Cyclopropanations of olefin-containing natural products for simultaneous arming and structure activity studies

Cyclopropanations of alkene-containing natural products that proceed under mild conditions are reported for simultaneous arming and structure-activity relationship studies. An alkynyl diazo ester under Rh(II) catalysis is employed for cyclopropanations of electron-rich olefins while an alkynyl sulfonium ylide is used for electron-poor olefins. This approach enables simultaneous natural product derivatization for SAR studies and arming (i.e., via alkyne attachment) for subsequent conjugation with reporter tags (e.g., biotin, fluorophores, photoaffinity labels) for mechanism of action studies including cellular target identification and proteome profiling experiments.     

Ligand fishing as a tool to screen natural products with anticancer potential

Cancer is the second leading cause of death in the world and its incidence is expected to increase with the aging of the world's population and globalization of risk factors. Natural products and their derivatives have provided a significant number of approved anticancer drugs and the development of robust and selective screening assays for the identification of lead anticancer natural products are essential in the challenge of developing personalized targeted therapies tailored to the genetic and molecular characteristics of tumors. To this end, a ligand fishing assay is a remarkable tool to rapidly and rigorously screen complex matrices, such as plant extracts, for the isolation and identification of specific ligands that bind to relevant pharmacological targets. In this paper, we review the application of ligand fishing with cancer-related targets to screen natural product extracts for the isolation and identification of selective ligands. We provide critical analysis of the system configurations, targets, and key phytochemical classes related to the field of anticancer research. Based on the data collected, ligand fishing emerges as a robust and powerful screening system for the rapid discovery of new anticancer drugs from natural resources. It is currently an underexplored strategy according to its considerable potential.     

Display cloning: functional identification of natural product receptors using cDNA-phage display.

BACKGROUND: The identification of cellular targets has traditionally been the starting point for natural product mode of action studies and has led to the understanding of many biological processes. Conventional experimental approaches have depended on cell-based screening and/or affinity chromatography. Although both of these techniques aid in the discovery of protein cellular targets, a method that couples protein identification with gene isolation would be extremely valuable. RESULTS: A procedure for the direct cloning of cellular proteins, based on their affinity for natural products, using cDNA phage display has been developed. The technique is referred to as display cloning because it involves the cloning of proteins displayed on the surface of a bacteriophage particle. The approach has been established by isolating a full-length gene clone of FKBP12 (FK506-binding protein) from a human brain cDNA library using a biotinylated FK506 probe molecule. During the affinity selection, the FKBP12 gene emerged as the dominant library member and was the only sequence identified after the second round of selection. CONCLUSIONS: The development of display cloning greatly facilitates the investigation of ligand-receptor interaction biology and natural product mode of action studies. This procedure utilizes heterologous protein display on infectious phage, which allows the amplification and repeated selection of putative sequences, leading to unambiguous target identification. In addition, the direct connection of a functional protein to its gene sequence eliminates the subsequent cloning step required with tissue homogenate or cell lysate affinity methods, allowing direct isolation of an expressible gene sequence.     

Electrophilic natural products and their biological targets

The study of biologically active natural products has resulted in seminal contributions to our understanding of living systems. In the case of electrophilic natural products, the covalent nature of their interaction has largely facilitated the identification of their biological binding partners. In this review, we provide a comprehensive compilation of electrophilic natural products from all major chemical classes together with their biological targets. Covering Michael acceptor systems, ring-strained compounds and other electrophiles, such as esters or carbamates, we highlight representative and instructive examples for over 20 electrophilic moieties. The fruitful cooperation of natural product chemistry, medicinal chemistry and chemical biology has produced a collection of well-studied examples for how electrophilic natural products exert their biological functions that range from antibiotic to antitumor effects. Special emphasis is put on the elucidation of their respective biological targets via activity-based protein profiling, which together with the recent advancements in mass spectrometry has been crucial to the success of the field. The wealth of naturally occurring electrophilic moieties and their chemical complexity enables binding of a large variety of biological targets, such as enzymes of all classes, nonenzymatic proteins, DNA and other cellular compounds. With approximately 30,000 genes in the human genome but only 266 confirmed protein drug targets, the study of biologically active, electrophilic natural products has the potential to provide insights into fundamental biological processes and to greatly aid the discovery of new drug targets.     

Oxazine ring construction: methods and applications to natural product synthesis

Although natural products containing a 1,2-oxazine ring are rare, more examples in this family of natural products have been discovered since trichodermamides A and B were reported in 2003. In addition to their structural novelty, these natural products possess very interesting bioactivities that are strongly dependent on their structures, making them attractive targets for investigating structure-activity relationships (SAR). In this feature article, we summarized the methodologies developed in recent years for constructing the 1,2-oxazine rings. Racemic and enantioselective total syntheses of trichodermamides A and B, based on these methodologies, are reviewed in detail.     

