Contra‐Thermodynamic,Photocatalytic E→Z Isomerization of Styrenyl Boron Species: Vectors to Facilitate Exploration of Two‐Dimensional Chemical Space

Designing strategies to access stereodefined olefinic organoboron species is an important synthetic challenge. Despite significant advances, there is a striking paucity of routes to Z‐α‐substituted styrenyl organoborons. Herein, this strategic imbalance is redressed by exploiting the polarity of the C(sp²)−B bond to activate the neighboring π system, thus enabling a mild, traceless photocatalytic isomerization of readily accessible E‐α‐substituted styrenyl BPins to generate the corresponding Z‐isomers with high fidelity. Preliminary validation of this contra‐thermodynamic E→Z isomerization is demonstrated in a series of stereoretentive transformations to generate Z‐configured trisubstituted alkenes, as well as in a concise synthesis of the anti‐tumor agent Combretastatin A4.     

Two‐Dimensional Boron Hydride Sheets: High Stability,Massless Dirac Fermions,and Excellent Mechanical Properties

Two‐dimensional (2D) boron sheets have been successfully synthesized in recent experiments, however, some important issues remain, including the dynamical instability, high energy, and the active surface of the sheets. In an attempt to stabilize 2D boron layers, we have used density functional theory and global minimum search with the particle‐swarm optimization method to predict four stable 2D boron hydride layers, namely the C2/m, Pbcm, Cmmm, and Pmmn sheets. The vibrational normal mode calculations reveal all these structures are dynamically stable, indicating potential for successful experimental synthesis. The calculated Young's modulus indicates a high mechanical strength for the C2/m and Pbcm phases. Most importantly, the C2/m, Pbcm, and Pmmn structures exhibit Dirac cones with massless Dirac fermions and the Fermi velocities for the Pbcm and Cmmm structures are even higher than that of graphene. The Cmmm phase is reported as the first discovery of Dirac ring material among boron‐based 2D structures. The unique electronic structure of the 2D boron hydride sheets makes them ideal for nanoelectronics applications.     

Palladium(0)‐Catalyzed Cross‐Coupling of Potassium (Z)‐2‐Chloroalk‐1‐enyl Trifluoroborates: A Chemo‐ and Stereoselective Access to (Z)‐Chloroolefins and Trisubstituted Alkenes

Original and ambivalent : We describe the preparation of a series of new potassium trifluoroborates 1 , and the study of their behaviour in a Pd⁰‐catalyzed cross‐coupling reaction. We found that compounds 1 are endowed with original properties as they behave as nucleophilic cross‐coupling partners chemoselectively, but also as ambivalent synthons. The usefulness of this methodology has been successfully illustrated by the first total synthesis of an N‐acyl spermidine.

     

Substrate‐Assisted,Transition‐Metal‐Free Diboration of Alkynamides with Mixed Diboron: Regio‐ and Stereoselective Access to trans‐1,2‐Vinyldiboronates

A substrate‐assisted diboration of alkynamides using the unsymmetrical pinacolato‐1,8‐diaminonaphthalenato diboron (pinBBdan) is described. The transition‐metal‐free reaction proceeds in a regio‐ and stereoselective fashion to exclusively afford trans ‐vinyldiboronates in good to excellent yields. Notably, Bdan and Bpin are installed on the α‐ and β‐carbon atoms, respectively.     

Stereocontrolled Synthesis of Adjacent Acyclic Quaternary‐Tertiary Motifs: Application to a Concise Total Synthesis of (−)‐Filiformin

Lithiation/borylation methodology has been developed for the synthesis of acyclic quaternary‐tertiary motifs with full control of relative and absolute stereochemistry, thus leading to all four possible isomers of a stereodiad. A novel intramolecular Zweifel‐type olefination enabled acyclic stereocontrol to be transformed into cyclic stereocontrol. These key steps have been applied to the shortest enantioselective synthesis of (?)‐filiformin to date (9 steps) with full stereocontrol.     

Total Synthesis and Conformational Study of Callyaerin A: Anti‐Tubercular Cyclic Peptide Bearing a Rare Rigidifying (Z)‐2,3‐ Diaminoacrylamide Moiety

The first synthesis of the anti‐TB cyclic peptide callyaerin A ( 1 ), containing a rare (Z)‐2,3‐diaminoacrylamide bridging motif, is reported. Fmoc‐formylglycine‐diethylacetal was used as a masked equivalent of formylglycine in the synthesis of the linear precursor to 1 . Intramolecular cyclization between the formylglycine residue and the N‐terminal amine in the linear peptide precursor afforded the macrocyclic natural product 1 . Synthetic 1 possessed potent anti‐TB activity (MIC₁₀₀=32 μm ) while its all‐amide congener was inactive. Variable‐temperature NMR studies of both the natural product and its all‐amide analogue revealed the extraordinary rigidity imposed by this diaminoacrylamide unit on peptide conformation. The work reported herein pinpoints the intrinsic role that the (Z)‐2,3‐diaminoacrylamide moiety confers on peptide bioactivity.     

