Toward a Template for Synthetic Actin-Targeting Macrolide Analogues That Inhibit Cancer Cell Invasiveness.

Actin barbed end-binding macrolides have been shown to inhibit cancer cell motility and invasion of extracellular matrix (ECM), evoking their potential utility as therapies for metastatic cancers. Unfortunately, the direct use of these compounds in clinical settings is impeded by their limited natural abundance, challenging total synthesis, and detrimental effects on normal tissues. To develop potent analogues of these compounds that are simpler to synthesize and compatible with cell-specific targeting systems, such as antibodies, we designed over 20 analogues of the acyclic side chain (tail) of the macrolide Mycalolide B. These analogues probed the contributions of four distinct regions of the tail towards the inhibition of actin polymerization and ECM invasion by human lung cancer A549 cells. We observed that two of these regions tolerate considerable substituent variability, and we identified a specific combination of substituents that leads to the optimal inhibition of the ECM invasion activity of A549 cells.

Enantioselective Rhodium-Catalyzed Pauson-Khand Reactions of 1,6-Chloroenynes with 1,1-Disubstituted Olefins.

An enantioselective rhodium(I)-catalyzed Pauson-Khand reaction (PKR) using 1,6-chloroenynes that contain challenging 1,1-disubstituted olefins is described. In contrast to the previous studies with these types of substrates, which are only suitable for a single type of tether and alkyne substituent, the new approach results in a more expansive substrate scope, including carbon and heteroatom tethers with polar and non-polar substituents on the alkene. DFT calculations provide critical insight into the role of the halide, which pre-polarizes the alkyne to lower the barrier for metallacycle formation and provides the proper steric profile to promote a favorable enantiodetermining interaction between substrate and chiral diphosphine ligand. Hence, the chloroalkyne enables the efficient and enantioselective PKR with 1,6-enynes that contain challenging 1,1-disubstituted olefins, thereby representing a new paradigm for enantioselective reactions involving 1,6-enynes.

Intramolecular Rhodium-Catalyzed [(3+2+2)] Carbocyclization Reactions with Dienylidenecyclopropanes: A Concise and Stereoselective Total Synthesis of the Sesquiterpene (+)-Zizaene.

The development of an intramolecular rhodium-catalyzed [(3+2+2)] carbocyclization reaction of alkylidenecyclopropanes (ACPs) tethered to 1,4- and 1,5-skipped dienes is described. This transformation offers a new approach for the construction of bridged tricyclic compounds with up to three quaternary centers, which are suitable for the synthesis of challenging bioactive natural products. For instance, the synthetic utility of this transformation is illustrated through a concise asymmetric total synthesis of the sesquiterpene (+)-zizaene in ten steps from a commercially available starting material.

Catalytic Enantioselective Alkylation of Prochiral Enolates.

The asymmetric alkylation of enolates is a particularly versatile method for the construction of α-stereogenic carbonyl motifs, which are ubiquitous in synthetic chemistry. Over the past several decades, the focus has shifted to the development of new catalytic methods that depart from classical stoichiometric stereoinduction strategies (e.g., chiral auxiliaries, chiral alkali metal amide bases, chiral electrophiles, etc.). In this way, the enantioselective alkylation of prochiral enolates greatly improves the step- and redox-economy of this process, in addition to enhancing the scope and selectivity of these reactions. In this review, we summarize the origin and advancement of catalytic enantioselective enolate alkylation methods, with a directed emphasis on the union of prochiral nucleophiles with carbon-centered electrophiles for the construction of α-stereogenic carbonyl derivatives. Hence, the transformative developments for each distinct class of nucleophile (e.g., ketone enolates, ester enolates, amide enolates, etc.) are presented in a modular format to highlight the state-of-the-art methods and current limitations in each area.

Truncated Actin-Targeting Macrolide Derivative Blocks Cancer Cell Motility and Invasion of Extracellular Matrix.

Cancer metastasis is a complex process involving highly motile tumor cells that breach tissue barriers, enter the bloodstream and lymphatic system, and disseminate throughout the body as circulating tumor cells. The primary cellular mechanism contributing to these critical events is the reorganization of the actin cytoskeleton. Mycalolide B (MycB) is an actin-targeting marine macrolide that can suppress proliferation, migration, and invasion of breast and ovarian cancer cells at low nanomolar doses. Through structure-activity relationship studies focused on the actin-binding tail region (C24-C35) of MycB, we identified a potent truncated derivative that inhibits polymerization of G-actin and severs F-actin by binding to actin's barbed end cleft. Biological analyses of this miniature MycB derivative demonstrate that it causes a rapid collapse of the actin cytoskeleton in ovarian cancer cells and impairs cancer cell motility and invasion of the extracellular matrix (ECM) by inhibiting invadopodia-mediated ECM degradation. These studies provide essential proof-of-principle for developing actin-targeting therapeutic agents to block cancer metastasis and establish a synthetically tractable barbed end-binding pharmacophore that can be further improved by adding targeting groups for precision drug design.

Regio- and Diastereoselective Rhodium-Catalyzed Allylic Substitution with Unstabilized Benzyl Nucleophiles.

