Access to the protoilludane core by gold-catalyzed allene-vinylcyclopropane cycloisomerization

The Rapid Synthesis of Colibactin Warhead Model Compounds Using New Metal-Free Photocatalytic Cyclopropanation Reactions Facilitates the Investigation of Biological Mechanisms

Herein, we report the synthesis of a series of colibactin warhead model compounds using two newly developed metal-free photocatalytic cyclopropanation reactions. These mild cyclopropanations expand the known applications of eosin within synthesis. A halogen atom transfer reaction mode has been harnessed so that dihalides can be used as the cyclopropanating agents. The colibactin warhead models were then used to provide new insight into two key mechanisms in colibactin chemistry. An explanation is provided for why the colibactin warhead sometimes undergoes a ring expansion-addition reaction to give fused cyclobutyl products while at other times nucleophiles add directly to the cyclopropyl unit (as when DNA adds to colibactin). Finally, we provide some evidence that Cu(II) chelated to colibactin may catalyze an important oxidation of the colibactin-DNA adduct. The Cu(I) generated as a result could then also play a role in inducing double strand breaks in DNA.

Asymmetric Total Synthesis of Shagenes A and B

We report the first total synthesis of shagenes A and B, which are tricyclic terpenoids containing a cis-substituted cyclopropane, via ring-closing metathesis of an enamide and Ir-catalyzed double-bond isomerization of an alkylidenecyclopropane. Chemo- and diastereoselectivity in the distorted cis-substituted structures were controlled by the alkylidenecyclopropane reactivity and using the ketone functionality as a remote directing group for the Ir catalyst, respectively. The total synthesis suggested the absolute configuration of shagenes.

Small rings in the bigger picture: ring expansion of three- and four-membered rings to access larger all-carbon cyclic systems

The release of the inherent ring strain of cyclobutane and cyclopropane derivatives allows a rapid build-up of molecular complexity. This review highlights the state-of-the-art of the ring expansions of three- and four-membered cycles and is organised by types of reactions with emphasis on the reaction mechanisms. Selected examples are discussed to illustrate the synthetic potential of this elegant synthetic tool.

Gold-Catalyzed Synthesis of Small Rings

Three- and four-membered rings, widespread motifs in nature and medicinal chemistry, have fascinated chemists ever since their discovery. However, due to energetic considerations, small rings are often difficult to assemble. In this regard, homogeneous gold catalysis has emerged as a powerful tool to construct these highly strained carbocycles. This review aims to provide a comprehensive summary of all the major advances and discoveries made in the gold-catalyzed synthesis of cyclopropanes, cyclopropenes, cyclobutanes, cyclobutenes, and their corresponding heterocyclic or heterosubstituted analogs.

Gold-Catalyzed Synthetic Strategies towards Four-Carbon Ring Systems

Four carbon ring systems are frequently present in natural products with remarkable biological activities such as terpenoids, alkaloids, and steroids. The development of new strategies for the assembly of these structures in a rapid and efficient manner has attracted the interest of synthetic chemists for a long time. The current research is focused mainly on the development of synthetic methods that can be performed under mild reaction conditions with a high tolerance to functional groups. In recent years, gold complexes have turned into excellent candidates for this aim, owing to their high reactivity, and are thus capable of promoting a wide range of transformations under mild conditions. Their remarkable efficiency has been thoroughly demonstrated in the synthesis of complex organic molecules from simple starting materials. This review summarizes the main synthetic strategies described for gold-catalyzed four-carbon ring formation, as well as their application in the synthesis of natural products.

Concise Total Synthesis of (+)-Atlanticone C

The first enantioselective total synthesis of (+)-atlanticone C is described. The complex tricyclic protoilludane core was rapidly assembled by a photochemical reaction cascade starting from an easily accessible indanone precursor (3 steps). Optimization of an enantioselective Corey–Bakshi–Shibata reduction permitted a catalytic chiral reso­lution of the racemic photoproduct (45% over two steps; up to 98% ee). The enantiomerically enriched photoproduct was efficiently transformed into the (+)-enantiomer of atlanticone C (10 steps; 18% yield), and the absolute configuration of naturally occurring (–)-atlanticone C was thereby determined.