Elucidation of Pseurotin Biosynthetic Pathway Points to Trans‐Acting C‐Methyltransferase: Generation of Chemical Diversity

Pseurotins comprise a family of structurally related Aspergillal natural products having interesting bioactivity. However, little is known about the biosynthetic steps involved in the formation of their complex chemical features. Systematic deletion of the pseurotin biosynthetic genes in A. fumigatus and in vivo and in vitro characterization of the tailoring enzymes to determine the biosynthetic intermediates, and the gene products responsible for the formation of each intermediate, are described. Thus, the main biosynthetic steps leading to the formation of pseurotin A from the predominant precursor, azaspirene, were elucidated. The study revealed the combinatorial nature of the biosynthesis of the pseurotin family of compounds and the intermediates. Most interestingly, we report the first identification of an epoxidase C‐methyltransferase bifunctional fusion protein PsoF which appears to methylate the nascent polyketide backbone carbon atom in trans.     

NMR Fingerprints of the Drug‐like Natural‐Product Space Identify Iotrochotazine A: A Chemical Probe to Study Parkinson’s Disease

The NMR spectrum of a mixture of small molecules is a fingerprint of all of its components. Herein, we present an NMR fingerprint method that takes advantage of the fact that fractions contain simplified NMR profiles, with minimal signal overlap, to allow the identification of unique spectral patterns. The approach is exemplified in the identification of a novel natural product, iotrochotazine A ( 1 ), sourced from an Australian marine sponge Iotrochota sp. Compound 1 was used as a chemical probe in a phenotypic assay panel based on human olfactory neurosphere‐derived cells (hONS) from idiopathic Parkinson’s disease patients. Compound 1 at 1 μM was not cytotoxic but specifically affected the morphology and cellular distribution of lysosomes and early endosomes.     

Synthesis of Cyclic Alkenylsiloxanes by Semihydrogenation: A Stereospecific Route to (Z)‐Alkenyl Polyenes

Cyclic alkenylsiloxanes were synthesized by semihydrogenation of alkynylsilanes—a reaction previously plagued by poor stereoselectivity. The silanes, which can be synthesized on multigram scale, undergo Hiyama–Denmark coupling to give (Z)‐alkenyl polyene motifs found in bioactive natural products. The ring size of the silane is crucial: five‐membered cyclic siloxanes also couple under fluoride‐free conditions, whilst their six‐membered homologues do not, enabling orthogonality within this structural motif.     

Synthesis of (Z)‐Trisubstituted Olefins by Decarboxylative Grob‐Type Fragmentations: Epothilone D,Discodermolide, and Peloruside A

Methyl‐branched (Z)‐trisubstituted olefins are found in many polyketides with interesting biological activity, such as epothilone D ( 1 ), discodermolide ( 3 ), and peloruside A ( 2 ). Despite the employment of numerous different strategies, this motif has often been the weak point in total synthesis. Thus, we present a novel hydroxide‐ induced Grob‐type fragmentation as an easy access to trisubstituted olefins. In our case, β‐mesyloxy δ‐lactones with three stereogenic centers were chosen whose fragmentation underlies a high stereoelectronic control. Major challenges in the syntheses were the installation of quaternary stereocenters, achieved by enzymatic desymmetrization of meso‐diesters and by aluminium‐promoted stereoselective rearrangement of chiral epoxides, respectively. Different aldol strategies were developed for the formation of the fragmentation precursors. Additionally a short survey about nucleophilic additions to aldehydes with quaternary α‐centers is presented.     

Determination of the Absolute Configuration of Super‐Carbon‐Chain Compounds by a Combined Chemical,Spectroscopic, and Computational Approach: Gibbosols A and B

Marine dinoflagellates produce remarkable organic molecules, particularly those with polyoxygenated long‐carbon‐chain backbones, namely super‐carbon‐chain compounds (SCCCs), characterized by the presence of numerous stereogenic carbon centers on acyclic polyol carbon chains. Even today, it is a challenge to determine the absolute configurations of these compounds. In this work, the planar structures and absolute configurations of two highly flexible SCCCs, featuring either a C₆₉‐ or C₇₁‐linear carbon backbone, gibbosols A and B, respectively, each containing thirty‐seven stereogenic carbon centers, were unambiguously established by a combined chemical, spectroscopic, and computational approach. The discovery of gibbosols A and B with two hydrophilic acyclic polyol chains represents an unprecedented class of SCCCs. A reasonable convergent strategy for the biosynthesis of these SCCCs was proposed.     