We have developed a highly regio- and diastereoselective rhodium-catalyzed allylic substitution of challenging alkyl-substituted secondary allylic carbonates with benzylzinc reagents, which are prepared from widely available benzyl halides. This process utilizes rhodium(III) chloride as a commercially available, high-oxidation state and bench-stable pre-catalyst to provide a rare example of a regio- and diastereoselective allylic substitution in the absence of an exogenous ligand. This reaction tolerates electronically diverse benzylzinc nucleophiles and an array of functionalized and/or challenging aliphatic allylic electrophiles. Finally, the configurational fluxionality of the rhodium-allyl intermediate is exploited to develop a novel diastereoselective process for the construction of vicinal acyclic ternary/ternary stereogenic centers, in addition to a cyclic ternary/quaternary derivative.

Regioselective and Stereospecific Rhodium-Catalyzed Allylic Cyanomethylation with an Acetonitrile Equivalent: Construction of Acyclic ß-Quaternary Stereogenic Nitriles.

A highly regioselective and stereospecific rhodium-catalyzed cyanomethylation of tertiary allylic carbonates for the construction of acyclic ß-quaternary stereogenic nitriles is described. This protocol represents the first example of a metal-catalyzed allylic substitution reaction using a triorganosilyl-stabilized acetonitrile anion, which permits access to several carbonyl derivatives that are challenging to prepare using conventional pronucleophiles. The synthetic utility of the stereospecific cyanomethylation is further exemplified through the construction of an intermediate utilized in the total synthesis of both (-)-epilaurene and (-)-α-cuparenone.

A natural solution to photoprotection and isolation of the potent polyene antibiotic, marinomycin A.

The photoprotection and isolation of marinomycin A using sporopollenin exine capsules (SpECs) derived from the spores of the plant Lycopodium clavatum is described. The marinomycins have a particularly short half-life in natural light, which severely impacts their potential biological utility given that they display potent antibiotic and anticancer activity. The SpEC encapsulation of the marinomycin A dramatically increases the half-life of the polyene macrodiolide to the direct exposure to UV radiation by several orders of magnitude, thereby making this a potentially useful strategy for other light sensitive bioactive agents. In addition, we report that the SpECs can also be used to selectively extract culture broths that contain the marinomycins, which provides a significantly higher recovery than with conventional XAD resins and provides concomitant photoprotection.

Copper-catalyzed desymmetrization of prochiral 4,4-disubstituted cyclopentenes via a site-selective allylic oxidation: a concise total synthesis of untenone A.

The desymmetrization of prochiral 4,4-disubstituted cyclopentenes via a site-selective copper-catalyzed allylic oxidation is described. This study provides a direct comparison of a series of known methods for allylic oxidation, and thus identifies ligand-free copper(i) iodide as the optimal catalyst for this particular process. Notably, this work offers a convenient approach to the preparation of γ-quaternary α,ß-unsaturated cyclopentenones, which permits an efficient three-step total synthesis of (±)-untenone A.

Dynamic Kinetic Resolution of Alkenyl Cyanohydrins Derived from α,ß-Unsaturated Aldehydes: Stereoselective Synthesis of E-Tetrasubstituted Olefins.

A novel dynamic kinetic resolution (DKR) of tetrasubstituted alkenyl cyanohydrins prepared from the corresponding α,ß-unsaturated aldehydes is described. The deprotonation of a geometrical mixture of tetrasubstituted alkenyl cyanohydrins with sodium diisopropylamide (NaDA) affords the allylic anions, which enables the equilibration of the E- and Z-olefins to permit the selective functionalization of the E-adduct. Theoretical studies indicate that the nature of the alkali metal cation is a critical component to lowering the barrier for interconversion between the two geometrical isomers, which provides the mechanistic basis for the DKR reaction. In addition, we demonstrate that the DKR reaction can be combined with a transition metal-catalyzed allylic substitution to generate a stereodefined E-tetrasubstituted olefin and quaternary center in a single cross-coupling reaction.

Enantioselective Rhodium-Catalyzed Allylic Alkylation of ß,γ-Unsaturated α-Amino Nitriles: Synthetic Homoenolate Equivalents.

An enantioselective rhodium-catalyzed allylic alkylation of ß,γ-unsaturated α-amino nitriles is described. This protocol provides a novel approach for the construction of ß-stereogenic carbonyl derivatives via the catalytic asymmetric alkylation of a homoenolate equivalent. The particularly challenging nature of this transformation is highlighted by the fact that three modes of selectivity must be manipulated, namely regio- and enantioselectivity, in addition to geometrical control. The γ-stereogenic cyanoenamine products can be readily hydrolyzed in situ to afford the ß-substituted carboxylic acids, which in turn provide expedient access to a number of related carbonyl derivatives. Additionally, control experiments indicate that the chiral rhodium-allyl intermediate facilitates the selective formation of the E-cyanoenamine products, which is critical since the Z-isomer affords significantly lower enantiocontrol.

Effects of Modulating Actin Dynamics on HER2 Cancer Cell Motility and Metastasis.

