Phytosiderophore pathway response in barley exposed to iron, zinc or copper starvation.

Efficient micronutrient acquisition is a critical factor in selecting micronutrient dense crops for human consumption. Enhanced exudation and re-uptake of metal chelators, so-called phytosiderophores, by roots of graminaceous plants has been implicated in efficient micronutrient acquisition. We compared PS biosynthesis and exudation as a response mechanism to either Fe, Zn or Cu starvation. Two barley (Hordeum vulgare L.) lines with contrasting micronutrient grain yields were grown hydroponically and PS exudation (LC-MS) and root gene expression (RNAseq) were determined after either Fe, Zn, or Cu starvation. The response strength of the PS pathway was micronutrient dependent and decreased in the order Fe > Zn > Cu deficiency. We observed a stronger expression of PS pathway genes and greater PS exudation in the barley line with large micronutrient grain yield suggesting that a highly expressed PS pathway might be an important trait involved in high micronutrient accumulation. In addition to several metal specific transporters, we also found that the expression of IRO2 and bHLH156 transcription factors was not only induced under Fe but also under Zn and Cu deficiency. Our study delivers important insights into the role of the PS pathway in the acquisition of different micronutrients.

Total synthesis of [13 C2 ]-labeled phytosiderophores of the mugineic and avenic acid families.

We, herein, report the synthesis of 13 C2 -labeled natural products from the mugineic acid and avenic acid family. These phytosiderophores ("plant iron carriers") are built up from non-proteinogenic amino acids and play a key role in micronutrient uptake in gramineous plants. In this work, two central building blocks are prepared from labeled starting materials (13 C2 -bromoacetic acid, 13 C2 -glycine) and further employed in our recently reported divergent, branched synthetic strategy delivering eight isotopically labeled phytosiderophores. The required labeled building blocks (13 C2 -l-allylglycine and a related hydroxylated derivative) were prepared via enantioselective phase-transfer catalysis and enantio- and diastereoselective aldol condensation with a chiral auxiliary, respectively, both potentially valuable themselves for other synthetic routes toward labeled (natural) products.

Surveying the mugineic acid family: Ion mobility - quadrupole time-of-flight mass spectrometry (IM-QTOFMS) characterization and tandem mass spectrometry (LC-ESI-MS/MS) quantification of all eight naturally occurring phytosiderophores.

Phytosiderophores (PS) are root exudates released by grass species (Poaceae) that play a pivotal role in iron (Fe) plant nutrition. A direct determination of PS in biological samples is of paramount importance in understanding micronutrient acquisition mediated by PS. To date, eight plant-born PS have been identified; however, no analytical procedure is currently available to quantify all eight PS simultaneously with high analytical confidence. With access to the full set of PS standards for the first time, we report comprehensive methods to both fully characterize (IM-QTOFMS) and quantify (LC-ESI-MS/MS) all eight naturally occurring PS belonging to the mugineic acid family. The quantitative method was fully validated, yielding linear results for all eight analytes, and no unwanted interferences with soil and plant matrices were observed. LOD and LOQ values determined for each PS were below 11 and 35 nmol L-1, respectively. The method's precision under reproducibility conditions (intra- and inter-day) of measurement was less than 2.5% RSD for all analytes. Additionally, all PS were annotated with high-resolution mass spectrometric fragment spectra and further characterized via drift tube ion mobility-mass spectrometry. The collision cross-sections obtained for primary ion species yielded a valuable database for future research focused on in-depth PS studies. The new quantitative method was applied to analyse root exudates from Fe-controlled and deficient barley, oat, rye, and sorghum plants. All eight PS, including mugineic acid (MA), 3"-hydroxymugineic acid (HMA), 3"-epi-hydroxymugineic acid (epi-HMA), hydroxyavenic acid (HAVA), deoxymugineic acid (DMA), 3"-hydroxydeoxymugineic acid (HDMA), 3"-epi-hydroxydeoxymugineic acid (epi-HDMA) and avenic acid (AVA) were for the first time successfully identified and quantified in root exudates of various graminaceous plants using a single analytical procedure. These newly developed methods can be applied to studies aimed at improving crop yield and micronutrient grain content for food consumption via plant-based biofortification.

A biomimetic approach for the concise total synthesis of greenwaylactams A-C.

A concise, racemic total synthesis of three sesquiterpenoid alkaloids (greenwaylactams A-C) exhibiting an unprecedented 8-membered benzolactam is disclosed. Key transformations of this work include the ring expansion through cleavage of an indole via Witkop oxidation, as well as an HFIP mediated cationic cyclisation to build up the pentacyclic carbon skeleton.