Ring-fused cyclobutanes via cycloisomerization of alkylidenecyclopropane acylsilanes

A novel Lewis acid-catalyzed cycloisomerization of alkylidenecyclopropane acylsilanes is disclosed. The readily available starting materials participate in tandem Prins addition/ring expansion/1,2-silyl shift to grant access to bicyclo[4.2.0]octanes and bicyclo[3.2.0]heptanes, which are common motifs in terpenoid natural products. Notably, the transformation relies on the ability of acylsilanes to act sequentially as acceptors and donors on the same carbon atom.

A novel Lewis acid-catalyzed cycloisomerization of alkylidenecyclopropane acylsilanes is disclosed that involves tandem Prins addition/ring expansion/1,2-silyl shift to grant access to bicyclo[4.2.0]octanes and bicyclo[3.2.0]heptanes.

Total Synthesis and Structural Revision of a Harziane Diterpenoid

The first total synthesis of nominal harziane diterpenoid 1 is disclosed, whose spectral characteristics did not match those of the reported natural product. Stereochemical analysis and subsequent synthesis of the epimeric tertiary alcohol led to reassignment of configuration of the natural product as shown for 2. At the heart of the synthesis is an enyne cycloisomerization that sets a key quaternary stereocenter within a cyclobutane with high diastereocontrol. The route features strategies for the synthesis of the highly congested 6-5-7-4 carbon skeleton characteristic of the caged harziane diterpenoids.

Concise Access to the Skeleton of Protoilludane Sesquiterpenes through a Photochemical Reaction Cascade: Total Synthesis of Atlanticone C

In a single photochemical operation (λ≥350 nm) an easily accessible indanone derivative was converted into a structurally complex precursor of the protoilludane sesquiterpenes. The product (60 % yield) contains all 15 carbon atoms of the skeleton in the required connectivity and was transformed into the natural product atlanticone C (9 steps, 6 % overall yield). In addition, it was shown that other protoilludanes, such as Δ⁶ -protoilludene and paesslerin A, can be prepared in a concise fashion via the photochemical key intermediate. The photochemical reaction cascade comprises an ortho photocycloaddition, a thermal disrotatory ring opening and a regioselective disrotatory [4π] photocyclization.

Total Syntheses of Allelopathic 4-Oxyprotoilludanes, Melleolides, and Echinocidins

Stereocontrolled total syntheses of allelopathic 4-oxyprotoilludane sesquiterpenes, melleolide, melleolide F, and echinocidins B and D were achieved. The curved 5/6/4 tricyclic system with an angular hydroxy group was built via three key transformations: (1) Me₃Al-catalyzed [2 + 2] cycloaddition of a ketene silyl acetal with a propiolate, (2) reductive ring-opening of a cyclic hemiketal, and (3) the intramolecular Morita-Baylis-Hillman reaction. This synthetic route represents a new and reliable strategy to obtain protoilludanes with several oxy-functional groups.

Rapid Assembly of Protoilludane Skeleton through Tandem Catalysis: Total Synthesis of Paesslerin A and Its Structural Revision

A multicomponent domino reaction involving three mechanistically distinct Tf₂NH-catalyzed reactions was developed. The reaction cascade enables the assembly of a skewed 5/6/4 tricyclic motif with migration of the reactive site with the assistance of a catalyst. The tricyclic product was used to achieve the first total synthesis of cytotoxic paesslerin A by regioselective C-H insertion of the sulfonyl carbenoid and base-promoted olefin isomerization. Our results led to the revision of the originally proposed tricyclic structure of paesslerin A.

