Stereospecific formal total synthesis of ecteinascidin 743

Recent advances in the synthesis and activity of analogues of bistetrahydroisoquinoline alkaloids as antitumor agents

Ecteinascidin 743 (Et-743), also known by the trade name Yondelis®, is the pioneering marine natural product to be successfully developed as an antitumor drug. Moreover, it is the first tetrahydroisoquinoline natural product used clinically for antitumor therapy since Kluepfel, a Canadian scientist, discovered the tetrahydroisoquinoline alkaloid (THIQ) naphthyridinomycin in 1974. Currently, almost a hundred natural products of bistetrahydroisoquinoline type have been reported. Majority of these bistetrahydroisoquinoline alkaloids exhibit diverse pharmacological activities, with some family members portraying potent antitumor activities such as Ecteinascidins, Renieramycins, Saframycins, Jorumycins, among others. Due to the unique chemical structure and exceptional biological activity of these natural alkaloids, coupled with their scarcity in nature, research seeking to provide material basis for further bioactivity research through total synthesis and obtaining compound leads with medicinal value through structural modification, remains a hot topic in the field of antitumor drug R&D. Despite the numerous reviews on the total synthesis of bistetrahydroisoquinoline natural products, comprehensive reviews on their structural modification are apparently scarce. Moreover, structural modification of bioactive natural products to acquire lead compounds with improved pharmaceutical characteristics, is a crucial approach for innovative drug discovery. This paper presents an up-to-date review of both structural modification and activity of bistetrahydroisoquinoline natural products. It highlights how such alkaloids can be used as antitumor lead compounds through careful chemical modifications. This review offers valuable scientific references for pharmaceutical chemists engaged in developing novel antitumor agents based on such alkaloid modifications, as well as those with such a goal in future.

Recent Advances in the Total Synthesis of the Tetrahydroisoquinoline Alkaloids (2002-2020)

The tetrahydroisoquinoline (THIQ) natural products constitute one of the largest families of alkaloids and exhibit a wide range of structural diversity and biological activity. Ranging from simple THIQ natural products to complex trisTHIQ alkaloids such as the ecteinascidins, the chemical syntheses of these alkaloids and their analogs have been thoroughly investigated due to their intricate structural features and functionalities, as well as their high therapeutic potential. This review describes the general structure and biosynthesis of each family of THIQ alkaloids as well as recent advancements of the total synthesis of these natural products from 2002 to 2020. Recent chemical syntheses that have emerged harnessing novel, creative synthetic design, and modern chemical methodology will be highlighted. This review will hopefully serve as a guide for the unique strategies and tools used in the total synthesis of THIQ alkaloids, as well as address the longstanding challenges in their chemical and biosynthesis.

Formal Synthesis of Ecteinascidin 743 from N-Cbz-l-tyrosine

A formal total synthesis of ecteinascidin 743 and lurbinectedin is achieved. Key features involve a Pictet-Spengler cyclization coupling of the tetrahydroisoquinoline and phenylalaninol moieties prepared by a common route with high yield and selectivity, a Parikh-Doering oxidation with good chemoselectivity and functionality tolerance, and a light-mediated A-ring elaboration of pentacyclic methoxyquinone substrates. By the approach, the known advanced intermediate (4-step conversion to Et-743) can be obtained conveniently in 21 total steps from N-Cbz-l-tyrosine.

Formal Synthesis of Ecteinascidin 743 via an Intramolecular Cascade Heck Reaction to Construct the Diazabicyclo[3.3.1]nonane Framework

A synthetic route to ecteinascidin 743 has been established via an intramolecular cascade Heck reaction to construct the diazabicyclo[3.3.1]nonane skeleton while controlling the two contiguous stereogenic centers. The strategically formed five-membered ring was oxidatively cleaved to generate a dialdehyde intermediate, from which the B ring of ecteinascidin 743 was constructed.

A brief overview of classical natural product drug synthesis and bioactivity

This manuscript briefly overviewed the total synthesis and structure–activity relationship studies of eight classical natural products, which emphasizes the important role of total synthesis in natural product-based drug development.

Total synthesis of the isoquinolinium metabolite ETM-204 of Trabectidin

Ecteinascidin-743 (Trabectidin, Trabectedin®, Yondelis®) is a synthetically obtained pharmaceutical drug originally isolated from a marine tunicate. Trabectedin is used for the chemotherapy of soft-tissue sarcoma and ovarian cancer. The isoquinolinium metabolite ETM-204 has been found in biotransformation and degradation studies of Trabectedin. We report the first total synthesis of ETM-204 and its full spectroscopic characterization confirming the postulated structure. Central elements of the 12-step synthesis starting from 2-methyl-6-nitrophenol are a Cu-mediated conversion of an iodoarene to a phenol, a Skattebøl-formylation, and a modified Pomeranz–Fritsch cyclization to assemble the isoquinoline ring. The pH-dependence of its visual absorbance could be clarified.

