Total synthesis and structural assignment of Curvicollide C and derivatives of Fusaequisin A

(–)-Curvicollide C, a polyketide metabolite isolated from the mycoparasitic fungus Podospora curvicolla (left, Illinois 2004), shows striking structural similarity to (–)-Fusaequisin A, isolated from the endophytic fungus Fusarium equiseti (right, Cameroon 2013). As these fungi originate from different habitats and remote geographic locations, and, as they also differ in their morphological characteristics and phylogenetic classification the question arises why these fungi produce secondary metabolites of close structural relationship. In both cases the relative configuration was only partially elucidated while the absolute configuration remained unknown. The central heterocycles were determined to exhibit an all-trans configuration. This inspired us to initiate a research project aiming towards the total synthesis of both natural products, also because the unique combination of structural characteristics, imposed a synthetic challenge. Herein we report the first total synthesis of (+)-Curvicollide C and of a non-natural diastereomer of Fusaequisin A. Cross-metathesis and Julia–Kocienski olefination were chosen as the key coupling steps resulting in three retrosynthetic fragments, namely the Western, the Central, and the Eastern fragment. The synthesis was desined to rely on late-stage coupling of the configurationally and constitutionally customizable Western and Eastern fragements with the Central fragment of defined absolute configuration. The resulting highly modular synthesis provided access to a collection of five individually synthesized diastereomers of Curvicollide C, which enabled the full structural assignment of Curvicollide C by comparative NMR spectroscopic analysis.Der polyketide Metabolit (–)-Curvicollid C, welcher aus dem mycoparasitären Pilz Podospora curvicolla (links, Illinois 2004) isoliert wurde, zeigt eine auffällige strukturelle Ähnlichkeit zu (–)-Fusaequisin A. Dieser Metabolit wurde aus dem endophytischem Pilz Fusarium equiseti (rechts, Cameroon 2013) gewonnen. Da die beiden genannten Pilze aus unterschiedlichen biologischen Habitaten und geographischen Lagen stammen und sich zudem in ihren morphologischen Charakteristka und ihrer phylogenetischen Klassifikation unterscheiden stellt sich die Frage warum diese Pilze Sekundärmetabolite mit einem derart hohen strukturellen Verwandschaftsgrad bilden. Für beide Naturstoffe wurde die relative Konfiguration nur teilweise aufgeklärt und die absolute Konfiguration verblieb unbekannt. Den zentralen Heterozyklen wurde eine all-trans Konfiguration zugeordnet. Die unvollständige Strukturaufklärung forderte uns heraus ein Forschungsprojekt mit dem Ziel der Totalsynthese beider Naturstoffe anzugehen. Zudem sahen wir eine synthetische Herausforderung in der einzigartigen Kombination von strukturellen Charakterisika der beiden Naturstoffe. Im Folgenden berichten wir über die Totalsynthese von (+)-Curvicollid C sowie eines nicht natürlichen Diastereomers von Fusaequisin A. Als zentrale Verknüpfungsschritte wurden eine Kreuzmetathese und eine Olefinierung gewählt, wobei drei retrosynthetische Fragmente resultierten, welche im Folgenden als West-, Zentral- und Ost-Fragment bezeichnet werden. Die Synthese wurde aufbauend auf der Idee geplant, die Verknüpfungen der Fragmente auf einer möglichst späten Stufe durchzuführen, wobei das West- und Ost-Fragment in ihrer Konfiguration und Konstitution flexibel anpassbar sein sollten und das Zentral-Fragment in einer festgelegten absoluten Konfiguration vorliegt. Die resultierende modulare Synthese ermöglichte den Zugang zu einer Kollektion von fünf einzeln synthetisierten Diastereomeren von Curvicollid C. Durch vergleichende NMR Analyse dieser Diastereomere konnte schließlich eine zuverlässige Strukturaufklärung von Curvicollid C erzielt werden

Theoretical analysis of cross-validation for estimating the risk of the k-Nearest Neighbor classifier

The present work aims at deriving theoretical guaranties on the behavior of some cross-validation procedures applied to the $k$-nearest neighbors ($k$NN) rule in the context of binary classification. Here we focus on the leave-$p$-out cross-validation (L$p$O) used to assess the performance of the $k$NN classifier. Remarkably this L$p$O estimator can be efficiently computed in this context using closed-form formulas derived by \cite{CelisseMaryHuard11}. We describe a general strategy to derive moment and exponential concentration inequalities for the L$p$O estimator applied to the $k$NN classifier. Such results are obtained first by exploiting the connection between the L$p$O estimator and U-statistics, and second by making an intensive use of the generalized Efron-Stein inequality applied to the L$1$O estimator. One other important contribution is made by deriving new quantifications of the discrepancy between the L$p$O estimator and the classification error/risk of the $k$NN classifier. The optimality of these bounds is discussed by means of several lower bounds as well as simulation experiments

Total Synthesis and Structural Assignment of Curvicollide C

The first total synthesis of (+)-curvicollide C has been accomplished. Cross-metathesis and Julia–Kocienski olefination were instrumental in the synthesis of 1,3-diene segments and allowed for a ternary-convergent synthetic design. A full structural assignment is proposed for (−)-curvicollide C, a uniquely structured polyketide of fungal origin

