Total Syntheses of Prothracarcin and Tomaymycin by Use of Palladium Catalyzed Carbonylation

Concise access to C2-ethylidene pyrrolo[1,4]benzodiazepine natural products

A 3-step route toward pyrrolo[1,4]benzodiazepine (PBD) antitumor antibiotic class members oxo-prothracarcin and boseongazepine B has been developed. This methodology also enables preparing oxo-tomaymycin in only 4 linear steps representing the shortest total synthesis known to date. The synthesis features an olefination with sterically demanding Julia-Kocienski reagents as the key step.

Stereoselective Olefination with Sterically Demanding Julia-Kocienski Reagents: Total Synthesis of Oxo-prothracarcin, Oxo-tomaymycin, and Boseongazepine B

Total syntheses of three pyrrolo[1,4]benzodiazepine anticancer antibiotic family members oxo-prothracarcin, oxo-tomaymycin, and boseongazepine B are described. The total syntheses feature late-stage stereoselective olefination employing modified Julia-Kocienski reagents that can be conveniently prepared in only two steps and allows for a significant reduction in the number of linear steps. Detailed density functional theory (DFT) studies explain the stereochemical outcome of the key step.

Isolation, chemistry, and biology of pyrrolo[1,4]benzodiazepine natural products

The isolation of the antitumor antibiotic anthramycin in the 1960s prompted extensive research into pyrrolo[1,4]benzodiazepines (PBD) as potential therapeutics for the treatment of cancers. Since then, nearly 60 PBD natural products have been isolated and evaluated with regard to their biological activity. Synthetic studies and total syntheses have enabled access to PBD analogues, culminating in the development of highly potent anticancer agents. This review provides a summary of the occurrence and biological activity of PBD natural products and covers the strategies employed for their total syntheses.

Palladium-Catalyzed Benzodiazepines Synthesis

This review is focused on palladium-catalyzed reactions as efficient strategies aimed at the synthesis of different classes of benzodiazepines. Several reaction typologies are reported including hydroamination, amination, C–H arylation, N-arylation, and the Buchwald–Hartwig reaction, depending on the different substrates identified as halogenated starting materials (activated substrates) or unactivated unsaturated systems, which then exploit Pd(0)- or Pd(II)-catalytic species. In particular, the use of the domino reactions, as intra- or intermolecular processes, are reported as an efficient and eco-compatible tool to obtain differently functionalized benzodiazepines. Different domino reaction typologies are the carboamination, aminoarylation, aminoacethoxylation, aminohalogenation, and aminoazidation.

Actinomycetes from the Red Sea Sponge Coscinoderma mathewsi: Isolation, Diversity, and Potential for Bioactive Compounds Discovery

The diversity of actinomycetes associated with the marine sponge Coscinoderma mathewsi collected from Hurghada (Egypt) was studied. Twenty-three actinomycetes were separated and identified based on the 16S rDNA gene sequence analysis. Out of them, three isolates were classified as novel species of the genera Micromonospora, Nocardia, and Gordonia. Genome sequencing of actinomycete strains has revealed many silent biosynthetic gene clusters and has shown their exceptional capacity for the production of secondary metabolites, not observed under classical cultivation conditions. Therefore, the effect of mycolic-acid-containing bacteria or mycolic acid on the biosynthesis of cryptic natural products was investigated. Sponge-derived actinomycete Micromonospora sp. UA17 was co-cultured using liquid fermentation with two mycolic acid-containing actinomycetes (Gordonia sp. UA19 and Nocardia sp. UA 23), or supplemented with pure mycolic acid. LC-HRESIMS data were analyzed to compare natural production across all crude extracts. Micromonospora sp. UA17 was rich with isotetracenone, indolocarbazole, and anthracycline analogs. Some co-culture extracts showed metabolites such as a chlorocardicin, neocopiamycin A, and chicamycin B that were not found in the respective monocultures, suggesting a mycolic acid effect on the induction of cryptic natural product biosynthetic pathways. The antibacterial, antifungal, and antiparasitic activities for the different cultures extracts were also tested.

The first crystal structure of the pyrrolo-[1,2-c]oxazole ring system

The title compound is the first crystal structure of the pyrrolo­[1,2-c]oxazole ring system. In the crystal, weak C—H⋯O and C—H⋯F hydrogen bonds link the mol­ecules into [001] chains and π–π stacking inter­actions consolidate the structure.

The title compound, C₇H₄F₃NO₂, 3-tri­fluoro­methyl-1H-pyrrolo­[1,2-c]oxazol-1-one, is the first crystal structure of the pyrrolo­[1,2-c]oxazole ring system: the fused ring system is almost planar (r.m.s. deviation = 0.006 Å). In the crystal, weak C—H⋯O and C—H⋯F hydrogen bonds link the mol­ecules into [001] chains and π–π stacking inter­actions consolidate the structure.