Graphic abstract

Catalytic Asymmetric Tandem Reaction of o-Alkynylbenzaldehydes, Amines, and Diazo Compounds

An efficient asymmetric tandem reaction of o-alkynylbenzaldehydes, amines, and diazo compounds catalyzed by chiral silver imidodiphosphate has been established. Chiral 1,2-dihydroisoquinoline analogues have a tertiary stereocenter at the C1 position, and substituents at the C3 position are available with up to 97% yields and 98% ee. These products can be elaborated into the corresponding β-aminophosphonates or PARP1-inhibitor analogues.

Synthesis of substituted 1,2-dihydroisoquinolines via Ni(ii) and Cu(i)/Ag(i) catalyzed double nucleophilic addition of arylamines to ortho-alkynyl donor-acceptor cyclopropanes (o-ADACs)

A concise approach for the synthesis of substituted 1,2-dihydroisoquinolines via double nucleophilic addition of primary arylamines to ortho-alkynyl donor-acceptor cyclopropanes (o-ADACs) in the presence of a catalytic Ni(ClO₄)₂·6H₂O and CuI/AgOTf system has been developed. Further applying this protocol, some of the derived malonates were converted into the corresponding monoesters under Krapcho decarboxylation reaction conditions. Thereafter, these esters were transformed into the respective acids and alcohols. In addition, multifunctionalized 4-(2,2,2-trifluoroacetyl) 1,2-dihydroisoquinolines were also obtained with excess TFAA.

Chemical Research on Antitumor Isoquinoline Marine Natural Products and Related Compounds

The biologically active, naturally occurring 1,2,3,4-tetrahydroisoquinoline-quinone (THIQ) family members isolated from Actinomycetes and marine organisms have been studied thoroughly over the past five decades. Among them, marine natural products along with their reduced compounds, such as renieramycins and ecteinascidins, have attracted interest due to their fantastic structures and meager availability in nature as well as their potent antitumor profiles. As part of our search for new anticancer metabolites through the isolation and characterization of anticancer THIQ compounds from Thai marine animals, we have developed a fascinating THIQ natural product chemistry and medicinal chemistry based on knowledge of the chemistry of saframycin antibiotics as well as their isolation, characterization, transformation, partial synthesis, and total synthesis. This review mainly presents our contributions during 1999-2019 to the field of research on biologically active renieramycin along with ecteinascidin marine natural products.

Construction of sulfur-containing moieties in the total synthesis of natural products

This review surveys the total syntheses of sulfur-containing natural products where sulfur atoms are introduced with different sulfurization agents to construct related sulfur-containing moieties.

Convergent Formal Synthesis of Ecteinascidin 743

A concise formal synthesis of ecteinascidin 743 is described. Key features involve the coupling of the multisubstituted tetrahydroisoquinoline and phenylalaninol moieties via a regio- and stereoselective Pictet-Spengler cyclization as well as the subsequent chemoselective MOM protection of the phenol group, which opens a rapid access to the desirable pentacycle. The synthesis successfully delivered the advanced intermediate with the characteristic macrolactone from sesamol in 23 steps.

Development of the Vinylogous Pictet-Spengler Cyclization and Total Synthesis of (±)-Lundurine A

A novel vinylogous Pictet-Spengler cyclization has been developed for the generation of indole-annulated medium-sized rings. The method enables the synthesis of tetrahydroazocinoindoles with a fully substituted carbon center, a prevalent structural motif in many biologically active alkaloids. The strategy has been applied to the total synthesis of (±)-lundurine A.

Asymmetric Synthesis of Isoquinoline Alkaloids: 2004-2015

In the past decade, the asymmetric synthesis of chiral nonracemic isoquinoline alkaloids, a family of natural products showing a wide range of structural diversity and biological and pharmaceutical activity, has been based either on continuation or improvement of known traditional methods or on new, recently developed, strategies. Both diastereoselective and enantioselective catalytic methods have been applied. This review describes the stereochemically modified traditional syntheses (the Pictet-Spengler, the Bischler-Napieralski, and the Pomeranz-Fritsch-Bobbitt) along with strategies based on closing of the nitrogen-containing ring B of the isoquinoline core by the formation of bonds between C₁-N₂, N₂-C₃, C₁-N₂/N₂-C₃, and C₁-N₂/C₄-C_4a atoms. Methods involving introduction of substituents at the C1 carbon of isoquinoline core along with syntheses applying various biocatalytic techniques have also been reviewed.

