Gram-Scale Enantioselective Formal Synthesis of Morphine through anortho-paraOxidative Phenolic Coupling Strategy

Asymmetric desymmetrization of 2,5-cyclohexadienones initiated by organocatalytic conjugate addition to 4-nitro-5-styrylisoxazoles

A highly regio-, enantio- and diastereo-selective strategy involving initial enantioselective conjugate addition to 4-nitro-5-styrylisoxazoles serves as a key step for the desymmetrization of 2,5-cyclohexadienones has been disclosed. We have designed a new class of 2,5-cyclohexadienones appended with 4-nitro-5-styrylisoxazoles to undergo organocatalytic asymmetric double or triple conjugate addition in a domino sequence depending on the substrate type leading to desymmetrization of the 2,5-cyclohexadienone core. The developed protocol allows the construction of a valuable hydrophenanthrene core or a unique bridged scaffold bearing multiple chiral centers with excellent enantio- (up to >99.5 : 0.5 er) and diastereo-control for diverse substrates.

Oxidative generation of isobenzofurans from phthalans: application to the formal synthesis of (±)-morphine

Treatment of phthalan derivatives with p-chloranil in dodecane in the presence of molecular sieves at 160-200 °C allowed the generation of unstabilized isobenzofurans, which underwent intramolecular Diels-Alder reaction to give endo cycloadducts exclusively. The cycloaddition turned out to be reversible, providing an equilibrium mixture of endo adducts when heating a substrate with a stereocenter on the tether. We also demonstrated the regioselective allylation of an oxygen-bridged cycloadduct upon exposure to EtAlCl2 in the presence of allyltrimethylsilane, and the conversion to Rice's intermediate completed a formal synthesis of (±)-morphine.

Recent advances in oxidative phenol coupling for the total synthesis of natural products

Covering: 2008 to 2023This review will describe oxidative phenol coupling as applied in the total synthesis of natural products. This review covers catalytic and electrochemical methods with a brief comparison to stoichiometric and enzymatic systems assessing their practicality, atom economy, and other measures. Natural products forged by C-C and C-O oxidative phenol couplings as well as from alkenyl phenol couplings will be addressed. Additionally, exploration into catalytic oxidative coupling of phenols and other related species (carbazoles, indoles, aryl ethers, etc.) will be surveyed. Future directions of this particular area of research will also be assessed.

Dearomative Access to (-)-Thebaine and Derivatives

A synthesis of the natural product thebaine is reported in eight steps from commercially available starting materials, hinging on the dearomatization and coupling of simple aromatic starting materials. This provides divergent access to two unnatural opioid derivatives and is aimed at the long-term development of synthetic opioid analogs of the "wonderdrug" Naloxone. Additionally, a formal enantioselective synthesis of all reported targets is disclosed that leverages a catalytic asymmetric dearomatization via anion-pairing catalysis.

Dearomative logic in natural product total synthesis

Covering: 2011 to 2022The natural world is a prolific source of some of the most interesting, rare, and complex molecules known, harnessing sophisticated biosynthetic machinery evolved over billions of years for their production. Many of these natural products represent high-value targets of total synthesis, either for their desirable biological activities or for their beautiful structures outright; yet, the high sp3-character often present in nature's molecules imparts significant topological complexity that pushes the limits of contemporary synthetic technology. Dearomatization is a foundational strategy for generating such intricacy from simple materials that has undergone considerable maturation in recent years. This review highlights the recent achievements in the field of dearomative methodology, with a focus on natural product total synthesis and retrosynthetic analysis. Disconnection guidelines and a three-phase dearomative logic are described, and a spotlight is given to nature's use of dearomatization in the biosynthesis of various classes of natural products. Synthetic studies from 2011 to 2021 are reviewed, and 425 references are cited.

