Fenchone Derivatives as a Novel Class of CB2 Selective Ligands: Design, Synthesis, X-ray Structure and Therapeutic Potential

Abstract

A series of novel cannabinoid-type derivatives were synthesized by the coupling of (1S,4R)-(+) and (1R,4S)-(-)-fenchones with various resorcinols/phenols. The fenchone-resorcinol derivatives were fluorinated using Selectfluor and demethylated using sodium ethanethiolate in dimethylformamide (DMF). The absolute configurations of four compounds were determined by X-ray single crystal diffraction. The fenchone-resorcinol analogs possessed high affinity and selectivity for the CB2 cannabinoid receptor. One of the analogues synthesized, 2-(2',6'-dimethoxy-4'-(2″-methyloctan-2″-yl)phenyl)-1,3,3-trimethylbicyclo[2.2.1]heptan-2-ol (1d), had a high affinity (K_i = 3.51 nM) and selectivity for the human CB2 receptor (hCB2). In the [³⁵S]GTPγS binding assay, our lead compound was found to be a highly potent and efficacious hCB2 receptor agonist (EC₅₀ = 2.59 nM, E_(max) = 89.6%). Two of the fenchone derivatives were found to possess anti-inflammatory and analgesic properties. Molecular-modeling studies elucidated the binding interactions of 1d within the CB2 binding site.

Keywords: cannabinoid agonists; fenchone; hCB2 receptor; inflammation.

Figure 1

Chemical structure of some CB2 receptor agonists.

Figure 2

Structural modifications employed in the SAR study.

Figure 3

Chemical structures of (1S,4R)-(+)-fenchone and (1R,4S)-(−)-fenchone.

Scheme 1

Synthesis of 2-(2′,6′-dimethoxy-4′-alkylphenyl)-1,3,3-trimethylbicyclo[2.2.1]heptan-2-ol analogs (1a–1f) ^a. ^a Reagents and conditions: (i) Potassium carbonate, anhydrous DMF and iodomethane. (ii) n-Butyllithium in hexane and anhydrous THF. (iii) Fenchone and anhydrous THF.

Scheme 2

Synthesis of 2-(3′-hexyl-2′,6′-dimethoxyphenyl)-1,3,3-trimethylbicyclo[2.2.1]heptan-2-ol analogs (2a, 2b) ^a. ^a Reagents and conditions: (i) Potassium carbonate, anhydrous DMF and iodomethane. (ii) n-Butyllithium in hexane and anhydrous THF. (iii) Fenchone and anhydrous THF.

Scheme 3

Synthesis of 2-(2′-methoxy-5′-alkylphenyl)-1,3,3-trimethylbicyclo[2.2.1]heptan-2-ol analogs (3a–3h) ^a. ^a Reagents and conditions: (i) Potassium carbonate, anhydrous DMF and iodomethane. (ii) n-Butyllithium in hexane and anhydrous THF. (iii) Fenchone and anhydrous THF.

Scheme 4

Synthesis of 2-(3′-fluoro-2′,6′-dimethoxy-4′-alkylphenyl)-1,3,3-trimethylbicyclo [2.2.1]heptan-2-ol analogs (4a–4d) ^a. ^a Reagents and conditions: (i) Selectfluor and CH₃CN.

Scheme 5

Synthesis of 2-(2′-hydroxy-6′-methoxy-4′-alkylphenyl)-1,3,3-trimethylbicyclo[2.2.1] heptan-2-ol (5a–5d) ^a. ^a Reagents and conditions: (i) Sodium ethanethiolate and anhydrous DMF.

Figure 4

In vitro interaction of the agonists HU-308, 1b and 1d with hCB2R. Ki values for CB2R were derived from displacement of [³H]CP-55,940 binding in membranes of CHO-K1 cells expressing human CB2R. Each curve represents the mean of three experiments performed in triplicate.

Figure 5

Mean log concentration–response curves of 1b and 1d for stimulation of [³⁵S]GTPγS binding to hCB2R CHO cell membranes. Each symbol represents the mean percentage increase in [³⁵S]GTPγS binding ± SE.

Figure 6

Anti-inflammatory and analgesic evaluation of HU-308 (light blue) compared to 1b (blue), 1d (dark blue) and vehicle control (black). (A) Swelling after six hours, (B) pain sensation after six hours, and (C) TNF-α serum levels after 24 hours. Statistical comparison was done by 1-way ANOVA (p-value = 0.05). * p < 0.05 comparing to control group. # p-value < 0.05 in the indicated comparison.

Figure 7

Orientation of docked ligands in the orthosteric site of CB2. The binding site of the CB2 cavity is represented by electrostatic potential surface. Ligands are shown in different colors; 5ZTY_ligand (pink), HU-308 (Yellow), HU-433 (Orange) and 1d (Green). TM means Transmembrane Helix. The residues of binding site and ligands are represented by thin and thick tubes, respectively. H-bonds and π–π interactions are represented by orange and cyan dotted lines, respectively.