Stereospecific synthesis of (2S)-2-methyl-3-(2',6'-dimethyl-4'-hydroxyphenyl)-propionic acid (Mdp) and its incorporation into an opioid peptide

Iridium catalysts with modular axial-unfixed biphenyl phosphine–oxazoline ligands: asymmetric hydrogenation of α,β-unsaturated carboxylic acids

Axial-unfixed biphenyl phosphine–oxazoline ligands were successfully applied to Ir-catalyzed asymmetric hydrogenation of α,β-unsaturated carboxylic acids.

Synthesis of neoclerodane diterpenes and their pharmacological effects

Salvinorin A is a neoclerodane diterpene that has been shown to be an agonist at kappa opioid receptors. Its unique structure makes it an attractive target for synthetic organic chemists due to its seven chiral centers and diterpene scaffold. This molecule is also interesting to pharmacologists because it is a non-serotonergic hallucinogen, and the first opioid ligand discovered that lacks a basic nitrogen. There have been several total synthesis approaches to salvinorin A, and these will be detailed within this chapter. Additionally, research efforts have concentrated on structure modification of the salvinorin A scaffold through semi-synthetic methods. Most modifications have focused on the manipulation of the acetate at C-2 and the furan ring. However, chemistry has also been developed to generate analogs at the C-1 ketone, the C-4 methyl ester, and the C-17 lactone. The synthetic methodologies developed for the salvinorin A scaffold will be described, as well as specific analogs with interesting biological activities.

Neuropharmacology of the naturally occurring kappa-opioid hallucinogen salvinorin A

Salvia divinorum is a perennial sage native to Oaxaca, Mexico, that has been used traditionally in divination rituals and as a treatment for the "semimagical" disease panzón de borrego. Because of the intense "out-of-body" experiences reported after inhalation of the pyrolized smoke, S. divinorum has been gaining popularity as a recreational hallucinogen, and the United States and several other countries have regulated its use. Early studies isolated the neoclerodane diterpene salvinorin A as the principal psychoactive constituent responsible for these hallucinogenic effects. Since the finding that salvinorin A exerts its potent psychotropic actions through the activation of KOP receptors, there has been much interest in elucidating the underlying mechanisms behind its effects. These effects are particularly remarkable, because 1) salvinorin A is the first reported non-nitrogenous opioid receptor agonist, and 2) its effects are not mediated by the 5-HT(2A) receptor, the classic target of hallucinogens such as lysergic acid diethylamide and mescaline. Rigorous investigation into the structural features of salvinorin A responsible for opioid receptor affinity and selectivity has produced numerous receptor probes, affinity labels, and tools for evaluating the biological processes responsible for its observed psychological effects. Salvinorin A has therapeutic potential as a treatment for pain, mood and personality disorders, substance abuse, and gastrointestinal disturbances, and suggests that nonalkaloids are potential scaffolds for drug development for aminergic G-protein coupled receptors.

Further studies of tyrosine surrogates in opioid receptor peptide ligands

A series of opioid peptide ligands containing modified N-terminal tyrosine (Tyr) residues was prepared and evaluated against cloned human mu, delta, and kappa opioid receptors. This work extends the recent discovery that (S)-4-carboxamidophenylalanine (Cpa) is an effective tyrosine bioisostere. Amino acids containing negatively charged functional groups in place of tyrosine's phenolic hydroxyl lacked receptor affinity, while exchange of Tyr for (S)-4-aminophenylalanine was modestly successful. Peptides containing the new amino acids, (S)-4-carboxamido-2,6-dimethylphenylalanine (Cdp) and (S)-beta-(2-aminobenzo[d]thiazol-6-yl)alanine (Aba), displayed binding (K(i)) and functional (EC(50)) profiles comparable to the parent ligands at the three receptors. Cdp represents the best performing Tyr surrogate in terms of overall activity, while Cpa and Aba show a subtle proclivity toward the delta receptor.

Novel endomorphin-2 analogs with mu-opioid receptor antagonist activity

A series of position 4-substituted endomorphin-2 (Tyr-Pro-Phe-Phe-NH2) analogs containing 3-(1-naphthyl)-alanine (1-Nal) or 3-(2-naphthyl)-alanine (2-Nal) in L- or D-configuration, was synthesized. The opioid activity profiles of these peptides were determined in the mu-opioid receptor representative binding assay and in the Guinea-Pig Ileum assay/Mouse Vas Deferens assay (GPI/MVD) bioassays in vitro, as well as in the mouse hot-plate test of analgesia in vivo. In the binding assay the affinity of all new analogs for the mu-opioid receptor was reduced compared with endomorphin-2. The two most potent analogs were [D-1-Nal(4)]- and [D-2-Nal4]endomorphin-2, with IC50 values 14 +/- 1.25 and 19 +/- 2.1 nM, respectively, compared with 1.9 +/- 0.21 nM for endomorphin-2. In the GPI assay these analogs were found to be weak antagonists and they were inactive in the MVD assay. The in vitro GPI assay results were in agreement with those obtained in the in vivo hot-plate test. Antinociception induced by endomorphin-2 was reversed by concomitant intracerebroventricula (i.c.v.) administration of [D-1-Nal4]- and [D-2-Nal4]-endomorphin-2, indicating that these analogs were mu-opioid antagonists. Their antagonist activity was compared with that of naloxone. At a dose 5 microg per animal naloxone almost completely inhibited antinociceptive action of endomorphin-2, while [D-1-Nal4]endomorphin-2 in about 46%.

