Stereospecific total synthesis of somocystinamide A

Enantioselective Synthesis of Chiral 2-Nitroallylic Amines via Cooperative Cation-Binding Catalysis

Chiral allylic amines are valuable building blocks for biologically important compounds and natural products. In this study, we present the use of cooperative cation-binding catalysis as an efficient method for synthesizing chiral allylic amines. By utilizing a chiral oligoEG and potassium fluoride as a cation-binding catalyst and base, respectively, a wide range of biologically relevant chiral 2-nitroallylic amines are obtained with excellent enantioselectivities (up to >99 % ee) through the organocatalytic asymmetric aza-Henry-like reaction of β-monosubstituted and β,β-disubstituted nitroalkenes with α-amidosulfones as imine precursors. Extensive experimental studies are presented to illustrate plausible mechanisms. Preliminary use of a chiral 2-nitroallylic amine as a Michael acceptor demonstrated its potential application for diversity-oriented synthesis of bioactive compounds.

Total Synthesis of Laucysteinamide A, a Monomeric Congener of Somocystinamide A

Laucysteinamide A (4) is a marine natural product isolated from the cyanobacterium Caldora penicillata and contains structural motifs found in promising cancer drug leads. The first total synthesis of 4 and its analogues was achieved, which also enabled a concise formal synthesis of somocystinamide A (3), a dimeric congener of 4 that previously showed extremely potent antiproliferative activities. This work provides further insights on structure-activity relationships in this class of natural products.

PQQ-dependent Dehydrogenase Enables One-pot Bi-enzymatic Enantio-convergent Biocatalytic Amination of Racemic sec-Allylic Alcohols

The asymmetric amination of secondary racemic allylic alcohols bears several challenges like the reactivity of the bi-functional substrate/product as well as of the α,β-unsaturated ketone intermediate in an oxidation-reductive amination sequence. Heading for a biocatalytic amination cascade with a minimal number of enzymes, an oxidation step was implemented relying on a single PQQ-dependent dehydrogenase with low enantioselectivity. This enzyme allowed the oxidation of both enantiomers at the expense of iron(III) as oxidant. The stereoselective amination of the α,β-unsaturated ketone intermediate was achieved with transaminases using 1-phenylethylamine as formal reducing agent as well as nitrogen source. Choosing an appropriate transaminase, either the (R)- or (S)-enantiomer was obtained in optically pure form (>98 % ee). The enantio-convergent amination of the racemic allylic alcohols to one single allylic amine enantiomer was achieved in one pot in a sequential cascade.

Bacteria and bacterial anticancer agents as a promising alternative for cancer therapeutics

Cancer is the leading cause of deaths worldwide, though significant advances have occurred in its diagnosis and treatment. The development of resistance against chemotherapeutic agents, their side effects, and non-specific toxicity urge to screen for the novel anticancer agent. Hence, the development of novel anticancer agents with a new mechanism of action has become a major scientific challenge. Bacteria and bacterially produced bioactive compounds have recently emerged as a promising alternative for cancer therapeutics. Bacterial anticancer agents such as antibiotics, bacteriocins, non-ribosomal peptides, polyketides, toxins, etc. These are adopted different mechanisms of actions such as apoptosis, necrosis, reduced angiogenesis, inhibition of translation and splicing, and obstructing essential signaling pathways to kill cancer cells. Also, live tumor-targeting bacteria provided a unique therapeutic alternative for cancer treatment. This review summarizes the anticancer properties and mechanism of actions of the anticancer agents of bacterial origin and antitumor bacteria along with their possible future applications in cancer therapeutics.

Enantioselective synthesis of chiral α-alkynylated thiazolidones by tandem S-addition/acetalization of alkynyl imines

A SPINOL-derived chiral phosphoric acid catalyzed asymmetric formal [2 + 3]-annulation of in situ generated alkynyl imines and 1,4-dithiane-2,5-diol has been developed to afford enantiopure α-alkynylated thiazolidones with up to 72% yield and 98.5 : 1.5 er. This tandem annulation involved a tandem S-addition of alkynyl imines/intramolecular acetalization, followed by PDC-mediated oxidation. The α-alkynylated thiazolidones could facilely afford the corresponding chiral α-alkynylated or α-alkenylated cyclic sulfoxides via further elaboration.