Enantioselective Total Synthesis of Cepharatines via Bioinspired Ring Reconstruction

We describe enantioselective total syntheses of cepharatines A-D, members of the hasubanan alkaloid family, which feature an unusual tetracyclic skeleton including an azabicyclo[3.3.1]nonane motif. A key reaction is a regio-divergent oxidative phenolic coupling reaction that affords the tricyclic core structure of hasubanan with different substitution patterns on the A-ring, including the all-carbon quaternary stereogenic center at C13, in a single step. The characteristic tetracyclic azabicyclo[3.3.1]nonane motif was constructed by means of a bioinspired cascade reaction involving the retro-aza-Michael reaction/hemiaminal formation.

Morphine alkaloids: History, biology, and synthesis

This chapter provides a short overview of the history of morphine since it's isolation by Sertürner in 1805. The biosynthesis of the title alkaloid as well as all total and formal syntheses of morphine and codeine published after 1996 are discussed in detail. The last section of this chapter provides a detailed overview of medicinally relevant derivatives of the title alkaloid.

Asymmetric Intramolecular Dearomatization of Nonactivated Arenes with Ynamides for Rapid Assembly of Fused Ring System under Silver Catalysis

Arene dearomatization is a straightforward method for converting an aromatic feedstock into functionalized carbocycles. Enantioselective dearomatizations of chemically inert arenes, however, are quite limited and underexplored relative to those of phenols and indoles. We developed a method for diazo-free generation of silver-carbene species from an ynamide and applied it to the dearomatization of nonactivated arenes. Transiently generated norcaradiene could be trapped by intermolecular [4 + 2] cycloaddition, synthesizing polycycles with five consecutive stereogenic centers. This protocol constitutes the first highly enantioselective reaction based on the diazo-free generation of silver-carbene species. Mechanistic investigations revealed a dearomatization followed by two different classes of pericyclic reactions, as well as the origin of the chemo- and enantioselectivity.

Mechanism of Iodine(III)-Promoted Oxidative Dearomatizing Hydroxylation of Phenols: Evidence for a Radical-Chain Pathway

The oxidative dearomatization of phenols with the addition of nucleophiles to the aromatic ring induced by hypervalent iodine(III) reagents and catalysts has emerged as a highly useful synthetic approach. However, experimental mechanistic studies of this important process have been extremely scarce. In this report, we describe systematic investigations of the dearomatizing hydroxylation of phenols using an array of experimental techniques. Kinetics, EPR spectroscopy, and reactions with radical probes demonstrate that the transformation proceeds by a radical-chain mechanism, with a phenoxyl radical being the key chain-carrying intermediate. Moreover, UV and NMR spectroscopy, high-resolution mass spectrometry, and cyclic voltammetry show that before reacting with the phenoxyl radical, the water molecule becomes activated by the interaction with the iodine(III) center, causing the Umpolung of this formally nucleophilic substrate. The radical-chain mechanism allows the rationalization of all existing observations regarding the iodine(III)-promoted oxidative dearomatization of phenols.

DARK Classics in Chemical Neuroscience: Heroin and Desomorphine

Opioids are arguably one of the most important pharmacologic classes, mainly due to their rich history, their useful and potent analgesic effects, and also, just as importantly, their "Dark Side", constituted by their reinforcing properties that have led countless of users to a spiral of addiction, biological dependence, tolerance, withdrawal syndromes, and death. Among the most significant abused and addictive known opioids are heroin and desomorphine, both synthetic derivatives of morphine that belong to the 4,5-epoxymorphinan structural chemical group of the opioid family drugs. These agents share not only structural, pharmacological, and epidemiological features but also a common geographical distribution. A drop in Afghan heroin production and its "exports" to Russia gave rise to widespread consumption of desomorphine in ex-Soviet republics during the first decade of the 21st century, representing an economical and accessible alternative for misusers to this sort of derivative. Herein we review the state of the art of history, chemistry and synthesis, pharmacology, and impact on society of these "cursed cousins".

