Synthesis of 8-aminomorphans with high KOR affinity

2-Azabicyclo[3.3.1]nonanes (morphans) with a (3,4-dichlorophenyl)acetyl group at 2-position and a pyrrolidino moiety at 8-position were designed as conformationally restricted analogs of piperidine-based KOR agonists. The synthesis started with 4-oxopiperidine-2-carboxylic acid comprising 13 reaction steps. At first the ketone 10 was transformed into diester 7 bearing a propionate side chain. Dieckmann condensation of diester 7 to afford bicyclic enolester 14 and subsequent Krapcho deethoxycarbonylation represent the key steps of the synthesis. The enantiomeric pyrrolidines (1S,5R,8R)-5a and (1R,5S,8S)-5a were separated by chiral HPLC. The eutomer (1S,5R,8R)-5a showed high KOR affinity (K_i = 18 nM) and selectivity over MOR, DOR and σ₂ receptors. It was concluded that the dihedral angle of the KOR pharmacophore N(pyrrolididine)-C-C-N(acyl) of (1S,5R,8R)-5a (68°) is close to the bioactive conformation of the flexible KOR agonist 3.

Synthesis and pharmacological evaluation of enantiomerically pure endo-configured KOR agonists with 2-azabicyclo[3.2.1]octane scaffold

Conformationally restricted bicyclic KOR agonists 10 with an endo-configured amino moiety were synthesized to analyze the bioactive conformation of conformationally flexible KOR agonists such as 2-5. A seven-step synthesis starting with (S)-configured 4-oxopiperidine-2-carboxylate 13 was developed. cis- and trans-configured diesters 12 were obtained in a 3 : 1 ratio via hydrogenation of the α,β-unsaturated ester 14. After establishment of the bicyclic scaffold, a diastereoselective reductive amination of ketone 11 provided exclusively the endo-configured bicyclic amines 10a,b. The 3 : 1 mixtures of enantiomers were separated by chiral HPLC, respectively, leading to enantiomerically pure KOR agonists (1S,5S,7R)-10a,b and (1R,5R,7S)-10a,b (ent-10a,b). The KOR affinity was determined in receptor binding studies with the radioligand [3H]U-69 593. The high KOR affinity of endo-configured amines 10a (Ki = 7 nM) and 10b (Ki = 13 nM) indicates that the dihedral angle of the KOR pharmacophoric element N(pyrrolidine)-C-C-N(phenylacetyl) of 42° is close to the bioactive conformation of more flexible KOR agonists. It should be noted that changing the configuration of potent and selective KOR agonists 10a and 10b led to potent and selective σ1 ligands (e.g. ent-10aKi(σ1) = 10 nM).

Diastereoselective synthesis of conformationally restricted KOR agonists

In order to analyze the bioactive conformation of flexible KOR agonists the ethylenediamine KOR pharmacophore was conformationally constrained by incorporation into a bicyclic system. For this purpose, 2-azabicyclo[3.2.1.]octan-7-amines were designed, synthesized and pharmacologically evaluated. The primary amine 14 as first key intermediate was prepared in a six-step synthesis starting with methyl cyclopent-3-enecarboxylate 9. Whereas phenylacetamides failed to provide bicyclic compounds, the intramolecular nucleophilic substitution of the sulfonamide 25 was initiated by deprotonation with NaH affording the bicyclic compound 26 in 72% yield. The three-step introduction of the pharmacophoric pyrrolidine ring started with nucleophilic substitution of exo-configured tosylate 26 with NaN3, which unexpectedly occurred under retention of configuration leading to exo-configured azide 31. The final KOR agonists 35 and 36 with exo-configured amino moieties were obtained by removal of the N-tosyl moiety of 33 and introduction of the second pharmacophoric element by acylation with dihalophenylacetyl chlorides. The KOR affinity of the pyrrolidine 35a is in the high nanomolar range (Ki = 862 nM). The low KOR affinity is explained by a non-appropriate dihedral angle of 137°/141° of the N(pyrrolidine)-C-C-N(acyl) system. As observed for stereoisomers of potent KOR agonists, phenylacetamide 35a and more importantly sulfonamides 33a and 33b show moderate affinity at σ1 receptors (Ki = 109-208 nM).

Stereoselective synthesis of conformationally restricted KOR agonists based on the 2,5-diazabicyclo[2.2.2]octane scaffold

It has been postulated that the KOR affinity depends on the dihedral angle of the ethylenediamine pharmacophore. Herein, 2,5-diazabicyclooctanes bearing a pyrrolidino moiety in the 7-position were envisaged to study KOR agonists with a conformationally rigid ethylenediamine pharmacophore and thus a defined N(pyrrolidine)-C7-C1-N2 dihedral angle. The first approach with an intramolecular addition at the chiral sulfinylimines 9 failed to give bicyclic products. The key step in the second approach was a Dieckmann analogous cyclization providing mixed methyl silyl ketals 11a-e as key intermediates. The highest KOR affinity was found for the 2,5-dibenzyl substituted derivatives (S,R,S)-16a (K_i = 31 nM) and (R,S,R)-16a (K_i = 74 nM) with the pyrrolidine ring oriented towards N-5. The high KOR affinity of (S,R,S)-16a is unexpected, since the KOR pharmacophoric ethylenediamine system adopts a dihedral angle of about 160°, which is quite different from the angle of the energetically most favored conformer of the flexible and potent KOR agonist 2. (S,R,S)-16a represents a KOR agonist with moderate selectivity over MOR (8-fold) and DOR (5-fold), but high selectivity over both σ receptor subtypes. In the [³⁵S]GTPγS assay (S,R,S)-16a reacted as a full KOR agonist with an EC₅₀ value of 240 nM.