Biosynthesis of cyanobacterin, a paradigm for furanolide core structure assembly.

The γ-butyrolactone motif is found in many natural signaling molecules and other specialized metabolites. A prominent example is the potent aquatic phytotoxin cyanobacterin, which has a highly functionalized γ-butyrolactone core structure. The enzymatic machinery that assembles cyanobacterin and structurally related natural products (herein termed furanolides) has remained elusive for decades. Here, we elucidate the biosynthetic process of furanolide assembly. The cyanobacterin biosynthetic gene cluster was identified by targeted bioinformatic screening and validated by heterologous expression in Escherichia coli. Full functional evaluation of the recombinant key enzymes in vivo and in vitro, individually and in concert, provided in-depth mechanistic insights into a streamlined C-C bond-forming cascade that involves installation of compatible reactivity at seemingly unreactive Cα positions of amino acid precursors. Our work extends the biosynthetic and biocatalytic toolbox for γ-butyrolactone formation, provides a general paradigm for furanolide biosynthesis and sets the stage for their targeted discovery, biosynthetic engineering and enzymatic synthesis.

Spin Dynamics of Flavoproteins.

This short review reports the surprising phenomenon of nuclear hyperpolarization occurring in chemical reactions, which is called CIDNP (chemically induced dynamic nuclear polarization) or photo-CIDNP if the chemical reaction is light-driven. The phenomenon occurs in both liquid and solid-state, and electron transfer systems, often carrying flavins as electron acceptors, are involved. Here, we explain the physical and chemical properties of flavins, their occurrence in spin-correlated radical pairs (SCRP) and the possible involvement of flavin-carrying SCRPs in animal magneto-reception at earth's magnetic field.

On-the-Fly Mass Spectrometry in Digital Microfluidics Enabled by a Microspray Hole: Toward Multidimensional Reaction Monitoring in Automated Synthesis Platforms.

We report an approach for the online coupling of digital microfluidics (DMF) with mass spectrometry (MS) using a chip-integrated microspray hole (µSH). The technique uses an adapted electrostatic spray ionization (ESTASI) method to spray a portion of a sample droplet through a microhole in the cover plate, allowing its chemical content to be analyzed by MS. This eliminates the need for chip disassembly or the introduction of capillary emitters for MS analysis, as required by state-of-the-art. For the first time, this allows the essential advantage of a DMF device─free droplet movement─to be retained during MS analysis. The broad applicability of the developed seamless coupling of DMF and mass spectrometry was successfully applied to the study of various on-chip organic syntheses as well as protein and peptide analysis. In the case of a Hantzsch synthesis, we were able to show that the method is very well suited for monitoring even rapid chemical reactions that are completed in a few seconds. In addition, the strength of the low resource consumption in such on-chip microsyntheses was demonstrated by the example of enzymatic brominations, for which only a minute amount of a special haloperoxidase is required in the droplet. The unique selling point of this approach is that the analyzed droplet remains completely movable after the MS measurement and is available for subsequent on-DMF chip processes. This is illustrated here for the example of MS analysis of the starting materials in the corresponding droplets before they are combined to investigate the reaction progress by DMF-MS further. This technology enables the ongoing and almost unlimited tracking of multistep chemical processes in a DMF chip and offers exciting prospects for transforming digital microfluidics into automated synthesis platforms.

Synthesis, Anticancer Potential and Comprehensive Toxicity Studies of Novel Brominated Derivatives of Bacterial Biopigment Prodigiosin from Serratia marcescens ATCC 27117.

Prodigiosins (prodiginines) are a class of bacterial secondary metabolites with remarkable biological activities and color. In this study, optimized production, purification, and characterization of prodigiosin (PG) from easily accessible Serratia marcescens ATCC 27117 strain has been achieved to levels of 14 mg/L of culture within 24 h. Furthermore, environmentally friendly bromination of produced PG was used to afford both novel mono- and dibrominated derivatives of PG. PG and its Br derivatives showed anticancer potential with IC50 values range 0.62-17.00 µg/mL for all tested cancer cell lines and induction of apoptosis but low selectivity against healthy cell lines. All compounds did not affect Caenorhabditiselegans at concentrations up to 50 µg/mL. However, an improved toxicity profile of Br derivatives in comparison to parent PG was observed in vivo using zebrafish (Danio rerio) model system, when 10 µg/mL applied at 6 h post fertilization caused death rate of 100%, 30% and 0% by PG, PG-Br, and PG-Br2, respectively, which is a significant finding for further structural optimizations of bacterial prodigiosins. The drug-likeness of PG and its Br derivatives was examined, and the novel Br derivatives obey the Lipinski's "rule of five", with an exemption of being more lipophilic than PG, which still makes them good targets for further structural optimization.

