Discovery of Cyanobacterial Natural Products Containing Fatty Acid Residues*.

In recent years, extensive sequencing and annotation of bacterial genomes has revealed an unexpectedly large number of secondary metabolite biosynthetic gene clusters whose products are yet to be discovered. For example, cyanobacterial genomes contain a variety of gene clusters that likely incorporate fatty acid derived moieties, but for most cases we lack the knowledge and tools to effectively predict or detect the encoded natural products. Here, we exploit the apparent absence of a functional ß-oxidation pathway in cyanobacteria to achieve efficient stable-isotope-labeling of their fatty acid derived lipidome. We show that supplementation of cyanobacterial cultures with deuterated fatty acids can be used to easily detect natural product signatures in individual strains. The utility of this strategy is demonstrated in two cultured cyanobacteria by uncovering analogues of the multidrug-resistance reverting hapalosin, and novel, cytotoxic, lactylate-nocuolin A hybrids-the nocuolactylates.

BrtB is an O-alkylating enzyme that generates fatty acid-bartoloside esters.

Esterification reactions are central to many aspects of industrial and biological chemistry. The formation of carboxyesters typically occurs through nucleophilic attack of an alcohol onto the carboxylate carbon. Under certain conditions employed in organic synthesis, the carboxylate nucleophile can be alkylated to generate esters from alkyl halides, but this reaction has only been observed transiently in enzymatic chemistry. Here, we report a carboxylate alkylating enzyme - BrtB - that catalyzes O-C bond formation between free fatty acids of varying chain length and the secondary alkyl halide moieties found in the bartolosides. Guided by this reactivity, we uncovered a variety of natural fatty acid-bartoloside esters, previously unrecognized products of the bartoloside biosynthetic gene cluster.

Design, synthesis and biological evaluation of novel C-29 carbamate celastrol derivatives as potent and selective cytotoxic compounds.

Celastrol and its derivatives have been reported for their potent anticancer activity. Among other celastrol analogues, novel carbamate derivatives were designed and synthesised, and their biological activity on the viability of human cancer cell lines was evaluated. Additionally, a preliminary structure-activity relationship study was conducted. Derivative 18 showed the highest activity on cancer cell viability, combined with the best selectivity between malignant cells and non-malignant fibroblasts. Preliminary mechanistic studies of its anti-tumour action indicated that compound 18 has an antiproliferative effect on SKOV-3 human ovarian cancer cells (IC50 = 0.54 µM). The results also suggested that its potent anticancer activity is mediated by apoptosis, and that this process was mainly the result of the activation of the extrinsic apoptotic pathway. Moreover, our results demonstrated the potential of derivative 18 as a new agent for combinatorial drug therapy for ovarian cancer.

Oleanane-, ursane-, and quinone methide friedelane-type triterpenoid derivatives: Recent advances in cancer treatment.

Natural pentacyclic triterpenoids (PTs) have been often reported to exhibit a wide range of biological activities. Among them, the anticancer and anti-inflammatory activities are the most studied. Over the last two decades, the number of publications reporting the anticancer effects of PTs has risen exponentially, reflecting the increasing interest in these natural products for the development of new antineoplastic drugs. Among of the most investigated PTs regarding their anticancer properties are oleanane-, ursane and friedelane-types, including oleanolic, glycyrrhetinic, ursolic and asiatic acids, and celastrol, among others. The extensive research in this field shows that the anticancer effects of PTs are mediated by several mechanisms, as they modulate a diverse range of molecular targets and signaling pathways, involved in cancer cell proliferation and survival. Considering the anticancer potential of this class of compounds, a number of semisynthetic derivatives has been synthetized aiming to improve their therapeutic activity and pharmacokinetic properties, and decrease their toxicity. Some of these new semisynthetic derivatives have shown improved anticancer activity in various cancer cell lines and animal models compared with the parent compound. Moreover, some of these compounds have been assessed in clinical trials, proving to be safe for human use. This review updates the most recent findings on the semisynthetic derivatives of oleanane-, ursane- and quinone methide friedelane-type PTs with anticancer activity. A brief introduction concerning the PTs and their anticancer activity is given, and the main semisynthetic modifications that have been performed between 2012 and early 2017 are reviewed and discussed.

Novel celastrol derivatives with improved selectivity and enhanced antitumour activity: Design, synthesis and biological evaluation.

Celastrol is one of the most active antitumour compounds among the natural triterpenoids. It has been reported to be highly active against a wide variety of tumours and to affect multiple cellular pathways. A series of new celastrol derivatives, including compounds bearing a urea group, have been synthesised and analysed for their biological activity against human cancer cell lines. Several compounds presented a stronger growth inhibition effect than celastrol on the cell lines studied. Among them, compound 24 was the most promising derivative, as it exhibited both a remarkable antiproliferative activity and an improved selectivity in tumour versus non-tumour cells. The anticancer molecular mechanism of compound 24 in the human ovary cancer cell line SKOV-3 was further studied and the results showed that compound 24 induced apoptosis through the activation of the extrinsic death receptor pathway. Interestingly, the results revealed that compound 24 might be able to decrease the levels of dysfunctional p53. The assays also suggested that compound 24 is an Hsp90 inhibitor, and that the Akt/mTOR pathway might be involved in the downstream regulation that leads to its antiproliferative activity. Moreover, a synergistic anticancer effect was evidenced when SKOV-3 cells were simultaneously treated with compound 24 and cisplatin. Taken together, these results suggest that compound 24 may be a promising lead for the development of new cancer therapies.

Bismuth compounds in medicinal chemistry.

In recent years, the chemical potential of bismuth and bismuth compounds has been actively exploited. Bismuth salts are known for their low toxicity, making them potential valuable reagents for large-scale synthesis, which becomes more obvious when dealing with products such as active pharmaceutical ingredients or synthetic intermediates. Conversely, bismuth compounds have been widely used in medicine. After extensive use in the treatments of syphilis and other bacterial infections before the advent of modern antibiotics, bismuth compounds remain important for the treatment of several gastrointestinal disorders and also exhibit antimicrobial properties and cytotoxic activity, among others. This review updates relevant advances in the past few years, concerning the application of bismuth reagents and catalysts in innovative synthetic processes for the preparation of compounds of medicinal interest, as well as the preparation, biological evaluation and potential medicinal uses of bismuth compounds.