Controlling the Selectivity Patterns of Au-Catalyzed Cyclization-Migration Reactions

As little as 2 mol % of (XPhos)AuNTf₂ catalyzes the transformation of a broad range of o-acetylene-substituted styrenes into 1,2-dihydronaphthalenes. Our data suggests that this transformation occurs via a gold-stabilized cyclopropyl carbinyl cation, which triggers either a [1,2] carboxylate shift or a less favorable [1,2] aryl shift. The relative rates of these migrations can be controlled by the identity of the ligand or by stabilizing the mesomeric cation.

Protoilludane, Illudalane, and Botryane Sesquiterpenoids from the Endophytic Fungus Phomopsis sp. TJ507A

To explore the chemical diversity of metabolites from endophytic fungi, the strain Phomopsis sp. TJ507A, isolated from the medicinal plant Phyllanthus glaucus, was investigated. A 2,3- seco-protoilludane-type sesquiterpenoid (1), eight protoilludane-type sesquiterpenoids (2-9), four illudalane-type sesquiterpenoids (10a/10b, 11, and 12), and a botryane-type sesquiterpenoid (13) in addition to seven known sesquiterpenoids (14-20) were identified from the liquid culture of the fungus. Structures of the isolated compounds, including their absolute configurations, were elucidated based on extensive spectroscopic analyses, a modified Mosher analysis, electronic circular dichroism (ECD) calculations, and [Rh₂(OCOCF₃)₄]-induced ECD spectra as well as X-ray crystallographic analyses. Compound 1 represents the first example of a naturally occurring sesquiterpenoid containing the unusual 2,3- seco-protoilludane scaffold. Compounds 1 ( p < 0.001); 2-6, 15, and 18 ( p < 0.01); and 7, 9, and 20 ( p < 0.05) displayed β-site amyloid precursor protein cleaving enzyme 1 (BACE1) inhibitory activities ranging from 19.4% to 43.8% at the concentration of 40 μM. LY2811376 was used as the positive control with an inhibitory activity of 38.6% ( p < 0.01). Furthermore, none of these compounds showed obvious hepatotoxicity at concentration of 40 μM.

Enantioselective gold(i)-catalyzed rearrangement of cyclopropyl-substituted 1,6-enynes into 2-oxocyclobutyl-cyclopentanes

A gold(i)-catalyzed cycloisomerization/ring expansion sequence allows the highly enantioselective synthesis of 2-oxocyclobutylcyclopentane derivatives from cyclopropyl-substituted enynes (up to 99% ee).

Gold(i)-catalyzed addition of aldehydes to cyclopropylidene bearing 6-aryl-1,5-enynes

A diastereoselective, gold-catalyzed cascading cycloisomerization of alkylidene cyclopropane bearing 1,5-enynes that terminates in a cyclo-addition of aldehydes has been developed. This diastereoselective reaction provides convergent access to novel polycyclic molecular structures (18 examples), and tolerates a diverse scope of aldehydes. Mechanistic studies reveal that the catalytic cycle rests at a digold off-cycle intermediate, one of which was isolated.

Platinum-catalyzed cycloisomerizations of a common enyne: a divergent entry to cyclopropane sesquiterpenoids. Formal synthesis of sarcandralactone A

A common enyne scaffold, resembling the structures of natural elemanes was found to be an excellent substrate for highly regioselective cycloisomerizations to produce diverse cyclopropane sesquiterpenoids. Platinum-catalysis was utilized to produce either lindenane or myliol cores, found in natural products, starting from enyne acetate 10 and its corresponding allene 12 respectively. Based on this concept, a second generation strategy allows the formal synthesis of sarcandralactone A.

Chemoenzymatic Total Syntheses of the Enantiomers of the Protoilludanes 8-Deoxydihydrotsugicoline and Radudiol

Chemoenzymatic and stereoselective total syntheses of the non-natural enantiomeric forms of the recently isolated protoilludane natural products 8-deoxydihydrotsugicoline (1) and radudiol (2) (viz. ent-1 and ent-2, respectively) are reported. The key steps involve the Diels-Alder cycloaddition of cyclopent-2-en-1-one to the acetonide derived from enantiomerically pure and enzymatically derived cis-1,2-dihydrocatechol 3, elaboration of the resulting adduct to the tricyclic ketone 12, and a photochemically promoted rearrangement of this last compound to the octahydro-1H-cyclobuta[e]indenone 13.