3D-Non-destructive Imaging through Heavy-Metal Eosin Salt Contrast Agents.

Conventional histology is a destructive technique based on the evaluation of 2D slices of a 3D biopsy. By using 3D X-ray histology these obstacles can be overcome, but their application is still restricted due to the inherently low attenuation properties of soft tissue. In order to solve this problem, the tissue can be stained before X-ray computed tomography imaging (CT) to enhance the soft tissue X-ray contrast. Evaluation of brominated fluorescein salts revealed a mutual influence of the number of bromine atoms and the cations applied on the achieved contrast enhancement. The dibromo fluorescein barium salt turned out to be the ideal X-ray contrast agent, allowing for 3D imaging and subsequent complementing counterstaining applying standard histological techniques.

Quarternary α-cyanobenzylsulfonamides: A new subclass of CAA fungicides with excellent anti-Oomycetes activity.

A bioisosteric carboxamide - sulfonamide replacement explored during the optimization of an insecticide lead compound led to the surprising discovery of a formerly unknown subclass of the Carboxylic Acid Amide (CAA) fungicides, which is the very first CAA fungicide group without a carboxamide function. In this paper we present invention pathway, racemic and stereoselective synthesis routes, structure-activity relationship studies as well as resistance profile of this novel family of fungicides.

α-Functionalization of Ketones via a Nitrogen Directed Oxidative Umpolung.

Reversing the polarity in molecules is a versatile tool for expanding the boundaries of structural space. Despite a manifold of different umpolung methods available today, overcoming the inherent reactivity still remains a constant challenge in organic chemistry. The oxidative α-functionalization of ketones by external nucleophiles constitute such an example. Herein, we present a hypervalent F-iodane mediated umpolung of pyridyl ketones triggered by Lewis base/Lewis acid noncovalent interactions. A wide variety of external nucleophiles are introduced with high regioselectivity applying this substrate-directing concept.

Biocatalysis Fueled by Light: On the Versatile Combination of Photocatalysis and Enzymes.

Enzymes catalyze a plethora of highly specific transformations under mild and environmentally benign reaction conditions. Their fascinating performances attest to high synthetic potential that is often hampered by operational obstacles such as in vitro cofactor supply and regeneration. Exploiting light and combining it with biocatalysis not only helps in overcoming these drawbacks, but the fruitful liaison of these two fields of "green chemistry" also offers opportunities to unlock new synthetic reactivities. In this review we provide an overview of the wide variety of photo-biocatalysis, ranging from the photochemical delivery of electrons required in redox biocatalysis and photochemical cofactor and reagent (re)generation to direct photoactivation of enzymes enabling reactions unknown in nature. We highlight synthetically relevant transformations such as asymmetric reactions facilitated by the combination of light as energy source and enzymes' catalytic power.

Fluorine as a Traceless Directing Group for the Regiodivergent Synthesis of Indoles and Tryptophans.

Despite ample evidence for the unique reactivity offered by hypervalent F-iodanes, mechanistic investigations fall far behind. In order to shed light on the unusual behavior of such F-reagents, we conducted computational and experimental studies on the chemodivergent transformation of styrenes. We identified the spirocyclic F-cyclopropane as the common intermediate for both the C, H-fluorination and C, H-amination pathways. The fate of this key compound is determined by the extent of cationic charge delocalization controlled by the N-substituents. Exploiting this phenomenon, a multitude of different transformations have become available, leading, i.e., to the regiodivergent synthesis of indoles and tryptophans.

NXS, Morpholine, and HFIP: The Ideal Combination for Biomimetic Haliranium-Induced Polyene Cyclizations.