Rh(i)-Catalyzed enantioselective and scalable [4 + 2] cycloaddition of 1,3-dienes with dialkyl acetylenedicarboxylates

An asymmetric intermolecular [4 + 2] cycloaddition of 1,3-dienes with dialkyl acetylenedicarboxylates, which was catalyzed by a rhodium(i)-chiral phosphoramidite complex, was developed. This protocol provided a highly enantioselective access to prepare carbonyl substituted cyclohexa-1,4-dienes with up to 96% yield and >99% ee. Notably, a cycloaddition on the 10 g scale gave the product in 92% yield and with 99% ee, which showed great potential for the scale-up synthesis of carbonyl substituted cyclohexa-1,4-dienes. In addition, oxidative aromatizations and hydrolysis of the products were also investigated.

Enantioselective synthesis of cis-hydrobenzofurans bearing all-carbon quaternary stereocenters and application to total synthesis of (‒)-morphine

(‒)-Morphine, which is selected as an essential medicine by World Health Organization, is widely applied in the treatment of the pain-related diseases. Due to its synthetically challenging molecular architecture and important clinical role, extensive synthetic studies of morphine-type alkaloids have been conducted. However, catalytic asymmetric total synthesis of (‒)-morphine remains a long-standing challenge. Here, we disclose an efficient enantioselective total synthesis of (‒)-morphine in a longest linear sequence of 16 steps. The key transformation features a highly enantioselective Robinson annulation enabled by our spiro-pyrrolidine catalyst to rapidly construct the densely functionalized cis-hydrodibenzofuran framework containing vicinal stereocenters with an all-carbon quaternary center. This asymmetric approach provides an alternative strategy for the synthesis of (‒)-morphine and its analogues.

(‒)-Morphine is an essential medicine selected by the World Health Organization, however its catalytic asymmetric syntheses have been rarely reported. Here, the authors developed an intramolecular enantioselective Michael addition leading to (‒)-morphine in a longest linear sequence of 16 steps.

A Nine-Step Formal Synthesis of (±)-Morphine

A nine-step stereoselective formal synthesis of (±)-morphine from readily available o-vanillin is presented. The carbocyclic structure of morphine was quickly assembled through an orchestration of the intermolecular Diels-Alder/Claisen/Friedel-Crafts sequential reaction. This approach involves many one-pot procedures and no protecting groups, and only a few chromatographic purifications are required.

Synthesis of the core structure of phalarine

Two new palladium-catalyzed reactions enabled the synthesis of the core structure of phalarine.

Asymmetric synthesis of (-)-naltrexone

(-)-Naltrexone, an opioid antagonist used extensively for the management of drug abuse, is derived from naturally occurring opioids. Herein, we report the first asymmetric synthesis of (-)-naltrexone that does not proceed through thebaine. The synthesis starts with simple, achiral precursors with catalytic enantioselective Sharpless dihydroxylation employed to introduce the stereogenic centers. A Rh(i)-catalyzed C-H alkenylation and torquoselective electrocyclization cascade provides the hexahydro isoquinoline bicyclic framework that serves as the precursor to the morphinan core. The acidic conditions used for Grewe cyclization not only provide the morphinan framework, but also cause a hydride shift resulting in the introduction of the C-6 oxo functionality present in (-)-naltrexone. The C-14 hydroxyl group is installed by an efficient two-step sequence of Pd-mediated ketone to enone dehydrogenation followed by C-H allylic oxidation using Cu(ii) and O₂, a method that has not previously been reported either for the synthesis or semi-synthesis of opioids. The longest linear sequence is 17 steps, and because the stereogenic centers in the product rely on Sharpless asymmetric dihydroxylation, the route could be used to access either enantiomer of the natural product, which have disparate biological activities. The route also may be applicable to the preparation of opioid derivatives that could not be easily prepared from the more fully elaborated and densely functionalized opioid natural products that have traditionally served as the starting inputs.