A novel cyclic enkephalin analogue with potent opioid antagonist activity

2',6'-Dimethyl substitution of the Tyr(1) residue in opioid agonist peptides and deletion of the N-terminal amino group, as achieved by replacement of Tyr(1) with 3-(2,6-dimethyl-4-hydroxyphenyl)propanoic acid (Dhp), have been shown to produce opioid antagonists. To examine the effect of beta-methylation of Dhp(1) in opioid peptides on the activity profile, stereoselective syntheses of (3S)- and (3R)-3-methyl-3-(2,6-dimethyl-4-hydroxyphenyl)propanoic acid [(3S)- and (3R)-Mdp] were carried out. In comparison with the cyclic parent antagonist peptide Dhp-c[D-Cys-Gly-Phe(pNO(2))-D-Cys]NH(2), the methylated analogue (3S)-Mdp-c[D-Cys-Gly-Phe(pNO(2))-D-Cys]NH(2) showed higher micro, delta and kappa antagonist potencies in functional assays and higher binding affinities for micro, delta and kappa opioid receptors (K(i)(micro)=2.03 nM; K(i)(delta)=2.34 nM; K(i)(kappa)=49.5 nM), whereas the corresponding (3R)-Mdp(1)-analogue was less potent by 1-2 orders of magnitude.

Characterization of antinociceptive activity of novel endomorphin-2 and morphiceptin analogs modified in the third position

In the present study we investigated and compared the in vivo analgesia of centrally administered endomorphin-2 and morphiceptin, and their analogs modified in position 3. Two series of analogs were synthesized by introducing unnatural aromatic amino acids in the D configuration: 3-(1-naphthyl)-D-alanine (D-1-Nal), 3-(2-naphthyl)-D-alanine (D-2-Nal), 3-(4-chlorophenyl)-D-alanine (D-ClPhe), 3-(3,4-dichlorophenyl)-D-alanine (D-Cl2Phe). Antinociceptive activity of endomorphin-2, morphiceptin, and their analogs was compared in the mouse hot-plate test, performed after i.c.v. administration of the peptides at a dose of 10 microg/animal. The best results were obtained for two morphiceptin analogs, [D-Phe3]morphiceptin and [D-1-Nal3]morphiceptin, which showed greatly improved analgesic activity, as compared to morphiceptin. In the endomorphin-2 series none of the modifications produced analogs more potent than the parent compound, but [D-1-Nal3]endomorphin-2 was the best analog. Antinociception induced by endomorphin-2 was reversed by concomitant i.c.v. administration of [D-Phe3]endomorphin-2, [D-2-Nal3]endomorphin-2, and [D-2-Nal3]morphiceptin, indicating that these analogs were weak mu-opioid antagonists.

Conversion of delta-, kappa- and mu-receptor selective opioid peptide agonists into delta-, kappa- and mu-selective antagonists

2',6'-Dimethyl substitution of the Tyr(1) residue of opioid agonist peptides and deletion of the positively charged N-terminal amino group or its replacement with a methyl group has recently been shown to represent a general structural modification to convert opioid peptide agonists into antagonists. This conversion requires the syntheses of opioid peptide analogues containing either 3-(2,6-dimethyl-4-hydroxyphenyl)propanoic acid (Dhp) or (2S)-2-methyl-3-(2,6-dimethyl-4-hydroxyphenyl)propanoic acid [(2S)-Mdp] in place of Tyr(1). Using this approach, delta-, kappa- and mu-selective opioid peptide agonist peptides were successfully converted into corresponding delta-, kappa- and mu-selective antagonists, whereby receptor selectivity was often maintained or even improved. Thus, two (2S)-Mdp(1)-analogues of the delta-selective cyclic enkephalin analogue H-Tyr-c[D-Pen-Gly-Phe(pF)-Pen]-Phe-OH turned out to be potent and selective delta antagonists. Most successful was the development of kappa antagonists derived from dynorphin A (Dyn A), including the highly potent and selective kappa-antagonist [(2S)-Mdp(1)]Dyn A(1-11)-NH(2) (dynantin) and the enzymatically stable octapeptide analogue [(2S)-Mdp(1),MeArg(7),D-Leu(8)]Dyn A(1-8)-NH(2). The (2S)-Mdp(1)-analogues of dynorphin B and alpha-neoendorphin also were kappa antagonists and may be useful as pharmacological tools in studies of kappa receptor subtypes. Finally, the Dhp(1)-analogues of the mu-selective cyclic enkephalin analogue H-Tyr-c[N(epsilon ),N(beta)-carbonyl-D-Lys(2),Dap(5)]enkephalinamide and of endomorphin-2 were moderately potent mu opioid antagonists.

New scaffolds in the development of mu opioid-receptor ligands

A new class of mu selective receptor antagonists has been developed using a combinatorial approach based on previously reported Dmt-Tic dipeptide ligands. Modified tetrahydroisoquinoline (Tiq) residues were reacted with different electrophiles in order to create novel molecules that would mimic the original dipeptide. A specific class of thioureas bearing basic pyrrolidine residues were shown to give good binding affinities. Further alkylation of the pyrrolidine ring with benzyl derivatives also proved to increase the mu binding affinity. In addition, it was demonstrated that mu binding was enhanced by the presence of polar groups around the benzyl ring having hydrogen-bonding character (donor/acceptor). This new class of ligands represents a novel scaffold in the development of opioid analogues.