Metal-catalyzed cycloisomerization as a powerful tool in the synthesis of complex sesquiterpenoids

Metal-catalyzed cycloisomerization reactions can be regarded as “artificial sesquiterpenoid cyclase” tools in modern organic synthesis, and their progress is reviewed.

Gold-catalyzed cycloisomerization of 1,6-diyne esters to 1H-cyclopenta[b]naphthalenes, cis-cyclopenten-2-yl δ-diketones, and bicyclo[3.2.0]hepta-1,5-dienes

A synthetic method to chemoselectively prepare 1H-cyclopenta[b]naphthalenes, cis-cyclopenten-2-yl δ-diketones, and bicyclo[3.2.0]hepta-1,5-dienes efficiently by gold-catalyzed cycloisomerization of 1,6-diyne esters is described. These three product classes were accessed divergently by taking advantage of the electronic and steric differences between a phosphine and NHC (NHC = N-heterocyclic carbene) ligand in the respective gold(I) complexes and that of gold(III) complex combined with substrate substitution patterns and optimized reaction conditions. In the presence of [PhCNAuIPr](+)SbF6(-) (IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidine) as the catalyst, substrates with a pendant aryl group at the acetate alkynyl position were found to undergo preferential 1,3-acyloxy migration/5-exo-dig cyclization/Friedel-Crafts reaction to give 1H-cyclopenta[b]naphthalenes. In contrast, the analogous reactions with PicAuCl2 (Pic =2-picolinate) were found to proceed by selective 1,3-acyloxy migration/5-exo-dig cyclization/1,5-acyl migration to afford cis-cyclopenten-2-yl δ-diketones. Changing the catalyst to [MeCNAu(JohnPhos)](+)SbF6(-) (JohnPhos = (1,1'-biphenyl-2-yl)-di-tert-butylphosphine) and the acetate alkynyl position from an aryl to vinyl substituent in the starting ester led to 1,3-acyloxy migration/5-exo-dig cyclization/Prins-type [2 + 2]-cycloaddition to provide bicyclo[3.2.0]hepta-1,5-dienes.

Gold-catalyzed diastereoselective cycloisomerization of alkylidene-cyclopropane-bearing 1,6-diynes

An unprecedented gold-catalyzed diastereoselective cycloisomerization of 1,6-diynes bearing an alkylidene cyclopropane moiety has been developed. This methodology enables rapid access to a variety of 1,2-trimethylenenorbornanes, which are important building blocks in the preparations of abiotic and sesquiterpene core structures.

Gold(I) as an artificial cyclase: short stereodivergent syntheses of (-)-epiglobulol and (-)-4β,7α- and (-)-4α,7α-aromadendranediols

Three natural aromadendrane sesquiterpenes, (-)-epiglobulol, (-)-4β,7α-aromadendranediol, and (-)-4α,7α-aromadendranediol, have been synthesized in only seven steps in 12, 15, and 17 % overall yields, respectively, from (E,E)-farnesol by a stereodivergent gold(I)-catalyzed cascade reaction which forms the tricyclic aromadendrane core in a single step. These are the shortest total syntheses of these natural compounds.

Gold-catalyzed enantioselective ring-expanding cycloisomerization of cyclopropylidene bearing 1,5-enynes

An enantioselective ring-expanding cycloisomerization of 1,5-enynes bearing a cyclopropylidene moiety has been developed. This methodology provides a new approach to bicyclo[4.2.0]octanes, a structural motif present in many biologically active natural products.

Electrophilic activation of allenenes and allenynes: analogies and differences between Brønsted and Lewis acid activation

This review focuses on electrophilic activation of allenes by an acid. Key mechanistic features are presented together with recent synthetic applications.