Recent advances in the dearomative functionalisation of heteroarenes.

Dearomatisation reactions of (hetero)arenes have been widely employed as efficient methods to obtain highly substituted saturated cyclic compounds for over a century. In recent years, research in this area has shifted towards effecting additional C-C bond formation during the overall dearomative process. Moving away from classical hydrogenation-based strategies a wide range of reagents were found to be capable of initiating dearomatisation through nucleophilic addition (typically a reduction) or photochemically induced radical addition. The dearomatisation process gives rise to reactive intermediates which can be intercepted in an intra- or intermolecular fashion to deliver products with significantly increased molecular complexity when compared to simple dearomatisation. In this Perspective recent examples and strategies for the dearomative functionalisation of heteroaromatic systems will be discussed.

A Vicinal Diol Approach for the Total Synthesis of Molestin E, ent-Sinulacembranolide A and ent-Sinumaximol A.

In this work an approach for the synthesis of furanocembranoid natural products containing the C-7,8-diol moiety is disclosed. This culminated in the first total synthesis of the natural product molestin E, together with ent-sinulacembranolide A and ent-sinumaximol A as well as a thorough exploration of their chemistry. Late-stage ring-closure of the C-7,8-diols to the corresponding epoxides was also demonstrated. Key features of this synthetic strategy include a stereoselective Baylis-Hillman reaction, ring-closing metathesis and Shiina macrolactonisation. Chiral-pool materials were deployed to ensure the desired absolute stereochemistry which was confirmed by late-stage single crystal X-ray diffraction.

Evolution of the Dearomative Functionalization of Activated Quinolines and Isoquinolines: Expansion of the Electrophile Scope.

Herein we disclose a mild protocol for the reductive functionalisation of quinolinium and isoquinolinium salts. The reaction proceeds under transition-metal-free conditions as well as under rhodium catalysis with very low catalyst loadings (0.01 mol %) and uses inexpensive formic acid as the terminal reductant. A wide range of electrophiles, including enones, imides, unsaturated esters and sulfones, ß-nitro styrenes and aldehydes are intercepted by the in situ formed enamine species forming a large variety of substituted tetrahydro(iso)quinolines. Electrophiles are incorporated at the C-3 and C-4 position for quinolines and isoquinolines respectively, providing access to substitution patterns which are not favoured in electrophilic or nucleophilic aromatic substitution. Finally, this reactivity was exploited to facilitate three types of annulation reactions, giving rise to complex polycyclic products of a formal [3+3] or [4+2] cycloaddition.

Evolution of the Dearomative Functionalization of Activated Quinolines and Isoquinolines: Expansion of the Electrophile Scope.

Herein we disclose a mild protocol for the reductive functionalisation of quinolinium and isoquinolinium salts. The reaction proceeds under transition-metal-free conditions as well as under rhodium catalysis with very low catalyst loadings (0.01 mol %) and uses inexpensive formic acid as the terminal reductant. A wide range of electrophiles, including enones, imides, unsaturated esters and sulfones, ß-nitro styrenes and aldehydes are intercepted by the in situ formed enamine species forming a large variety of substituted tetrahydro(iso)quinolines. Electrophiles are incorporated at the C-3 and C-4 position for quinolines and isoquinolines respectively, providing access to substitution patterns which are not favoured in electrophilic or nucleophilic aromatic substitution. Finally, this reactivity was exploited to facilitate three types of annulation reactions, giving rise to complex polycyclic products of a formal [3+3] or [4+2] cycloaddition.

A Unified Approach to Phytosiderophore Natural Products.

This work reports on the concise total synthesis of eight natural products of the mugineic acid and avenic acid families (phytosiderophores). An innovative "east-to-west" assembly of the trimeric products resulted in a high degree of divergence enabling the formation of the final products in just 10 or 11 steps each with a minimum of overall synthetic effort. Chiral pool starting materials (l-malic acid, threonines) were employed for the outer building blocks while the middle building blocks were accessed by diastereo- and enantioselective methods. A highlight of this work consists in the straightforward preparation of epimeric hydroxyazetidine amino acids, useful building blocks on their own, enabling the first synthesis of 3''-hydroxymugineic acid and 3''-hydroxy-2'-deoxymugineic acid.

Synthesis of human long-term metabolites of dehydrochloromethyltestosterone and oxymesterone.

