Marine indole alkaloid diversity and bioactivity. What do we know and what are we missing?

Synthesis and Cheminformatics-Directed Antibacterial Evaluation of Echinosulfonic Acid-Inspired Bis-Indole Alkaloids

Synthetic efforts toward complex natural product (NP) scaffolds are useful ones, particularly those aimed at expanding their bioactive chemical space. Here, we utilised an orthogonal cheminformatics-based approach to predict the potential biological activities for a series of synthetic bis-indole alkaloids inspired by elusive sponge-derived NPs, echinosulfone A (1) and echinosulfonic acids A-D (2-5). Our work includes the first synthesis of desulfato-echinosulfonic acid C, an α-hydroxy bis(3'-indolyl) alkaloid (17), and its full NMR characterisation. This synthesis provides corroborating evidence for the structure revision of echinosulfonic acids A-C. Additionally, we demonstrate a robust synthetic strategy toward a diverse range of α-methine bis(3'-indolyl) acids and acetates (11-16) without the need for silica-based purification in either one or two steps. By integrating our synthetic library of bis-indoles with bioactivity data for 2048 marine indole alkaloids (reported up to the end of 2021), we analyzed their overlap with marine natural product chemical diversity. Notably, the C-6 dibrominated α-hydroxy bis(3'-indolyl) and α-methine bis(3'-indolyl) analogues (11, 14, and 17) were found to contain significant overlap with antibacterial C-6 dibrominated marine bis-indoles, guiding our biological evaluation. Validating the results of our cheminformatics analyses, the dibrominated α-methine bis(3'-indolyl) alkaloids (11, 12, 14, and 15) were found to exhibit antibacterial activities against methicillin-sensitive and -resistant Staphylococcus aureus. Further, while investigating other synthetic approaches toward bis-indole alkaloids, 16 incorrectly assigned synthetic α-hydroxy bis(3'-indolyl) alkaloids were identified. After careful analysis of their reported NMR data, and comparison with those obtained for the synthetic bis-indoles reported herein, all of the structures have been revised to α-methine bis(3'-indolyl) alkaloids.

Marine Streptomyces-Derived Novel Alkaloids Discovered in the Past Decade

Natural alkaloids originating from actinomycetes and synthetic derivatives have always been among the important suppliers of small-molecule drugs. Among their biological sources, Streptomyces is the highest and most extensively researched genus. Marine-derived Streptomyces strains harbor unconventional metabolic pathways and have been demonstrated to be efficient producers of biologically active alkaloids; more than 60% of these compounds exhibit valuable activity such as antibacterial, antitumor, anti-inflammatory activities. This review comprehensively summarizes novel alkaloids produced by marine Streptomyces discovered in the past decade, focusing on their structural features, biological activity, and pharmacological mechanisms. Future perspectives on the discovery and development of novel alkaloids from marine Streptomyces are also provided.

Catecholamine Derivatives: Natural Occurrence, Structural Diversity, and Biological Activity

Catecholamines (CAs) are aromatic amines containing a 3,4-dihydroxyphenyl nucleus and an amine side chain. Representative CAs included the endogenous neurotransmitters epinephrine, norepinephrine, and dopamine. CAs and their derivatives are good resources for the development of sympathomimetic or central nervous system drugs, while they also provide ligands important for G-protein coupled receptor (GPCR) research. CAs are of broad interest in the fields of chemical, biological, medical, and material sciences due to their high adhesive capacities, chemical reactivities, metal-chelating abilities, redox activities, excellent biocompatibilities, and ease of degradability. Herein, we summarize CAs derivatives isolated and identified from microorganisms, plants, insects, and marine invertebrates in recent decades, alongside their wide range of reported biological activities. The aim of this review is to provide an overview of the structural and biological diversities of CAs, the regularity of their natural occurrences, and insights toward future research and development pertinent to this important class of naturally occurring compounds.

Deciphering the quest in the divergent total synthesis of natural products

The divergent synthesis of natural products is rapidly developing towards achieving the goal of efficiency and economy in total synthesis. However, presently, the sustainable development of the synthesis of natural products does not permit the linear synthesis of a single target. In this case, divergent total synthesis is based on the identification of an advanced intermediate with structural features that can be mapped in more than two molecules. However, the identification of this intermediate and its scalable synthesis in enantiopure form are challenging. Herein, we present the details of the ingenious efforts by researchers in the last six years toward the divergent synthesis of two to as many as eight natural products initially via a single route, and then diverging from a common intermediate and further branching out toward several natural products. The planning and strategies adopted can serve as guidelines for the future development of efficient divergent routes aimed at achieving higher efficiency toward multiple targets, causing divergent synthesis to become an accepted common practice.

A Terphenyllin Derivative CHNQD-00824 from the Marine Compound Library Induced DNA Damage as a Potential Anticancer Agent

With the emergence of drug resistance and the consequential high morbidity and mortality rates, there is an urgent need to screen and identify new agents for the effective treatment of cancer. Terphenyls-a group of aromatic hydrocarbons consisting of a linear 1,4-diaryl-substituted benzene core-has exhibited a wide range of biological activities. In this study, we discovered a terphenyllin derivative-CHNQD-00824-derived from the marine compound library as a potential anticancer agent. The cytotoxic activities of the CHNQD-00824 compound were evaluated against 13 different cell lines with IC50 values from 0.16 to 7.64 μM. Further study showed that CHNQD-00824 inhibited the proliferation and migration of cancer cells, possibly by inducing DNA damage. Acridine orange staining demonstrated that CHNQD-00824 promoted apoptosis in zebrafish embryos. Notably, the anti-cancer effectiveness was verified in a doxycin hydrochloride (DOX)-induced liver-specific enlargement model in zebrafish. With Solafinib as a positive control, CHNQD-00824 markedly suppressed tumor growth at concentrations of 2.5 and 5 μM, further highlighting its potential as an effective anticancer agent.

Cyclotheonellazoles D-I, Potent Elastase Inhibitory Thiazole-Containing Cyclic Peptides from Theonella sp. (2131)

Six new thiazole-containing cyclic peptides, the cyclotheonellazoles D-I (1-6), were isolated from the Australian marine sponge Theonella sp. (2131) with their structures assigned by comprehensive 1D and 2D NMR spectroscopic and MS spectrometric analyses, Marfey's derivatization studies, and comparison with time-dependent density functional theory (TDDFT) calculated ECD data. The Type 2 azole-homologated peptides herein comprise up to five nonproteinogenic amino acids, including the protease transition state mimic α-keto-β-amino acid residue 3-amino-4-methyl-2-oxohexanoic acid (Amoha), while 1-3 also contain a terminal hydantoin residue not previously found in cyclotheonellazoles. The keramamides A (7) and L (8) were reisolated affording expanded exploration of their biological activities. The peptides were examined for protease inhibitory activities against two mammalian serine proteases (elastase and chymotrypsin) and SARS-CoV-2 3-chymotrypsin-like protease (3CLpro), a validated antiviral therapeutic target for COVID-19. Peptides 1-6 and keramamide A (7) displayed potent nanomolar inhibition of elastase (IC50 16.0 to 61.8 nM), while 7 also contained modest inhibition of chymotrypsin and SARS-CoV-2 3CLpro (IC50 0.73 and 1.1 μM, respectively). The cyclotheonellazoles D-E (1-3) do not affect the viability of human breast, ovarian, and colon cancer cells (>100 μM), with the cytotoxicity previously reported for keramamide L (8) not replicated (inactive >20 μM).