The DARK Side of Total Synthesis: Strategies and Tactics in Psychoactive Drug Production

Humankind has used and abused psychoactive drugs for millennia. Formally, a psychoactive drug is any agent that alters cognition and mood. The term "psychotropic drug" is neutral and describes the entire class of substrates, licit and illicit, of interest to governmental drug policy. While these drugs are prescribed for issues ranging from pain management to anxiety, they are also used recreationally. In fact, the current opioid epidemic is the deadliest drug crisis in American history. While the topic is highly politicized with racial, gender, and socioeconomic elements, there is no denying the toll drug mis- and overuse is taking on this country. Overdose, fueled by opioids, is the leading cause of death for Americans under 50 years of age, killing ca. 64,000 people in 2016. From a chemistry standpoint, the question is in what ways, if any, did organic chemists contribute to this problem? In this targeted review, we provide brief historical accounts of the main classes of psychoactive drugs and discuss several foundational total syntheses that ultimately provide the groundwork for producing these molecules in academic, industrial, and clandestine settings.

Total Synthesis of the Norhasubanan Alkaloid Stephadiamine

(+)-Stephadiamine is an unusual alkaloid isolated from the vine Stephania japonica. It features a norhasubanan skeleton, and contains two adjacent α-tertiary amines, which renders it an attractive synthetic target. Here, we present the first total synthesis of stephadiamine, which hinges on an efficient cascade reaction to implement the aza[4.3.3]propellane core of the alkaloid. The α-aminolactone moiety in a highly hindered position was installed via Tollens reaction and Curtius rearrangement. Useful building blocks for the asymmetric synthesis of morphine and (nor)hasubanan alkaloids are introduced.

Total Synthesis of (+)-Gracilamine Based on an Oxidative Phenolic Coupling Reaction and Determination of Its Absolute Configuration

The enantioselective total synthesis of (+)-gracilamine (1) is described. The strategy features a diastereoselective phenolic coupling reaction followed by a regioselective intramolecular aza-Michael reaction to construct the ABCE ring system. The configuration at C3a in 1 was controlled by the stereocenter at C9a, which was selectively generated (91 % ee) by an organocatalytic enantioselective aza-Friedel-Crafts reaction developed by our research group. This synthesis revealed that the absolute configuration of (+)-gracilamine is 3aR, 4S, 5S, 6R, 7aS, 8R, 9aS.

Programmed serial stereochemical relay and its application in the synthesis of morphinans

Herein we report a rationally designed, serial point-to-axial and axial-to-point stereoinduction and its integration into multi-step and target-oriented organic synthesis. In this proof-of-concept study, the configurational stability of several carefully designed atropisomeric intermediates and the fidelity of their unconventional stereoinductions were systematically investigated. The highly functionalized prepared synthetic intermediate was further applied in a novel chemical method to access the morphinans and it is potentially applicable to other structurally related alkaloids.

Asymmetric Transfer Hydrogenation of 1,3-Alkoxy/Aryloxy Propanones Using Tethered Arene/Ru(II)/TsDPEN Complexes

A series of propanones containing combinations of aryloxy and alkoxy substituents at the 1- and 3-positions were reduced to the alcohols via asymmetric transfer hydrogenation using a tethered Ru(II)/TsDPEN catalyst. The enantioselectivities of the reductions reveal a complex pattern of electronic and steric effects which, when used in a matched combination, can lead to the formation of products of up to 68% ee (84:16 er) from this highly challenging class of substrate.

Selective construction of quaternary stereocentres in radical cyclisation cascades triggered by electron-transfer reduction of amide-type carbonyls

Radical cyclisation cascades triggered by electron-transfer to amide-type carbonyls using SmI₂–H₂O–LiBr, result in the selective construction of quaternary carbon stereocentres.

Rapid construction of the 6/6/5 tricyclic framework via a tandem radical cyclization reaction and its application to the synthesis of 5-epi-7-deoxy-isoabietenin A

A tandem radical cyclization towards the 6/6/5 tricyclic skeleton, which exists in numerous natural products, was developed in modest to good yields.