Marine Natural Products as Models to Circumvent Multidrug Resistance

Multidrug resistance (MDR) to anticancer drugs is a serious health problem that in many cases leads to cancer treatment failure. The ATP binding cassette (ABC) transporter P-glycoprotein (P-gp), which leads to premature efflux of drugs from cancer cells, is often responsible for MDR. On the other hand, a strategy to search for modulators from natural products to overcome MDR had been in place during the last decades. However, Nature limits the amount of some natural products, which has led to the development of synthetic strategies to increase their availability. This review summarizes the research findings on marine natural products and derivatives, mainly alkaloids, polyoxygenated sterols, polyketides, terpenoids, diketopiperazines, and peptides, with P-gp inhibitory activity highlighting the established structure-activity relationships. The synthetic pathways for the total synthesis of the most promising members and analogs are also presented. It is expected that the data gathered during the last decades concerning their synthesis and MDR-inhibiting activities will help medicinal chemists develop potential drug candidates using marine natural products as models which can deliver new ABC transporter inhibitor scaffolds.

An expedient approach to 1,2-dihydroisoquinoline derivatives via cobalt catalysed 6-endo dig cyclization followed by Mannich condensation of o-alkynylarylaldimines

A highly effective 6-endo dig cyclisation of o-alkynylaldimines to 1,2-dihydroisoquinolines has been described via direct and nitro Mannich condensation using inexpensive and readily available cobalt chloride as catalyst. This strategy provides an effective procedure for the synthesis of substituted 1,2-dihydroisoquinolines derivatives in moderate to high yields. An addition of pronucleophiles, such as nitromethane, acetone and α-hydroxyacetone, to o-alkynylarylaldimines has been achieved via isoquinolinium intermediate.

Ecteinascidins. A review of the chemistry, biology and clinical utility of potent tetrahydroisoquinoline antitumor antibiotics

The ecteinascidin family comprises a number of biologically active compounds, containing two to three tetrahydroisoquinoline subunits. Although isolated from marine tunicates, these compounds share a common pentacyclic core with several antimicrobial compounds found in terrestrial bacteria. Among the tetrahydroisoquinoline natural products, ecteinascidin 743 (Et-743) stands out as the most potent antitumor antibiotics that it is recently approved for treatment of a number of soft tissue sarcomas. In this article, we will review the backgrounds, the mechanism of action, the biosynthesis, and the synthetic studies of Et-743. Also, the development of Et-743 as an antitumor drug is discussed.

Stereoselective synthesis of functionalized 1,2,3,4-tetrahydroisoquinolines (THIQs) via highly diastereoselective Ugi three-component reactions (U3CRs) with chiral 3,4-dihydroisoquinolines (DHIQs)

A highly diastereoselective Ugi three-component reaction (U3CR) of chiral 3,4-dihydroisoquinolines has been developed to synthesize enantiopure 1,2,3,4-tetrahydroisoquinolines (THIQs).

Alkaloids from marine invertebrates as important leads for anticancer drugs discovery and development

The present review describes research on novel natural antitumor alkaloids isolated from marine invertebrates. The structure, origin, and confirmed cytotoxic activity of more than 130 novel alkaloids belonging to several structural families (indoles, pyrroles, pyrazines, quinolines, and pyridoacridines), together with some of their synthetic analogs, are illustrated. Recent discoveries concerning the current state of the potential and/or development of some of them as new drugs, as well as the current knowledge regarding their modes of action, are also summarized. A special emphasis is given to the role of marine invertebrate alkaloids as an important source of leads for anticancer drug discovery.

Cancer wars: natural products strike back

Natural products have historically been a mainstay source of anticancer drugs, but in the 90's they fell out of favor in pharmaceutical companies with the emergence of targeted therapies, which rely on antibodies or small synthetic molecules identified by high throughput screening. Although targeted therapies greatly improved the treatment of a few cancers, the benefit has remained disappointing for many solid tumors, which revitalized the interest in natural products. With the approval of rapamycin in 2007, 12 novel natural product derivatives have been brought to market. The present review describes the discovery and development of these new anticancer drugs and highlights the peculiarities of natural product and new trends in this exciting field of drug discovery.