Pyrrolobenzodiazepine Dimers as Antibody–Drug Conjugate (ADC) Payloads

The pyrrolobenzodiazepine (PBD) ring system was first discovered in the 1960s and is found in several naturally occurring potent anti-tumour antibiotics. The mode of action of PBDs involves sequence-selective [purine–guanine–purine (PuGPu)] alkylation in the minor groove of DNA through covalent binding from guanine N2 to the PBD C11-position. Dimerization of the PBD ring system gives molecules that can cross-link DNA, which leads to a substantial increase in potency and DNA binding affinity and an extension of sequence-selectivity compared with monomers. PBD dimers feature as the cytotoxic component of numerous ADCs being evaluated in clinical trials. PBD-ADC clinical candidates loncastuximab tesirine, camidanlumab tesirine and rovalpituzumab tesirine employ a PBD N10 linkage while vadastuximab talirine uses a C2-linkage. The PBD dimer scaffold is versatile and offers many opportunities to diversify the ADC platform, with extensive research being performed worldwide to develop the next generation of PBD payload–linker molecules. The search for new PBD payload–linker molecules has mainly focused on changes in payload structure (e.g. PBD C2 modification and macrocyclisation), alternative conjugation strategies (e.g. haloacetamides, ‘click’ approaches and pyridyl disulphides), non-peptide triggers in the linker (e.g. disulphides) and non-cleavable derivatives (i.e. payload release through antibody degradation).

Natural product syntheses via carbonylative cyclizations

This review summarizes the application of various transition metal-catalyzed/mediated carbonylative cyclization reactions in natural product total synthesis.

Enantioselective total synthesis of pyrrolo-[2,1-c][1,4]-benzodiazepine monomers (S)-(−)-barmumycin and (S)-(+)-boseongazepine B

An efficient enantioselective total synthesis of pyrrolo-[2,1-c][1,4]benzodiazepine (PBD) monomers (S)-(−)-barmumycin and (S)-(+)-boseongazepine B was achieved through a stereocontrolled strategy, which relies on a proline catalysed asymmetric α-amination and ester α-ethylenation.

Limazepines A-F, pyrrolo[1,4]benzodiazepine Antibiotics from an Indonesian Micrococcus sp

In our screening of Indonesian microorganisms for novel bioactive natural products we have isolated seven new compounds, designated as limazepines A, B1 and B2 (isolated as an isomeric mixture), C, D, E, and F, from the culture broth of Micrococcus sp. strain ICBB 8177. In addition, the known natural products prothracarcin and 7-O-succinylmacrolactin A, as well as two previously reported synthetic compounds, 2-amino-3-hydroxy-4-methoxybenzoic acid methyl ester and 4-ethylpyrrole-2-carboxaldehyde, were obtained from the extract. Chemical structures were determined by spectroscopic methods and by comparison with the NMR data of structurally related compounds. The limazepines belong to the growing group of the pyrrolo[1,4]benzodiazepine antitumor antibiotics isolated from various soil bacteria. Limazepines B1/B2 mixture, C, and E were active against the Gram-positive bacterium Staphylococcus aureus and the Gram-negative bacterium Escherichia coli. Limazepine D was also active against S. aureus, but was not active against E. coli. Interestingly, only the limazepines B1/B2 mixture and D were active against Pseudomonas aeruginosa.

A novel approach to the synthesis of cytotoxic C2-C3 unsaturated pyrrolo[2,1-c]benzodiazepines (PBDs) with conjugated acrylyl C2-substituents

A concise synthesis of three novel C2-C3 unsaturated pyrrolo[2,1-c][1,4]benzodiazepine analogues (18-20) containing conjugated acrylyl C2-substituents is reported that utilises Heck coupling to install the C2-acrylyl side chains. These analogues possess significant cytotoxicity according to the NCI 60-cell line screen with 18 surpassing anthramycin (1) in potency.

Application of the Stille coupling reaction to the synthesis of C2-substituted endo–exo unsaturated pyrrolo[2,1-c][1,4]benzodiazepines (PBDs)

The Stille coupling reaction has been used to introduce novel vinyl, alkynyl and heterocyclic substituents to the C2-position of pyrrolo[2,1-c][1,4]benzodiazepine dilactams. Sodium borohydride reduction followed by N10-SEM deprotection has provided five analogues (6b, 8a-d) that contain C2-endo/exo-unsaturation and novel C2-substituents. These analogues have significant multilog cytotoxicity profiles in the NCI 60-Cell Line screen, and provide new SAR data for the PBD family.

Solid-phase synthesis of pyrrolo[2,1-c][1,4]benzodiazepine-5,11-diones

The solid-phase synthesis of biologically important pyrrolo[2,1-c][1,4]benzodiazepine-5,11-diones using Wang resin through amide formation and reductive cyclization procedures is described. Further, N10-substituents have been introduced in the final products and these have been cleaved from the solid support in good yields.