Total Synthesis and Structural Assignment of Curvicollide C

The first total synthesis of (+)-curvicollide C has been accomplished. Cross-metathesis and Julia–Kocienski olefination were instrumental in the synthesis of 1,3-diene segments and allowed for a ternary-convergent synthetic design. A full structural assignment is proposed for (−)-curvicollide C, a uniquely structured polyketide of fungal origin

Total Synthesis and Structural Assignment of Curvicollide C

The first total synthesis of (+)-curvicollide C has been accomplished. Cross-metathesis and Julia–Kocienski olefination were instrumental in the synthesis of 1,3-diene segments and allowed for a ternary-convergent synthetic design. A full structural assignment is proposed for (−)-curvicollide C, a uniquely structured polyketide of fungal origin

(4S,5S,6R,E)-3,5-Dimethyl-6-vinylhept-2-ene-1,4,7-triol

The title compound, C11H20O3, was obtained in the course of the total syntheses of curvicollides A–C and features the same relative configuration for the central lactone moiety as that reported for the latter compounds. In the crystal, molecules are linked via O—H...O hydrogen bonds: all of the OH groups act as donors as well as acceptors for these bonds, hence each molecule is bound to six surrounding molecules and a three-dimensional network is formed. The absolute structure was confirmed by refinement of the Flack parameter

(2R,4S,5S)-5-Hydroxy-4-methyl-3-oxohept-6-en-2-yl benzoate

The title compound, C15H18O4, which crystallizes with two mol­ecules in the asymmetric unit, was obtained in the course of the total synthesis of curvicollides A–C and fusaequisin A. It features the relative configuration of the Western aldol part of the natural products. In the crystal, mol­ecules are linked by C—H...O hydrogen bonds

Toward Optimized 89Zr-Immuno-PET: Side-by-Side Comparison of [89Zr]Zr-DFO-, [89Zr]Zr-3,4,3-(LI-1,2-HOPO)- and [89Zr]Zr-DFO*-Cetuximab for Tumor Imaging: Which Chelator Is the Most Suitable?

89Zr represents a highly favorable positron emitter for application in immuno-PET (Positron Emission Tomography) imaging. Clinically, the 89Zr4+ ion is introduced into antibodies by complexation with desferrioxamine B. However, producing complexes of limited kinetic inertness. Therefore, several new chelators for 89Zr introduction have been developed over the last years. Of these, the direct comparison of the most relevant ones for clinical translation, DFO* and 3,4,3-(LI-1,2-HOPO), is still missing. Thus, we directly compared DFO with DFO* and 3,4,3-(LI-1,2-HOPO) immunoconjugates to identify the most suitable agent stable 89Zr-complexation. The chelators were introduced into cetuximab, and an optical analysis method was developed, enabling the efficient quantification of derivatization sites per protein. The cetuximab conjugates were efficiently obtained and radiolabeled with 89Zr at 37 °C within 30 min, giving the [89Zr]Zr-cetuximab derivatives in high radiochemical yields and purities of >99% as well as specific activities of 50 MBq/mg. The immunoreactive fraction of all 89Zr-labeled cetuximab derivatives was determined to be in the range of 86.5–88.1%. In vivo PET imaging and ex vivo biodistribution studies in tumor-bearing animals revealed a comparable and significantly higher kinetic inertness for both [89Zr]Zr-3,4,3-(LI-1,2-HOPO)-cetuximab and [89Zr]Zr-DFO*-cetuximab, compared to [89Zr]Zr-DFO-cetuximab. Of these, [89Zr]Zr-DFO*-cetuximab showed a considerably more favorable pharmacokinetic profile with significantly lower liver and spleen retention than [89Zr]Zr-3,4,3-(LI-1,2-HOPO)-cetuximab. Since [89Zr]Zr-DFO* demonstrates a very high kinetic inertness, paired with a highly favorable pharmacokinetic profile of the resulting antibody conjugate, DFO* currently represents the most suitable chelator candidate for stable 89Zr-radiolabeling of antibodies and clinical translation

αvβ3-Specific Gold Nanoparticles for Fluorescence Imaging of Tumor Angiogenesis

This paper reports on the development of tumor-specific gold nanoparticles (AuNPs) as theranostic tools intended for target accumulation and the detection of tumor angiogenesis via optical imaging (OI) before therapy is performed, being initiated via an external X-ray irradiation source. The AuNPs were decorated with a near-infrared dye, and RGD peptides as the tumor targeting vector for αvβ3-integrin, which is overexpressed in tissue with high tumor angiogenesis. The AuNPs were evaluated in an optical imaging setting in vitro and in vivo exhibiting favorable diagnostic properties with regards to tumor cell accumulation, biodistribution, and clearance. Furthermore, the therapeutic properties of the AuNPs were evaluated in vitro on pUC19 DNA and on A431 cells concerning acute and long-term toxicity, indicating that these AuNPs could be useful as radiosensitizers in therapeutic concepts in the future