Dearomatization strategies in the synthesis of complex natural products

Evolution in the field of the total synthesis of natural products has led to exciting developments over the last decade. Numerous chemoselective and enantioselective methodologies have emerged from total syntheses, resulting in efficient access to many important natural product targets. This Review highlights recent developments concerning dearomatization, a powerful strategy for the total synthesis of architecturally complex natural products wherein planar, aromatic scaffolds are converted to three-dimensional molecular architectures.     

Solution-phase preparation of a 560-compound library of individual pure mappicine analogues by fluorous mixture synthesis

Solution-phase mixture synthesis has efficiency advantages and favorable reaction kinetics. Applications of this technique, however, have been discouraged by the difficulty in obtaining individual, pure final products by using conventional separation and identification processes. Introduced here is a new strategy for mixture synthesis that addresses the separation and identification problems. Members of a series of organic substrates are paired with a series of fluorous tags of different chain lengths. The tagged starting materials are then mixed and taken through a multistep reaction process. Fluorous chromatography is used to demix the tagged product mixtures on the basis of the fluorine content of the tags to provide the individual pure components of the mixture, which are detagged to release the final products. The utility of fluorous mixture synthesis is demonstrated by the preparation of a 560-membered library of analogues of the natural product mappicine. A seven-component mixture is carried through a four-step mixture synthesis (two one-pot and two parallel steps) to incorporate two additional points of diversity onto the tetracyclic core. Methods for analysis and purification of the intermediates are established for the quality control of the mixture synthesis.     

Development of a flexible strategy towards FR900482 and the mitomycins

FR900482 and the mitomycins are two intriguing classes of alkaloid natural products that have analogous biological mechanisms and obvious structural similarity. Both classes possess potent anticancer activity, a feature that has led to their investigation and implementation for the clinical treatment of human cancer. Given the structural similarity between these natural products, we envisioned a common synthetic strategy by which both classes could be targeted through assembling the mitomycin skeleton prior to further oxidative functionalization. Realization of this strategy with respect to FR900482 was accomplished through the synthesis of 7-epi-FR900482, which displayed equal potency relative to the natural product against two human cancer cell lines. With the challenging goal of a synthesis of either mitomycin or FR900482 in mind, several methodologies were explored. While not all of these methods ultimately proved useful for our synthetic goal, a number of them led to intriguing findings that provide a more complete understanding of several methodologies. In particular, amination via π-allyl palladium complexes for the synthesis of tetrahydroquinolines, eight-membered heterocycle formation via carbonylative lactamization, and amination through late-stage C-H insertion via rhodium catalysis all featured prominently in our synthetic studies.     

Fluorescent profiling of natural product producers

The identification of natural product producer organisms remains a problem for both isolation and natural product classification. A concise screen is developed through fluorescent modification of a set of natural products that offer a common activity. Through real-time multicolor microscopy, the processing, storage, and effects of a natural product are rapidly screened at the level of the strain and individual organism.     

Recent progress on the total synthesis of natural products in China

An overview of natural products synthesis in Mainland China during the past 10 years is provided. This review only emphasizes the first total synthesis of molecules of contemporary interest and syntheses that helped to correct structures. In addition, some significant results on the novel synthesis and structure-activity relationship (SAR) studies of several natural products are introduced.     

Rhodium-catalyzed arylative and alkenylative cyclization of 1,5-enynes induced by geminal carbometalation of alkynes

A rhodium(I)-catalyzed addition-cyclization of 1,5-enynes with aryl- and alkenylboronic acids has been developed. The reaction allows for efficient C-C coupling of multiple reactive components while accomplishing a net R,H-addition in a single step under mild conditions. In the presence of [Rh(OH)(COD)]2 catalyst and triethylamine base in methanol solvent, a range of 1,5-enynes undergo an intermolecular addition with a wide variety of aryl- and alkenylboronic acids and concomitant endo-selective cyclization to yield 1-aryl and alkenyl-substituted cyclopentene derivatives as the products. Deuterium labeling studies suggest that the reaction involves formation of a rhodium vinylidene complex with the terminal alkyne of the enyne substrate. The subsequent migration of the aryl or alkenyl group from the rhodium center to the alpha-carbon of the vinylidene ligand gives a vinyl rhodium complex, a formal 1,1-carbometalation process of the alkyne. This vinyl rhodium then adds to the pendent alkene, and the protodemetalation of the resulting rhodium enolate affords the product.     