Biosurfactants Produced by Marine Microorganisms with Therapeutic Applications

Marine microorganisms possess unique metabolic and physiological features and are an important source of new biomolecules, such as biosurfactants. Some of these surface-active compounds synthesized by marine microorganisms exhibit antimicrobial, anti-adhesive and anti-biofilm activity against a broad spectrum of human pathogens (including multi-drug resistant pathogens), and could be used instead of existing drugs to treat infections caused by them. In other cases, these biosurfactants show anti-cancer activity, which could be envisaged as an alternative to conventional therapies. However, marine biosurfactants have not been widely explored, mainly due to the difficulties associated with the isolation and growth of their producing microorganisms. Culture-independent techniques (metagenomics) constitute a promising approach to study the genetic resources of otherwise inaccessible marine microorganisms without the requirement of culturing them, and can contribute to the discovery of novel biosurfactants with significant biological activities. This paper reviews the most relevant biosurfactants produced by marine microorganisms with potential therapeutic applications and discusses future perspectives and opportunities to discover novel molecules from marine environments.

Enantioselective synthesis of β-substituted chiral allylic amines via Rh-catalyzed asymmetric hydrogenation

Rh–DuanPhos complex-catalyzed asymmetric hydrogenation of α-alkenyl substituted enamides has been developed, which provides a readily accessible approach to chiral allylic amines with excellent enantioselectivities and high regioselectivities.

Asymmetric additions of thioglycolates and N-Boc aldimines catalyzed by a bifunctional tertiary-amine squaramide

A highly enantioselective asymmetric addition reaction of thioglycolates and N-Boc aldimines was promoted by a bifunctional tertiary-amine squaramide catalyst.

Microbial amphiphiles: a class of promising new-generation anticancer agents

Developing new classes of anticancer molecules has always been a major scientific challenge owing to multidrug resistance of cancer cells to conventional chemotherapeutic agents. Microbial amphiphiles, particularly lipopeptides and glycolipids, have recently emerged as potential new-generation anticancer agents, owing to low toxicity, high efficacy and easy biodegradability. They exhibit anticancer activities by retarding cell cycle progression, inhibiting crucial signaling pathways such as Akt, extracellular signal-regulated kinase/c-Jun N-terminal kinase (ERK/JNK) and Janus kinase/signal transducer and activator of transcription (JAK/STAT), reducing angiogenesis, activating natural killer T (NKT) cells and inducing apoptosis through death receptors in cancer cells. It has been well established that the oncogenic signals of cancer cells are amplified by the overexpression of various membrane-bound receptors such as epidermal growth factor receptor (EGFR), vascular endothelial growth factor receptor (VEGFR), platelet-derived growth factor receptor (PDGFR) and insulin-like growth factor receptor (IGFR). Microbial amphiphiles, upon interaction with the cell membrane, are believed to suppress the activities of these cell surface receptors by fatty acid chain mediated membrane destabilization. This review analyzes the modes and mechanisms of action of these green molecules for application as potential anticancer agents.

Evaluation of Streptomyces coelicolor A3(2) as a heterologous expression host for the cyanobacterial protein kinase C activator lyngbyatoxin A

Filamentous marine cyanobacteria are extremely rich sources of bioactive natural products and often employ highly unusual biosynthetic enzymes in their assembly. However, the current lack of techniques for stable DNA transfer into these filamentous organisms, combined with the absence of heterologous expression strategies for nonribosomal cyanobacterial gene clusters, prohibit the creation of mutant strains or the heterologous production of these cyanobacterial compounds in other bacteria. In this study, we evaluated the capability of a derivative of the model actinomycete Streptomyces coelicolor A3(2) to express enzymes involved in the biosynthesis of the protein kinase C activator lyngbyatoxin A from a Hawaiian strain of Moorea producta (previously classified as Lyngbya majuscula). Despite large differences in GC content between these two bacteria and the presence of rare TTA/UUA leucine codons in lyngbyatoxin ORFs we were able to achieve expression of the cytochrome P450 monooxygenase LtxB and reverse prenyltransferase LtxC in S. coelicolor M512 and confirmed the in vitro functionality of S. coelicolor overexpressed LtxC. Attempts to express the entire lyngbyatoxin A gene cluster in S. coelicolor M512 were not successful because of transcript termination observed for the ltxA gene, which encodes a large nonribosomal peptide synthetase. However, these attempts did show a detectable level of cyanobacterial promoter recognition in Streptomyces. Successful expression of lyngbyatoxin A proteins in Streptomyces provides a new platform for biochemical investigation of natural product enzymes from Moorea strains.

