramolecular Diels-Alder reaction of a transiently formed o-quinone methide, which proceeds stereospecifically with the formation of the tricyclic cannabinoid template. Introduction of the iodo group is the last step in the sequence and is compatible with the use of (125)I-radiolabel.

Cannabinergic ligands

The understanding of the pharmacology surrounding the cannabinergic system has seen many advances since the discovery of the CB1 receptor in the mammalian brain and the CB2 receptor in the periphery. Among these advances is the discovery of the endogenous ligands arachidonoylethanolamide (anandamide) and 2-arachidonoylglycerol amide (2-AG), which are selective agonists for the CB1 and CB2 receptors, respectively. These endogenous neuromodulators involved in the cannabinergic system are thought to be produced on demand and are metabolized by the enzymes fatty acid amide hydrolase (FAAH) and monoacylglycerol lipase (MAG lipase). Recently, we characterized a reuptake system that facilitates the transport of anandamide across the cell membrane and subsequently developed selective inhibitors of this transport, which have been found to have therapeutic potential as analgesic and peripheral vasodilators. The cannabinergic proteins currently being explored, which include the CB1 and CB2 receptors, FAAH and the anandamide transporter, are excellent targets for the development of therapeutically useful drugs for a range of conditions including pain, loss of appetite, immunosuppression, peripheral vascular disease and motor disorders. As cannabinoid research has progressed, various potent and selective cannabimimetic ligands, targeting these four cannabinoid proteins, have been designed and synthesized. Many of these ligands serve as important molecular probes, providing structural information regarding the binding sites of the cannabinergic proteins, as well as pharmacological tools, which have been playing pivotal roles in research aimed at understanding the biochemical and physiological aspects of the endocannabinoid system. This review will focus on some of the current cannabinergic ligands and probes and their pharmacological and therapeutic potential.

Enantiomeric resolution of a novel chiral cannabinoid receptor ligand

The enantiomeric resolution of a racemic novel cannabinoid receptor ligand conformationally restricted at the southern aliphatic chain was accomplished using a ChiralPak AD column. Both enantiomers were tested for their competitive binding to the rat brain CB1, mouse spleen CB2 and human CB2 receptors. The levorotatory isomer showed exceptionally high affinity for the CB1 receptor with a seven-fold selectivity over CB2.

Synthesis of tetrahydrocannabinols based on an indirect 1,4-addition strategy

The synthetic procedure presented for the preparation of the title compounds requires 1,4-addition of bulky cuprates to cyclohexenones and subsequent reaction with electrophiles. However, the enolates generated by BF(3).OEt(2)-assistance suffer from lack of nucleophilicity. To circumvent this problem, we developed an indirect method consisting of the following three steps: (1) iodination of the cyclohexenones at the alpha position; (2) BF(3).OEt(2)-assisted 1,4-addition of cuprates (Ar(2)Cu(CN)Li(2), Ar = aryl) followed by quenching the enolates with water; (3) reaction of the alpha-iodo-beta-aryl-cylohexanones thus formed with EtMgBr to generate magnesium enolates. The enolates thus generated in this way showed a high reactivity toward ClP(O)(OEt)(2) to furnish enol phosphates. The aforementioned procedure was also applied to a synthesis of optically active Delta(9)-tetrahydrocannabinol. In addition, a naphthalene analogue of the latter compound was also synthesized in a similar way.

From cannabis to cannabinergics: new therapeutic opportunities

The molecular basis of cannabinoid activity is better understood since the discovery of the CB(1) receptor in the mammalian brain and the CB(2) receptor in peripheral tissues. Subsequently, an endogenous CB(1) receptor ligand, arachidonylethanolamide (anandamide), was isolated from porcine brain and shown to be metabolized by the enzyme arachidonylethanolamide amidohydrolase or fatty acid amide hydrolase. Recently, we have characterized a reuptake system for the transport of anandamide across the cell membrane, and have shown that selective inhibition of this transporter is associated with analgesia and peripheral vasodilation. The four cannabinoid system proteins, including the CB(1) and CB(2) receptors, fatty acid amide hydrolase, and the anandamide transporter, are excellent targets for the development of novel medications for various conditions, including pain, immunosuppression, peripheral vascular disease, appetite enhancement or suppression, and motor disorders. During the last decade, numerous selective ligands for each of these proteins were designed and synthesized. Many of these agents serve as important molecular probes, providing structural information about their binding sites, as well as pharmacological tools imparting information about the roles of their targets in physiological and disease states. All of the above compounds that modulate the functions of the endocannabinoid system can be collectively described under the term cannabinergics, regardless of chemical classification or type of resultant pharmacological action.