A Cascade Strategy Enables a Total Synthesis of (±)-Morphine

Morphine has been a target for synthetic chemists since Robinson proposed its correct structure in 1925, resulting in a large number of total syntheses of morphine alkaloids. Here we report a total synthesis of (±)‐morphine that employs two key strategic cyclizations: 1) a diastereoselective light‐mediated cyclization of an O‐arylated butyrolactone to form a tricyclic cis‐fused benzofuran and 2) a cascade ene–yne–ene ring closing metathesis to forge the tetracyclic morphine core. This approach enables a short and stereoselective synthesis of morphine in an overall yield of 6.6 %.

Dearomative dihydroxylation with arenophiles

Aromatic hydrocarbons are some of the most elementary feedstock chemicals, produced annually on a million metric ton scale, and are used in the production of polymers, paints, agrochemicals and pharmaceuticals. Dearomatization reactions convert simple, readily available arenes into more complex molecules with broader potential utility, however, despite substantial progress and achievements in this field, there are relatively few methods for the dearomatization of simple arenes that also selectively introduce functionality. Here we describe a new dearomatization process that involves visible-light activation of small heteroatom-containing organic molecules-arenophiles-that results in their para-cycloaddition with a variety of aromatic compounds. The approach uses N-N-arenophiles to enable dearomative dihydroxylation and diaminodihydroxylation of simple arenes. This strategy provides direct and selective access to highly functionalized cyclohexenes and cyclohexadienes and is orthogonal to existing chemical and biological dearomatization processes. Finally, we demonstrate the synthetic utility of this strategy with the concise synthesis of several biologically active compounds and natural products.

Synthesis of ent-ketorfanol via a C-H alkenylation/torquoselective 6π electrocyclization cascade

The asymmetric synthesis of ent-ketorfanol from simple and commercially available precursors is reported. A Rh(I) -catalyzed intramolecular CH alkenylation/torquoselective 6π electrocyclization cascade provides a fused bicyclic 1,2-dihydropyridine as a key intermediate. Computational studies were performed to understand the high torquoselectivity of the key 6π electrocyclization. The computational results demonstrate that a conformational effect is responsible for the observed selectivity. The ketone functionality and final ring are introduced in a single step by a redox-neutral acid-catalyzed rearrangement of a vicinal diol to give the requisite carbonyl, followed by intramolecular Friedel-Crafts alkylation.

The quest for a practical synthesis of morphine alkaloids and their derivatives by chemoenzymatic methods

We became interested in approaches to morphine in the early 1990s following our immersion into the new program on the enzymatic dihydroxylation of aromatics. Larry Kwart, a former classmate of one of us at Rice University, who worked with our group at Virginia Tech in the mid-1980s, introduced to us the use of blocked mutants of Pseudomonas putida (Pp39D) for the production of arene-cis-dihydrodiols. Larry had gained expertise in microbiology from a postdoctoral stay with David Gibson, who discovered this unique enzymatic transformation, and he helped us to establish a strong program in chemoenzymatic synthesis that continues to this day. Without his pioneering effort, none of our accomplishments in chemoenzymatic synthesis, including the various approaches to morphine, would have materialized. Here we trace the evolution of our approaches to morphine alkaloids and some commercial opiate-derived medicinal agents. The design features and chronology of our approaches are discussed in a way that allows the reader to appreciate a number of errors that were made in conception as well as in execution. Experience acquired from many failed or less-than-effective attempts has finally led to an "almost reasonable" total synthesis, the key concept being based on our very first but unsuccessful attempt more than two decades ago. The irony of this accomplishment has not been lost on us. Each section of this Account presents a summary of distinctly different approaches to morphine alkaloids. Each ends with a short and philosophical lesson that was (or should have been) learned in the process. We intend for this Account to offer more than the history of a search for the perfect design solution to a synthetic problem. In today's era of rapid and often careless publication of results, it should serve also as a reminder that the success and the integrity of synthetic ventures depends on perseverance, adjustment of strategy, improvements of previous attempts, and serious attention to the quality of experimental data. Although somewhat satisfied with our latest accomplishment in morphinan synthesis, we plan to improve our design in the hope that a six-step synthesis is no longer in the realm of fantasy. With more than 20 years of effort in this area, our continuing involvement may qualify as obsession.