In contrast to Nature that accomplishes polyene cyclizations seemingly with ease, such transformations are difficult to conduct in the lab. In our program dealing with the development of selective halogenations of alkenes, we now asserted that standard X+ reagents are perfectly suited for the biomimetic cation-π cyclization of both electron rich and poor linear polyenes in the presence of the Lewis base morpholine and the Lewis acid HFIP. The method stands out due to its broad substrate scope and practicability together with high chemical yields and excellent selectivities, even for highly challenging chloriranium-induced polyene cyclizations.

Hypervalent iodine(iii) fluorinations of alkenes and diazo compounds: new opportunities in fluorination chemistry.

The fluorination of organic molecules is a rapidly evolving and exciting field in synthesis, which still poses huge challenges despite the advances made in the past decades. Hypervalent iodine(iii) reagents, which have already proven their versatility as synthetic tools in organic chemistry, are currently on the rise in fluorination chemistry. With their ability to break new mechanistic grounds, they grant access to completely new reactivities and thus also to novel fluorinated structural scaffolds. This review aims to provide an overview of the achievements made in the iodine(iii) mediated fluorinations of aliphatic Csp2-carbon atoms with special focus on the opportunities provided by this exciting class of hypervalent substances.

Characterization of a Cyanobacterial Haloperoxidase and Evaluation of its Biocatalytic Halogenation Potential.

Vanadium-dependent haloperoxidases (VHPOs) are a class of halogenating enzymes found in fungi, lichen, algae, and bacteria. We report the cloning, purification, and characterization of a functional VHPO from the cyanobacterium Acaryochloris marina (AmVHPO), including its structure determination by X-ray crystallography. Compared to other VHPOs, the AmVHPO features a unique set of disulfide bonds that stabilize the dodecameric assembly of the protein. Easy access by high-yield recombinant expression, as well as resistance towards organic solvents and temperature, together with a distinct halogenation reactivity, make this enzyme a promising starting point for the development of biocatalytic transformations.

Advances in Iodine(III)-Mediated Halogenations: A Versatile Tool to Explore New Reactivities and Selectivities.

Within the repertoire of organic chemical transformations, the halogenation of substrates is among the most versatile, reliable, and broadly applicable reactions. Although a multitude of different methods are known today, there is still a huge demand for novel and, in particular, catalytic halogenation methods that exhibit new reactivities and selectivities. The class of hypervalent iodanes meets exactly these needs and thus offers a great opportunity to fuel this highly desirable direction within the field of halogenation chemistry. This Concept gives a short overview of recent examples focusing on selective and/or mechanistically unusual halogenations.

A Fluorination/Aryl Migration/Cyclization Cascade for the Metal-Free Synthesis of Fluoro-Benzoxazepines.

Fluorinated organic molecules are of high interest for many applications across chemical and medical disciplines. Efficient methods for the synthesis of such compounds are thus needed. Within this work, application of the bench-stable cyclic hypervalent iodine(III) fluoro reagent 1 facilitated the development of an efficient, metal-free method for the preparation of the novel class of 4-fluoro-1,3-benzoxazepines starting from readily available styrenes. The efficacy and broad applicability of this concept is demonstrated by the synthesis of 20 structurally diverse congeners in high yields, regio-, and diastereoselectivities. The presented method provides complementary chemoselectivity when compared to the common, commercially available electrophilic fluorination reagents, such as selectfluor. First mechanistic investigations with isotopically labeled substrates reveal a complex reaction mechanism, proceeding via an unusual fluorination/1,2-aryl migration/cyclization cascade.

Iodine(III)-catalyzed rearrangements of imides: a versatile route to α,α-dialkylated α-hydroxy carboxylamides.

A tertiary hydroxy group α to a carboxyl moiety comprises a key structural motif in many bioactive substances. With the herein presented metal-free rearrangement of imides triggered by hypervalent λ(3)-iodane, an easy and selective way to gain access to such a compound class, namely α,α-disubstituted-α-hydroxy carboxylamides, was established. Their additional methylene bromide side chain constitutes a useful handle for rapid diversification, as demonstrated by a series of further functionalizations. Moreover, the in situ formation of an iodine(III) species under the reaction conditions was proven. Our findings clearly corroborate that hypervalent λ(3)-benziodoxolones are involved in these organocatalytic reactions.

