A Modular Synthesis of Salvileucalin B Structural Domains

Rapid Access to Divergent Fused Polycycles Via One-Pot A3 Coupling and Intramolecular Diels-Alder Reaction

Divergent nitrogen-containing fused polycyclic ring systems are constructed from simple starting materials via a one-pot aldehyde-alkyne-amine (A3) coupling and intramolecular Diels-Alder reaction. This domino reaction directly furnishes linear 5/5/5 and 5/5/6, or nonlinear 5/5/6/5, polycyclic rings containing an oxa-bridged fused 5/5 bicycle and a 1,6-enyne substructure. One-step derivation of the oxa-bridged 5/5 bicycle leads to a polyfunctionalized 5/5 bicycle with tetrahydrofuran fused back-to-back to pyrroline or a 6/5 bicycle with the hexahydro-1H-isoindole structure, while cycloisomerizing the enyne substructure adds an extra fused 5-membered ring to afford functionalized linear 5/5/5/5 or 5/5/5/5/5 fused ring systems from selected substrates. In addition, the one-pot product can be designed so that the alkyne moiety is hydroalkoxylated to form an additional heterocyle in a linear 5/5/5/6 or nonlinear 5/5/6/5/5 ring system. This diversity-oriented synthetic approach thus allows rapid access to an under-explored structural space for discovery of new biological or non-biological activities or functions.

Asymmetric Synthesis and Biological Evaluation of Platensilin, Platensimycin, Platencin, and Their Analogs via a Bioinspired Skeletal Reconstruction Approach

Platensilin, platensimycin, and platencin are potent inhibitors of β-ketoacyl-acyl carrier protein synthase (FabF) in the bacterial and mammalian fatty acid synthesis system, presenting promising drug leads for both antibacterial and antidiabetic therapies. Herein, a bioinspired skeleton reconstruction approach is reported, which enables the unified synthesis of these three natural FabF inhibitors and their skeletally diverse analogs, all stemming from a common ent-pimarane core. The synthesis features a diastereoselective biocatalytic reduction and an intermolecular Diels-Alder reaction to prepare the common ent-pimarane core. From this intermediate, stereoselective Mn-catalyzed hydrogen atom-transfer hydrogenation and subsequent Cu-catalyzed carbenoid C-H insertion afford platensilin. Furthermore, the intramolecular Diels-Alder reaction succeeded by regioselective ring opening of the newly formed cyclopropane enables the construction of the bicyclo[3.2.1]-octane and bicyclo[2.2.2]-octane ring systems of platensimycin and platencin, respectively. This skeletal reconstruction approach of the ent-pimarane core facilitates the preparation of analogs bearing different polycyclic scaffolds. Among these analogs, the previously unexplored cyclopropyl analog 47 exhibits improved antibacterial activity (MIC80 = 0.0625 μg/mL) against S. aureus compared to platensimycin.

Total Synthesis of Vilmoraconitine

Vilmoraconitine belongs to one of the most complex skeleton types in the C19-diterpenoid alkaloids, which architecturally features an unprecedented heptacyclic core possessing a rigid cyclopropane unit. Here, we report the first total synthesis of vilmoraconitine relying on strategic use of efficient ring-forming reactions. Key steps include an oxidative dearomatization-induced Diels-Alder cycloaddition, a hydrodealkenylative fragmentation/Mannich sequence, and an intramolecular Diels-Alder cycloaddition.

Mild Intermolecular Synthesis of a Cyclopropane-Containing Tricyclic Skeleton: Unusual Reactivity of Isobenzopyryliums

Cyclopropanes embedded in a polycyclic bridged architecture are a versatile structural motif, but such complex frameworks often impose substantial synthetic challenges. Herein we introduce a new approach for the expedient access to such spring-loaded strained systems via an exceptionally mild intermolecular convergent process between the readily available isobenzopyryliums and vinyl boronic acids. Different from the typical conventional approaches, our protocol does not involve the highly active carbenoid intermediates or strong conditions in order to overcome the disfavored kinetic and thermodynamic problems. Instead, the key cyclopropane ring was formed between the well-positioned nucleophile and electrophile in the adduct from the regioselective [4+2] cycloaddition. Thus, this unusual process also represents a new reactivity of the versatile isobenzopyryliums. The choice of a Brønsted acid catalyst with proper acidity is crucial to the high efficiency and selectivity for this multiple bond-forming process. The strained products are precursors to other useful synthetic building blocks.