Molecular probes for the cannabinoid receptors

Cannabinoids produce most of their biochemical and pharmacological effects by interacting with CB1 and CB2 cannabinoid receptors, both of which are G-protein coupled membrane-bound functional proteins. CB1 is found in the central nervous system and in a variety of other organs including heart, vascular endothelium, uterus, vas deferens, testis and small intestine. Conversely, the CB2 receptor appears to be associated exclusively with the immune system and is found in the periphery of the spleen and other cells associated with immunochemical functions. Although both CB1 and CB2 have been cloned and the primary sequences are known, their three dimensional structures and the amino acid residues at the active site, critical for ligand recognition, binding and activation have not been characterized. In the absence of any X-ray crystallographic and NMR data, information on the structural requirements for ligand-receptor interactions is obtained with the help of suitably designed molecular probes. These ligands either interact with the receptor in a reversible fashion (reversible probes) or, alternatively, attach at or near the receptor active site with the formation of a covalent bond (irreversible probes). Subsequently, information related to ligand binding and receptor activation is further amplified with the help of receptor mutants and computer modeling. This review focuses on molecular probes related to the classical and non-classical cannabinoids that have been reported since the discovery of the first cannabinoid receptor over a decade ago.

Synthesis of functionalized cannabinoids

An effective synthesis of tricyclic, nonclassical cannabinoids has been developed on the basis of a cation-olefin cyclization that forms the two nonaromatic rings with the desired stereochemistry in a single step.

Cannabinoids, endogenous ligands and synthetic analogs

The investigation of natural and synthetic cannabinoid ligands, including (-)-Delta(9)-tetrahydrocannabinol, cannabinol, cannabidiol, HU-210, HU-211, CT3, CP 55, 940, WIN 55, 212-2, SR 14, 1716A, anandamide, 2-arachidonoylglycerol, and numerous novel analogs, has led to important findings that have contributed to a better understanding of the role of these compounds in physiological processes. Their potential use for medicinal purposes is also better understood as a result.

Design and synthesis of the CB1 selective cannabinoid antagonist AM281: a potential human SPECT ligand

In the search for a radioligand capable of imaging cannabinoid CB1 receptors in the living human brain by SPECT (single photon emission computed tomography),N-(morpholin-4-yl)-1-(2,4-dichlorophenyl)-5-(4-iodophenyl)-4-methyl-1H-pyrazole-3-carboxamide (AM281) was synthesized. This compound is an analog of the potent, CB1 receptor selective antagonist SR141716A [N-(piperidin-1-yl)-1-(2,4-dichlorophenyl)-5-(4-chlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide]. AM281 bound to brain and spleen membrane preparations (CB1 and CB2 receptors, respectively) with K(i) values of 12 nM and 4200 nM, respectively. AM281 also inhibited the response of guinea-pig small intestine preparation to a cannabinoid receptor agonist. Thus, AM281 behaves as a CB1 receptor selective antagonist. Methods for the rapid, high-yield synthesis and purification of [123I]AM281 were developed, and transaxially reconstructed brain SPECT images obtained after continuous infusion of [123I]AM281 in baboons. Thus [123I]AM281 may be suitable for imaging CB1 receptors in humans.

Structure-activity relationships of pyrazole derivatives as cannabinoid receptor antagonists

As a potent, specific antagonist for the brain cannabinoid receptor (CB1), the biarylpyrazole N-(piperidin-1-yl)-5-(4-chlorophenyl)-1-(2, 4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide (SR141716A; 1) was the lead compound for initiating studies designed to examine the structure-activity relationships of related compounds and to search for more selective and potent cannabimimetic ligands. A series of pyrazole derivatives was designed and synthesized to aid in the characterization of the cannabinoid receptor binding sites and also to serve as potentially useful pharmacological probes. Therapeutically, such compounds may have the ability to antagonize harmful side effects of cannabinoids and cannabimimetic agents. Structural requirements for potent and selective brain cannabinoid CB1 receptor antagonistic activity included (a) a para-substituted phenyl ring at the 5-position, (b) a carboxamido group at the 3-position, and (c) a 2,4-dichlorophenyl substituent at the 1-position of the pyrazole ring. The most potent compound of this series contained a p-iodophenyl group at the 5-position, a piperidinyl carboxamide at the 3-position, and a 2,4-dichlorophenyl group at the 1-position of the pyrazole ring. The iodinated nature of this compound offers additional utility as a gamma-enriching SPECT (single photon emission computed tomography) ligand that may be useful in characterizing brain CB1 receptor binding in vivo.