Chemistry of opium alkaloids. Part 44: synthesis and opioid receptor binding profile of substituted ethenoisomorphinans and ethenomorphinans

Opioid Antagonists from the Orvinol Series as Potential Reversal Agents for Opioid Overdose

The opioid crisis continues to claim many lives, with a particular issue being the ready availability and use (whether intentional or accidental) of fentanyl and fentanyl analogues. Fentanyl is both potent and longer-acting than naloxone, the standard of care for overdose reversal, making it especially deadly. Consequently, there is interest in opioid reversal agents that are better able to counter its effects. The orvinol series of ligands are known for their high-affinity binding to opioid receptors and often extended duration of action; generally, compounds on this scaffold show agonist activity at the kappa and the mu-opioid receptor. Diprenorphine is an unusual member of this series being an antagonist at mu and only a partial agonist at kappa-opioid receptors. In this study, an orvinol antagonist, 14, was designed and synthesized that shows no agonist activity in vitro and is at least as good as naloxone at reversing the effects of mu-opioid receptor agonists in vivo.

Diels-Alder Adducts of Morphinan-6,8-Dienes and Their Transformations

6,14-ethenomorphinans are semisynthetic opiate derivatives containing an ethylene bridge between positions 6 and 14 in ring-C of the morphine skeleton that imparts a rigid molecular structure. These compounds represent an important family of opioid receptor ligands in which the 6,14-etheno bridged structural motif originates from a [4 + 2] cycloaddition of morphinan-6,8-dienes with dienophiles. Certain 6,14-ethenomorphinans having extremely high affinity for opioid receptors are often non-selective for opioid receptor subtypes, but this view is now undergoing some revision. The agonist 20R-etorphine and 20R-dihydroetorphine are several thousand times more potent analgesics than morphine, whereas diprenorphine is a high-affinity non-selective antagonist. The partial agonist buprenorphine is used as an analgesic in the management of post-operative pain or in substitution therapy for opiate addiction, sometimes in combination with the non-selective antagonist naloxone. In the context of the current opioid crisis, we communicated a summary of several decades of work toward generating opioid analgesics with lesser side effects or abuse potential. Our summary placed a focus on Diels-Alder reactions of morphinan-6,8-dienes and subsequent transformations of the cycloadducts. We also summarized the pharmacological aspects of radiolabeled 6,14-ethenomorphinans used in molecular imaging of opioid receptors.

Thiazole constrained analogues of the thevinones: synthesis and structure

A simple synthesis of ring-constrained endoethenomorphinans possessing 2'-substituted thiazole ring 4-6 has been achieved by regio- and stereoselective Diels-Alder reaction of thiazolomorphinandienes 1-3 and methyl vinyl ketone in high yield (72, 64 and 87%, respectively). The structure of cycloaddition products was determined by high resolution mass spectrometry (HRMS), IR, 1D and 2D NMR techniques. Double-pulsed field gradient spin-echo-nOe and HMBC were found to be particularly powerful and indispensable tools in the exact structural elucidation of the presented new class of spatially constrained thevinones.

7α-Methoxy-carbonyl-6,7,8,14-tetra-hydro-6,14-endo-ethenothebaine

In the mol­ecule of the title compound, C₂₃H₂₇NO₅, the furan ring adopts an envelope conformation. Intra­molecular C—H⋯O inter­actions result in the formation of S(5) and S(6) motifs. In the crystal structure, weak inter­molecular C—H⋯O hydrogen bonds link the mol­ecules through C(6) and C(8) chains along the [100] and [010] directions, generating a two-dimensional network.

Formic acid catalysed rearrangement of 5beta-methyldihydrothevinols (= 3,6-dimethoxy-5,17-dimethyl-4,5-epoxy-6,14-ethanomorphinan-7-methanols): synthesis of new doubly bridged morphinan derivatives

We recently showed that substitution of a 5beta-methyl group allows the isolation of 14-alkenyldihydrocodeinones from the action of hot formic acid on certain dihydrothevinols. It has now been shown that, in those dihydrothevinols which are converted to stable side chain olefins by such treatment, introduction of a 5beta-methyl group results in the formation of new doubly bridged morphinan derivatives in addition to the dihydrocodeinones or olefins. The new morphinans have low affinities for opioid receptors.

Rearrangement of 5-trimethylsilylthebaine on treatment with L-selectride: an efficient synthesis of (+)-bractazonine

Treatment of 5-trimethylsilylthebaine with L-Selectride gave rise to a rearrangement to 10-trimethylsilylbractazonine through migration of the phenyl group, whereas treatment of thebaine with strong Lewis acids is known to lead to a similar rearrangement through migration of the alkyl bridge to give, after reduction, (+)-neodihydrothebaine. It is suggested that the rearrangement of the alkyl group of thebaine is favored due to the formation of a tertiary benzylic cation. However, for 5-trimethylsilylthebaine, the lithium ion of L-Selectride acts as the Lewis acid and the beta-silyl effect dominates in the stabilization of any positive charge. This rearrangement provides a clear example of the greater relative migratory aptitude of phenyl groups over alkyl groups, and provides an efficient synthesis of (+)-bractazonine from thebaine.

Molecular determinants of non-specific recognition of delta, mu, and kappa opioid receptors

Identification of the molecular determinants of recognition common to all three opioid receptors embedded in a single three-dimensional (3D) non-specific recognition pharmacophore has been carried out. The working hypothesis that underlies the computational study reported here is that ligands that bind with significant affinity to all three cloned opioid receptors, delta, mu, and kappa, but with different combinations of activation and inhibition properties at these receptors, could be promising behaviorally selective analgesics with diminished side effects. The study presented here represents the first step towards the rational design of such therapeutic agents. The common 3D pharmacophore developed for recognition of delta, mu, and kappa opioid receptors was based on the receptor affinities determined for 23 different opioid ligands that display no specificity for any of the receptor subtypes. The pharmacophore centers identified are a protonated amine, two hydrophobic groups, and the centroid of an aromatic group in a geometric arrangement common to all 23, non-specific, opioid ligands studied. Using this three-dimensional pharmacophore as a query for searching 3D structural databases, novel compounds potentially involved in non-specific recognition of delta, mu, and kappa opioid receptors were retrieved. These compounds can be valuable candidates for novel behaviorally selective analgesics with diminished or no side effects, and thus with potential therapeutic usefulness.