Prins-type macrocyclizations as an efficient ring-closing strategy in natural product synthesis

Prins-type macrocyclizations have recently emerged as a successful strategy in the synthesis of polyketide-derived natural products. This reaction provides a concise and selective means to form tetrahydropyran-containing macrocyclic rings of varying size. A high degree of functionality within the macrocycle is tolerated and the yields for these transformations are typically good to excellent. Since the initial report of a Prins macrocyclization reaction in 1979, examples of this approach did not re-emerge until 2008. However, the use of this method in natural product synthesis has rapidly gained momentum in the synthetic community, with multiple examples of this macrocyclization tactic reported in the recent literature.     

Synthesis of a beta-estradiol-biotin chimera that potently heterodimerizes estrogen receptor and streptavidin proteins in a yeast three-hybrid system

Small molecules that dimerize proteins in living cells provide powerful probes of biological processes and have potential as tools for the identification of protein targets of natural products. We synthesized 7-alpha-substituted derivatives of beta-estradiol tethered to the natural product biotin to regulate heterodimerization of estrogen receptor (ER) and streptavidin (SA) proteins expressed as components of a yeast three-hybrid system. Addition of an estradiol-biotin chimera bearing a 19-atom linker to yeast expressing DNA-bound ER-alpha or ER-beta LexA fusion proteins and wild-type SA protein fused to the B42 activation domain activated reporter gene expression by as much as 450-fold in vivo (10 muM ligand). Comparative analysis of lower affinity Y43A (biotin Kd approximately 100 pM) and W120A (biotin Kd approximately 100 nM) mutants of SA indicated that moderate affinity interactions can be readily detected with this system. Comparison of a 7-alpha-substituted estradiol-biotin chimera with a structurally similar dexamethasone-biotin chimera revealed that yeast expressing ER proteins can detect cognate ligands with up to 5-fold greater potency and 70-fold higher activity than yeast expressing analogous glucocorticoid receptor (GR) proteins. This approach may facilitate the identification of protein targets of biologically active small molecules screened against genetically encoded libraries of proteins expressed in yeast three-hybrid systems.     

Recent advances in target identification by natural product based chemical probes

Natural products have been extensively used to treat diseases throughout human history. These are mainly because natural products normally target biological macromolecules selectively. Target identification could help us to develop new therapeutic agents and discover new biological pathways underlying human diseases. Herein, we highlight some recent examples of using natural products and their derivatives as chemical probes to identify the molecular targets and elucidate mode of action.     

Development of Chemical Probes for Functional Analysis of Anticancer Saponin OSW‐1

Chemical probe‐based approaches have proven powerful in recent years in the target identification studies of natural products. OSW‐1 is a saponin class of natural products with highly potent and selective cytotoxicity against various cancer cell lines. Understanding its mechanism of action is important for the development of anticancer drugs with potentially novel target pathways. This account reviews recent progress in the development of OSW‐1 derived probes for exploring the mechanism of its action. The key to the probe development is a judicious choice of functionalization sites and a selective functionalization strategy. The types of probes include fluorescent probes for cellular imaging analysis and affinity probes for target identification analysis.     

Stereodivergent and Protecting‐Group‐Free Synthesis of the Helicascolide Family: A Rhodium‐Catalyzed Atom‐Economical Lactonization Strategy

Natural products of polyketide origin, in particular small‐sized lactones often possess a very broad range of impressive biological activities. An efficient way to demonstrate the concise access to six‐membered lactones was emphasized as part of a stereodivergent and protecting‐group‐free synthesis of all three representatives of the helicascolide family. This strategy features an atom‐economical and highly diastereoselective rhodium‐catalyzed “head‐to‐tail” lactonization by an intramolecular addition of ω‐allenyl‐substituted carboxylic acids to terminal allenes, including the selective construction of a new stereocenter in the newly formed core structures. The excellent selectivities with which the helicascolide precursors were obtained are remarkable, thus resulting in an expeditious and highly efficient natural product synthesis.     

Preparation of 3-hydroxyoxindoles with dimethyldioxirane and their use for the synthesis of natural products

This work describes a general protocol for the oxidation of indole and oxindole derivatives with dimethyldioxirane to give 3-hydroxyoxindoles present in many natural products. This strategy allowed us to synthesize the natural product 1, to carry out the first total synthesis of 4, a formal total synthesis of donaxaridine (5) and to achieve the synthesis of pyrroloindoline 8, a debromo analogue of the natural product flustraminol B (7).     

Rhodium‐Catalyzed Asymmetric Intramolecular Hydroamination of Allenes

The rhodium‐catalyzed asymmetric intramolecular hydroamination of sulfonyl amides with terminal allenes is reported. It provides selective access to 5‐ and 6‐membered N‐heterocycles, scaffolds found in a large range of different bioactive compounds. Moreover, gram scale reactions, as well as the application of suitable product transformations to natural products and key intermediates thereof are demonstrated.