We herein report the synthesis of the long-term metabolites "M4" (IUPAC: 4-chloro-17-hydroxymethyl-17-methyl-18-norandrosta-4,13-dien-3-ol) of dehydrochloromethyl-testosterone (DHCMT, Oral Turinabol) and "Oxy M9" (4-hydroxy-17ß-hydroxymethyl-17α-methyl-18-norandrosta-4,13-dien-3-one) of oxymesterone (Oranabol). Both compounds were derived from a common synthetic route starting from dehydroepiandrosterone acetate. Four different stereoisomers were evaluated for metabolite M4. The previously assigned structure could be corrected regarding the C-3 and C-17 stereocenters.

Development and validation of a simple online-SPE method coupled to high-resolution mass spectrometry for the analysis of stanozolol-N-glucuronides in urine samples.

Stanozolol is still the most commonly used illicit anabolic-androgenic steroid (AAS) in professional sports. Therefore, accurate and fast analysis and long detection windows are of great interest in the field of antidoping analysis. In this work, a very simple, fast, and highly sensitive online solid-phase extraction method coupled with liquid chromatography-high-resolution tandem mass spectrometry (HPLC-HRMSMS) for the analysis of stanozolol-N-glucuronides was developed. This fully validated procedure is characterized by only a few manual steps (dilution and addition of internal standard) in the sample preparation. A limit of identification (LOI) of 75 pg/mL, high accuracy (87.1%-102.1%), precision (3.1%-7.8%), and sensitivity was achieved. Furthermore, good linearity (> 0.99) and robustness, as well as no carry-over effects, could be observed. In addition to excellent confirmation analysis performance, this method shows sufficient potential for the identification and characterization of unknown metabolites. Using this method, it was possible to unambiguously confirm the presence of 1'N- and 2'N-stanozolol-glucuronide in human urine for the first time due to the access to reference material.

Synthesis of a human long-term oxymetholone metabolite.

A long-term metabolite of the doping agent oxymetholone (OXM-M2, 17ß-hydroxymethyl-2,17α-methyl-18-norandrost-13-en-3-one) which has been identified by GC-MS/MS was synthesized from commercially available materials. Two efficient synthetic routes to access both C-17 epimers of tentative metabolites were developed. The identity and molecular configuration of the in vivo metabolite: 17ß-hydroxymethyl-2α,17α-methyl-18-norandrost-13-en-3-one was confirmed by single crystal X-ray diffraction.

Magnolol dimer-derived fragments as PPARγ-selective probes.

Partial agonists of the transcription factor PPARγ (peroxisome proliferator-activated receptor γ) have shown potential for the treatment of metabolic and inflammatory conditions and novel activators serve as valuable tool and lead compounds. Based on the natural product magnolol (I) and recent structural information of the ligand-target interaction we have previously developed magnolol dimer (II) which has been shown to have enhanced affinity towards PPARγ and improved selectivity over RXRα (retinoid X receptor α), PPARγ's heterodimerization partner. In this contribution we report the synthesis and evaluation of three fragments of the dimeric lead compound by structural simplifications. Sesqui magnolol A and B (III and IV) were found to exhibit comparable activities to magnolol dimer (II) and selectivity over RXRα persisted. Computational studies suggest a common pharmacophore of the distinctive biphenyl motifs. Truncated magnolol dimer (V) on the other hand does not share this feature and was found to act as an antagonist.

Synthesis and structural elucidation of a dehydrochloromethyltestosterone metabolite.

The human urinary long-term metabolite "M3" (4-chloro-17ß-hydroxymethyl-17α-methyl-18-norandrost-13-en-3-ol) of the common doping agent DHCMT has thus far been detected via GC/MS-MS, creating ambiguities concerning its absolute configuration. Its structure was elucidated via the synthesis of all eight possible stereoisomers with 17ß-hydroxymethyl configuration. The highlights of the synthesis consist of a novel first generation approach to 4ß-chloro-5ß compounds as well as a divergent route which allows easy access to the remaining A-ring chlorohydrins.

Synthesis of 17ß-hydroxymethyl-17α-methyl-18-norandrosta-1,4,13-trien-3-one: A long-term metandienone metabolite.

The goal of this work was a good-yielding chemical synthesis of a metandienone metabolite which is of interest in doping analysis. 20ßOH-NorMD (IUPAC: 17ß-hydroxymethyl-17α-methyl-18-norandrosta-1,4,13-triene-3-one) has been identified as a long-term urinary metabolite which can be detected and attributed to metandienone up to almost 3weeks after exposure. The chemical synthesis of its epimer 20αOH-NorMD has been described before, as was an enzymatic synthesis of 20ßOH-NorMD, but no chemical synthesis was published.