Discovery and Surprises with Cyclizations, Cycloadditions, Fragmentations, and Rearrangements in Complex Settings

We discuss a number of synthesis routes to complex natural products recently reported from our group. Although the structures are quite varied, we demonstrate the research endeavor as a setting to examine the implementation of cyclizations, cycloadditions, rearrangements, and fragmentations. We showcase how the various transformations enabled access to key core structures and thereby allowed the rapid introduction of complexity. Two different routes to (-)-mitrephorone A, the first case discussed, led to the use of Koser's reagent to effect oxetane formation from diosphenol derivatives. Even though the Diels-Alder cycloaddition reaction represents one of the workhorses of complex molecule synthesis, there are opportunities provided by the complexity of secondary metabolites for discovery, study, and development. In our first approach to (-)-mitrephorone A, Diels-Alder cycloaddition provided access to fused cyclopropanes, while the second synthesis underscored the power of diastereoselective nitrile oxide cycloadditions to access hydroxy ketones. The successful implementation of the second approach required the rigorous stereocontrolled synthesis of tetrasubstituted olefins; this was accomplished by a highly stereoselective Cr-mediated reduction of dienes. The diterpenoid (+)-sarcophytin provided a stage for examining the Diels-Alder cycloaddition of two electron-deficient partners. The study revealed that in the system this unusual combination works optimally with the E,Z-dienoate and proceeds through an exo transition state to provide the desired cycloadduct. Our reported pallambin synthesis showcased the use of fulvene as a versatile building block for the core structure. Fulvene decomposition could be outcompeted by employing it as a diene and using a highly reactive dienophile, which affords a bicyclic product that can in turn be subjected to chemo- and stereoselective manipulations. The synthesis route proceeds with a C-H insertion providing the core structure en route to pallambin A and B. The studies resulting in our synthesis of gelsemoxonine highlight the use of the acid-catalyzed rearrangement/chelotropic extrusion of oxazaspiro[2.4]heptanes to access complex β-lactams, which are otherwise not readily prepared by extant methods in common use. Mechanistic investigations of the intriguing ring contraction supported by computational studies indicate that the reaction involves a concerted cleavage of the N-O bond and cyclopropane ring opening under the extrusion of ethylene. The synthesis of guanacastepenes focused on the use of cyclohexyne in [2+2]-cycloadditions with enolates. The resulting cyclobutene can be enticed to undergo ring opening to give a fused six-seven ring system. The cycloinsertion reaction of cyclohexyne developed for the first time proves useful as a general approach to complex fused ring systems.

Biomimetic Total Synthesis of (±)-Carbocyclinone-534 Reveals Its Biosynthetic Pathway

Carbocyclinone-534 is a new antibiotic produced after the metabolism of tapinarof. We identify a biomimetic total synthesis of carbocyclinone-534 in eight steps by taking advantage of an intermolecular Diels-Alder homodimerization/dehydrogenation/intramolecular Diels-Alder cycloaddition cascade. This synthetic sequence provides direct experimental evidence for revealing the biosynthetic pathway of carbocyclinone-534.

Total Synthesis of (-)-Mitrephorone A

The first synthesis of (-)-mitrephorone A is disclosed along with discussion and study of synthetic strategies. The natural product includes a highly congested hexacyclic ent-trachylobane diterpenoid framework featuring a rare, embedded oxetane. The synthetic analysis presented dissects a number of approaches for the synthesis of the central oxetane, including carbonyl-olefin photocycloadditions, Prins-type cyclizations, and oxidative ring closures. In the successful route, three [4 + 2] cycloadditions enable rapid construction of all carbocycles. A novel late-stage oxidative cyclization of a hydroxy diosphenol with Koser's reagent furnishes the pivotal oxetane moiety.

Propellanes-From a Chemical Curiosity to "Explosive" Materials and Natural Products

Propellanes are a unique class of compounds currently consisting of well over 10 000 representatives, all featuring two more or less inverted tetrahedral carbon atoms that are common to three bridging rings. The central single bond between the two bridgeheads is significantly weakened in the smaller entities, which leads to unusual reactivities of these structurally interesting propeller-like molecules. This Review highlights the synthesis of such propellanes and their occurrence in material sciences, natural products, and medicinal chemistry. The conversion of [1.1.1]propellane into bridgehead derivatives of bicyclo[1.1.1]pentane, including oligomers and polymers with bicyclo[1.1.1]penta-1,3-diyl repeat units, is also featured. A selection of natural products with larger propellane subunits are discussed in detail. Heteropropellanes and inorganic propellanes are also addressed. The historical background is touched in brief to show the pioneering work of David Ginsburg, Günther Snatzke, Kenneth B. Wiberg, Günter Szeimies, and others.

Enantioselective synthesis of the 5-6-7 carbocyclic core of the gagunin diterpenoids

A catalytic enantioselective double allylic alkylation reaction has been employed in the synthesis of the core of the gagunin diterpenoids. Enantioenriched material was advanced in 11 steps to afford the core of the highly oxygenated target, which includes two all-carbon quaternary stereocenters.

Probing for the Pharmacophore of the Cytotoxic Neoclerodane Salvileucalin B

The novel [4.3.1]propelladiene 2, which embodies the key structural elements of the pentacyclic core of the cytotoxic neoclerodane salvileucalin B (1), has been prepared using a rhodium-catalysed intramolecular Büchner reaction as the key step. Compound 2 and the readily obtained derivatives 12–17 all proved to be essentially inactive when tested against a panel of four human cancer cell lines. Furthermore, not one of these compounds was a P-gp inhibitor.