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

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.

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.

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.

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Chemistry of Renieramycins. Part 19: Semi-Syntheses of 22-O-Amino Ester and Hydroquinone 5-O-Amino Ester Derivatives of Renieramycin M and Their Cytotoxicity against Non-Small-Cell Lung Cancer Cell Lines

Two new series of synthetic renieramycins including 22-O-amino ester and hydroquinone 5-O-amino ester derivatives of renieramycin M were semi-synthesized and evaluated for their cytotoxicity against the metastatic non-small-cell lung cancer H292 and H460 cell lines. Interestingly, the series of 22-O-amino ester derivatives displayed a potent cytotoxic activity greater than the hydroquinone derivatives. The most cytotoxic derivative of the series was the 22-O-(N-Boc-l-glycine) ester of renieramycin M (5a: IC₅₀ 3.56 nM), which showed 7-fold higher potency than renieramycin M (IC₅₀ 24.56 nM) and 61-fold more than jorunnamycin A (IC₅₀ 217.43 nM) against H292 cells. In addition, 5a exhibited a significantly higher cytotoxic activity than doxorubicin (ca. 100 times). The new semi-synthetic renieramycin derivatives will be further studied and developed as potential cytotoxic agents for non-small-cell lung cancer treatment.

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.

Chemistry of Renieramycins. 17. A New Generation of Renieramycins: Hydroquinone 5-O-Monoester Analogues of Renieramycin M as Potential Cytotoxic Agents against Non-Small-Cell Lung Cancer Cells

A series of hydroquinone 5-O-monoester analogues of renieramycin M were semisynthesized via bishydroquinonerenieramycin M (5) prepared from renieramycin M (1), a major cytotoxic bistetrahydroisoquinolinequinone alkaloid isolated from the Thai blue sponge Xestospongia sp. All 20 hydroquinone 5-O-monoester analogues possessed cytotoxicity with IC₅₀ values in nanomolar concentrations against the H292 and H460 human non-small-cell lung cancer (NSCLC) cell lines. The improved cytotoxicity toward the NSCLC cell lines was observed from the 5-O-monoester analogues such as 5-O-acetyl ester 6a and 5-O-propanoyl ester 7e, which exhibited 8- and 10-fold increased cytotoxicity toward the H292 NSCLC cell line (IC₅₀ 3.0 and 2.3 nM, respectively), relative to 1 (IC₅₀ 24 nM). Thus, the hydroquinone 5-O-monoester analogues are a new generation of the renieramycins to be further developed as potential marine-derived drug candidates for lung cancer treatment.

Asymmetric synthesis of (−)-renieramycin T

A nonclassical renieramycin with a similar A-ring to ecteinascidin, (−)-renieramycin T, is synthesized through a convergent approach featuring a regio- and stereoselective condensation of isoquinoline and trisubstituted phenylalaninol partners. This practical strategy via the bis-THIQ intermediate will open a rapid access to the hybrid-type renieramycins, as well as ecteinascidins and their analogues.

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.