Structure of a putative fluorinated natural product from Streptomyces sp. TC1.

Fluorine-containing natural products are extremely rare. The recent report on the isolation and biological activity of the bacterial secondary metabolite 3-(3,5-di-tert-butyl-4-fluorophenyl)propionic acid was thus highly remarkable. The compound contained the first aromatic fluorine substituent known to date in any natural product. The promise to discover an enzyme capable of aromatic fluorination in the producing strain Streptomyces sp. TC1 prompted our immediate interest. A close inspection of the originally reported analytical data of the fluoro metabolite revealed inconsistencies that triggered us to validate the reported structure. The results of these efforts are presented in this communication.

Hypervalent Fluoro-iodane-Triggered Semipinacol Rearrangements: Synthesis of α-Fluoro Ketones.

Hypervalent fluoro-λ3-iodanes have emerged as versatile reagents that provide unusual fluorination selectivities under mild reaction conditions. Here, we report on adding a semipinacol rearrangement, fluorination, and aryl migration cascade reaction of styrene derivatives. Thus, various cyclopentanones became accessible in up to 96% yield, all bearing tertiary C,F-carbon centers adjacent to the ketone group. Such fluorinated structural motifs are difficult to build with previously established methods. Preliminary experiments on enantioselective processes validated that asymmetric transformations are likewise feasible.

Biomimetic chlorine-induced polyene cyclizations harnessing hypervalent chloroiodane-HFIP assemblies.

While bromo- and iodocyclizations have recently been successfully implemented, the challenging chlorocyclizations have been scantly investigated. We present a selective and generally applicable concept of chlorination-induced polyene cyclization by utilizing HFIP-chloroiodane networks mimicking terpene cyclases. A manifold of different alkenes was converted with excellent selectivities (up to d.r. >95 : 5). The cyclization platform was even extended to several structurally challenging terpenes and terpenoid carbon frameworks.

Enzyme-like polyene cyclizations catalyzed by dynamic, self-assembled, supramolecular fluoro alcohol-amine clusters.

Terpene cyclases catalyze one of the most powerful transformations with respect to efficiency and selectivity in natural product (bio)synthesis. In such polyene cyclizations, structurally highly complex carbon scaffolds are built by the controlled ring closure of linear polyenes. Thereby, multiple C,C bonds and stereocenters are simultaneously created with high precision. Structural pre-organization of the substrate carbon chain inside the active center of the enzyme is responsible for the product- and stereoselectivity of this cyclization. Here, we show that in-situ formed fluorinated-alcohol-amine supramolecular clusters serve as artificial cyclases by triggering enzyme-like reactivity and selectivity by controlling substrate conformation in solution. Because of the dynamic nature of these supramolecular assemblies, a broad range of terpenes can be produced diastereoselectively. Mechanistic studies reveal a finely balanced interplay of fluorinated solvent, catalyst, and substrate as key to establishing nature's concept of a shape-selective polyene cyclization in organic synthesis.

Strained cyclophane natural products: macrocyclization at its limits.

Cyclophane natural products comprise an intriguing class of structurally diverse compounds. As inherent for all cyclic compounds regardless of their origin, macrocyclization is naturally the most decisive step, which defines the overall efficiency of the synthetic pathway. Especially in small cyclophane molecules, this key step constitutes an even greater challenge. Due to the strain imparted by the macrocyclic system, free rotation of the benzene ring(s) is often restricted depending on both the constitution of the tether and the aromatic portions. Not surprisingly, the synthesis of natural cyclophanes with their often outstanding pharmaceutical activities and the inherent issues associated with their preparation has attracted much attention among the synthetic community. In particular, it stimulated the development of new strategies for the ring-closing step, as often otherwise well established and robust reactions fail to perform effectively. In this review, we describe the challenges synthetic chemists are facing during the synthesis of this small, but structurally and biologically fascinating class of natural products, concentrating on the representatives exhibiting configurational stability. The main focus will be on the different concepts for the installation of the macrocyclic system, in most cases the central problem in assembling these extremely rigid molecules.