The endeavor of total synthesis and its impact on chemistry, biology and medicine

The synthesis of urea in 1828 set in motion the discipline of organic synthesis in general and of total synthesis in particular, the art and science of synthesizing natural products, the molecules of living nature. Early endeavors in total synthesis had as their main objective the proof of structure of the target molecule. Later on, the primary goal became the demonstration of the power of synthesis to construct complex molecules through appropriately devised strategies, making the endeavor an achievement whose value was measured by its elegance and efficiency. While these objectives continue to be important, contemporary endeavors in total synthesis are increasingly focused on practical aspects, including method development, efficiency, and biological and medical relevance. In this article, the emergence and evolution of total synthesis to its present state is traced, selected total syntheses from the author's laboratories are highlighted, and projections for the future of the field are discussed.

Diastereoselective Pictet–Spengler Reactions of a Tethered 2-Aminoimidazole

The diastereoselective Pictet–Spengler reaction of aminopropyl-2-aminoimidazole with enantiopure aldehydes has been investigated. With amino acid-derived aldehydes, anti stereochemistry is favoured, with a diastereoselectivity up to 92 % achievable. The absolute stereochemistry of the products was determined through synthesis of a rigid derivative and from NMR data in combination with molecular modelling. The diastereoselectivity was shown to be dependent on the steric bulk of the amino acid side chain and independent of the nitrogen protecting group. Lewis acids catalysed the reaction but did not affect the diastereoselectivity.

Enantioselective synthesis of 2-substitued-tetrahydroisoquinolin-1-yl glycine derivatives via oxidative cross-dehydrogenative coupling of tertiary amines and chiral nickel(II) glycinate

The asymmetric synthesis of 2-substituted-tetrahydroisoquinolin-1-yl glycines was achieved by an oxidative cross-dehydrogenative coupling (CDC) reaction. This method for activation of the α-C-H bonds of amines with chiral nickel(II) glycinate using o-chloranil as the sole oxidant afforded highly diastereoselective coupling adducts. The decomposition of coupling adducts readily afforded 2-substituted-tetrahydroisoquinolin-1-yl glycine derivatives.

Total synthesis of ecteinascidin 743

A straightforward synthesis of ecteinascidin 743 was accomplished from readily available l-glutamic acid as a single chiral source. Our novel synthesis features a concise and convergent approach for construction of the B-ring, consisting of a sequence involving a stereoselective Heck reaction between a diazonium salt and an enamide, oxidative cleavage of the resulting alkene, and intramolecular ortho substitution of the phenol by an aldehyde.

Natural products synthesis: enabling tools to penetrate Nature's secrets of biogenesis and biomechanism

Selected examples from our laboratory of how synthetic technology platforms developed for the total synthesis of several disparate families of natural products was harnessed to penetrate biomechanistic and/or biosynthetic queries is discussed. Unexpected discoveries of biomechanistic reactivity and/or penetrating the biogenesis of naturally occurring substances were made possible through access to substances available only through chemical synthesis. Hypothesis-driven total synthesis programs are emerging as very useful conceptual templates for penetrating and exploiting the inherent reactivity of biologically active natural substances. In many instances, new enabling synthetic technologies were required to be developed. The examples demonstrate the often untapped richness of complex molecule synthesis to provide powerful tools to understand, manipulate and exploit Nature's vast and creative palette of secondary metabolites.

Synthetic study toward ecteinascidin 743: concise construction of the diazabicyclo[3.3.1]nonane skeleton and assembly of the pentacyclic core

Synthesis of the pentacyclic core of ecteinascidin 743 is described. This synthesis features concise construction of the diazabicyclo[3.3.1]nonane skeleton using gold(I)-catalyzed one-pot keto amide formation, acid-promoted enamide formation, and oxidative Friedel-Crafts cyclization as the key steps.

Recent synthetic approaches to 6,15-iminoisoquino[3,2-b]3-benzazocine compounds

Saframycins, safracins, renieramycins, cribrostatins, and esteinascidins are 6,15-iminoisoquino[3,2-b]3-benzazocine compounds that constitute the largest subgroup among the antitumor antibiotics belonging to the tetrahydroisoquinoline family. Their structural complexity has led to widespread synthetic attention to obtain them in both racemic and enantiopure forms. Publication in 1996 of the first total synthesis of ecteinascidin 743 by Corey's group was an important milestone, but the development of preparative protocols for these structures has continued, offering new possibilities to exploit the biological activity of the above-mentioned natural products and their analogues. This minireview is intended to update this progress following a methodological rather than a chronological organization. Besides of a brief description of the different strategies evolved from retrosynthetic analyses, which have been organized according to the order of bonding events that will link the precursors, semisynthetic approaches and a brief account of the total syntheses of ecteinascidin 743, have been analyzed.