The Cobalt Way to Angucyclinones: Asymmetric Total Synthesis of the Antibiotics (+)‐Rubiginone B2, (−)‐Tetrangomycin,and (−)‐8‐O‐Methyltetrangomycin

A cobalt(I)‐mediated convergent and asymmetric total synthesis of angucyclinones with an aromatic B ring has been developed. In the course of our research, we synthesized three naturally occurring anguclinone derivatives, namely, (+)‐rubiginone B₂ ( 1 ), (?)‐8‐O‐methyltetrangomycin ( 2 ), and (?)‐tetrangomycin ( 3 ). By combining 3‐hydroxybenzoic acid, 3‐methoxybenzoic acid, citronellal, and geraniol as starting materials in a convergent way, we were able to synthesize chiral triyne chains, which were cyclized with [CpCo(C₂H₄)₂] (Cp=cyclopentadienyl) by means of an intramolecular [2+2+2] cycloaddition to their corresponding tetrahydrobenzo[a]anthracenes. Successive oxidation and deprotection steps led to the above‐mentioned natural products 1 – 3 .     

A Flexible Approach to Azasugars: Asymmetric Total Syntheses of (+)‐Castanospermine, (+)‐7‐Deoxy‐6‐epi‐castanospermine,and (+)‐1‐epi‐Castanospermine

The asymmetric total synthesis of natural azasugars (+)‐castanospermine, (+)‐7‐deoxy‐6‐epi‐castanospermine, and synthetic (+)‐1‐epi‐castanospermine has been accomplished in nine to ten steps from a common chiral building block (S)‐ 8 . The method features a powerful chiral relay strategy consisting of a highly diastereoselective vinylogous Mukaiyama‐type reaction with either chiral or achiral aldehydes (≥95 % de; de=diastereomeric excess) and a diastereodivergent reduction of tetramic acids, which allows formation of three continuous stereogenic centers with high diastereoselectivities. The method also provides a flexible access to structural arrays of 5‐(α‐hydroxyalkyl)tetramic acids, such as 17/34 , and 5‐(α‐hydroxyalkyl)‐4‐hydroxyl‐2‐pyrrolidinones, such as 18 and 25/35 a . The method constitutes the first realization of the challenging chiral synthons A and D and thus of the conceptually attractive retrosynthetic analysis shown in Scheme 1 in a highly enantioselective manner.     

A Unified Approach to the Daucane and Sphenolobane Bicyclo[5.3.0]decane Core: Enantioselective Total Syntheses of Daucene,Daucenal, Epoxydaucenal B,and 14‐para‐Anisoyloxydauc‐4,8‐diene

Access to the bicyclo[5.3.0]decane core found in the daucane and sphenolobane terpenoids via a key enone intermediate enables the enantioselective total syntheses of daucene, daucenal, epoxydaucenal B, and 14‐para‐anisoyloxydauc‐4,8‐diene. Central aspects include a catalytic asymmetric alkylation followed by a ring contraction and ring‐closing metathesis to generate the five‐ and seven‐membered rings, respectively.     

Cassia obtusifolia MetE as a Cytosolic Target for Potassium Isolespedezate,a Leaf‐Opening Factor of Cassia plants: Target Exploration by a Compact Molecular‐Probe Strategy

Affinity chromatography by using ligand‐immobilized bead technology is generally the first choice for target exploration of a bioactive ligand. However, when a ligand has comparatively low affinity against its target, serious difficulties will be raised in affinity‐based target detection. We report here that the use of compact molecular probes (CMP) will be advantageous in such cases; it enables the retention of moderate affinity between the ligand and its target in contrast to immobilizing the ligand on affinity beads that will cause a serious drop in affinity to preclude target detection. In the CMP strategy, a CMP containing an azide handle is used for an initial affinity‐based labeling of target, and subsequent tagging by CuAAC with a large FLAG tag will give a tagged target protein. By using the CMP strategy, we succeeded in the identification of Cassia obtusifolia MetE as a cytosolic target protein of potassium isolespedezate ( 1 ), a moderately bioactive ligand.