Highly diastereoselective synthesis of α-difluoromethyl amines from N-tert-butylsulfinyl ketimines and difluoromethyl phenyl sulfone

The first highly efficient and stereoselective difluoromethylation of structurally diverse N-tert-butylsulfinyl ketimines has been achieved with an in situ generated PhSO(2)CF(2)(-) anion, which provides a powerful synthetic method for the preparation of a variety of structurally diverse homochiral α-difluoromethyl tertiary carbinamines, including α-difluoromethyl allylic amines and α-difluoromethyl propargylamines. The stereocontrol mode of the present diastereoselective difluoromethylation of ketimines was found to be different from that of other known fluoroalkylations of N-tert-butylsulfinyl aldimines, which suggests that a cyclic six-membered transition state may be involved in the reaction.

Biologically active secondary metabolites from marine cyanobacteria

Marine cyanobacteria are a rich source of complex bioactive secondary metabolites which derive from mixed biosynthetic pathways. Recently, several marine cyanobacterial natural products have garnered much attention due to their intriguing structures and exciting anti-proliferative or cancer cell toxic activities. Several other recently discovered secondary metabolites exhibit insightful neurotoxic activities whereas others are showing pronounced anti-inflammatory activity. A number of anti-infective compounds displaying activity against neglected diseases have also been identified, which include viridamides A and B, gallinamide A, dragonamide E, and the almiramides.

Marine natural product drug discovery: Leads for treatment of inflammation, cancer, infections, and neurological disorders

Natural products, secondary metabolites, isolated from plants, animals and microbes are important sources for bioactive molecules that in many cases have been developed into treatments for diseases. This review will focus on describing the potential for finding new treatments from marine natural products for inflammation, cancer, infections, and neurological disorders. Historically terrestrial natural products have been studied to a greater extent and such classic drugs as aspirin, vincristine and many of the antibiotics are derived from terrestrial natural products. The need for new therapeutics in the four areas mentioned is dire. Within the last 30 years marine natural products, with their unique structures and high level of halogenation, have shown many promising activities against the inflammatory response, cancer, infections and neurological disorders. The review will outline examples of such compounds and activities.

Transcriptional analysis of the jamaicamide gene cluster from the marine cyanobacterium Lyngbya majuscula and identification of possible regulatory proteins

Background

The marine cyanobacterium Lyngbya majuscula is a prolific producer of bioactive secondary metabolites. Although biosynthetic gene clusters encoding several of these compounds have been identified, little is known about how these clusters of genes are transcribed or regulated, and techniques targeting genetic manipulation in Lyngbya strains have not yet been developed. We conducted transcriptional analyses of the jamaicamide gene cluster from a Jamaican strain of Lyngbya majuscula, and isolated proteins that could be involved in jamaicamide regulation.

Results

An unusually long untranslated leader region of approximately 840 bp is located between the jamaicamide transcription start site (TSS) and gene cluster start codon. All of the intergenic regions between the pathway ORFs were transcribed into RNA in RT-PCR experiments; however, a promoter prediction program indicated the possible presence of promoters in multiple intergenic regions. Because the functionality of these promoters could not be verified in vivo, we used a reporter gene assay in E. coli to show that several of these intergenic regions, as well as the primary promoter preceding the TSS, are capable of driving β-galactosidase production. A protein pulldown assay was also used to isolate proteins that may regulate the jamaicamide pathway. Pulldown experiments using the intergenic region upstream of jamA as a DNA probe isolated two proteins that were identified by LC-MS/MS. By BLAST analysis, one of these had close sequence identity to a regulatory protein in another cyanobacterial species. Protein comparisons suggest a possible correlation between secondary metabolism regulation and light dependent complementary chromatic adaptation. Electromobility shift assays were used to evaluate binding of the recombinant proteins to the jamaicamide promoter region.

Conclusion

Insights into natural product regulation in cyanobacteria are of significant value to drug discovery and biotechnology. To our knowledge, this is the first attempt to characterize the transcription and regulation of secondary metabolism in a marine cyanobacterium. If jamaicamide is light regulated, this mechanism would be similar to other cyanobacterial natural product gene clusters such as microcystin LR. These findings could aid in understanding and potentially assisting the management of toxin production by Lyngbya in the environment.