Amplification of HER2 leads to development of HER2-positive (HER2+) cancers with high rates of metastasis compared to other cancer subtypes. The goal of this study was to probe the vulnerability of HER2+ cancer cells to a filamentous actin (F-actin) severing and capping toxin. The growth and viability of human HER2+ breast cancer (HCC1954) and ovarian cancer (SKOV3) cell lines were significantly impaired upon treatment with the marine macrolide mycalolide B (Myc B) at doses above 100 nanomolar. Further testing of Myc B in combination with the antibody-drug conjugate Trastuzumab-emtansine (T-DM1) led to improved killing of SKOV3 cells compared to either treatment alone. At sub-lethal doses, treatment of HER2+ cancer cells with Myc B resulted in rapid loss of leading edge protrusions and formation of aggresomes containing F-actin and the actin regulatory protein Cortactin. This correlated with robust inhibition of HER2+ cancer cell motility and invasion with Myc B treatment. In SKOV3 tumor xenograft assays, intratumoral injections of Myc B impaired HER2+ tumor growth and metastasis, with maximal effects observed in combination with systemic delivery of Trastuzumab. Metastasis of SKOV3 cells to the lungs following tail vein injection was also reduced by Myc B. Together, these findings provide rationale for targeting F-actin in combination with existing therapies for HER2+ cancers to reduce metastasis.

Asymmetric Rhodium-Catalyzed Allylic Substitution Reactions: Discovery, Development and Applications to Target-Directed Synthesis.

The transition metal-catalyzed allylic substitution reaction is a particularly versatile method for the construction of carbon-carbon and carbon-heteroatom bonds. In this regard, the rhodium-catalyzed variant has emerged as a powerful method for the regioselective and stereospecific allylic substitution of chiral nonracemic secondary and tertiary allylic carbonates with a variety of carbon- and heteroatom-based nucleophiles. In addition, recent developments have made the analogous enantioselective process possible using prochiral nucleophiles with achiral allylic electrophiles, which represents a significant advance in this area. In this Perspective, the discovery, development and applications of these conceptually orthogonal strategies to target-directed synthesis are discussed, with a particular emphasis given to those methods developed in our laboratory.

Rhodium-catalyzed [(3+2)+1] carbocyclizations of alkynylidenecyclopropanes with carbon monoxide: construction of polysubstituted bicyclohexa-2,5-dienones.

A rhodium-catalyzed carbocyclization reaction of alkynylidenecyclopropanes with carbon monoxide to prepare bicyclohexa-2,5-dienones is described. This protocol offers convenient access to doubly conjugated cyclic enones and is tolerant of functionalized alkynes and cyclopropanes. Furthermore, a photochemical rearrangement of a bicyclohexa-2,5-dienone facilitates the construction of a highly functionalized bicyclopentenone containing two contiguous stereogenic centres, which represents a versatile intermediate for target directed synthesis.

Intramolecular Thioether Migration in the Rhodium-Catalyzed Ene-Cycloisomerization of Alkenylidenecyclopropanes by a Metal-Mediated ß-Sulfide Elimination.

Reported is the first example of a rhodium-mediated ß-sulfide elimination, which represents a new mode of reactivity for late-transition-metal chemistry. This serendipitous discovery facilitates an ene-cycloisomerization of allylic-sulfide-containing alkenylidenecyclopropanes (ACPs) to afford five-membered carbo- and heterocyclic rings with concomitant intramolecular thioether migration. Interestingly, similar selectivity is obtained with both E- and Z-allylic sulfides and the reaction is also feasible with an allylic selenide. Mechanistic studies are consistent with an inner-sphere transfer of the sulfide, which is remarkable given the propensity for sulfides to poison transition-metal catalysts. Finally, this type of atom-economical rearrangement is envisioned to prompt the development of related processes given the utility of sulfides in target-directed synthesis.

Regio- and stereospecific rhodium-catalyzed allylic alkylation with an acyl anion equivalent: an approach to acyclic α-ternary ß,γ-unsaturated aryl ketones.

The regio- and stereospecific rhodium-catalyzed allylic alkylation of secondary allylic carbonates with cyanohydrin pronucleophiles facilitates the direct construction of acyclic α-ternary ß,γ-unsaturated aryl ketones. Interestingly, this study illustrates the impact of deaggregating agents on regiocontrol and the electronic nature of the aryl component to suppress olefin isomerization. In addition, we demonstrate that the dimethylamino substituent, which modulates the pKa of the α-ternary ß,γ-unsaturated aryl ketone, provides a useful synthetic handle for further functionalization via Kumada cross-coupling of the aryl trimethylammonium salt. Finally, the stereospecific alkylation of a chiral nonracemic secondary allylic carbonate affords the enantioenriched α-ternary aryl ketone, which was employed in a formal synthesis of trichostatic acid to illustrate that the allylic alkylation proceeds with net retention of configuration.

A Concise, Efficient and Scalable Total Synthesis of Thapsigargin and Nortrilobolide from (R)-(-)-Carvone.

A concise, efficient and scalable synthesis of thapsigargin and nortrilobolide from commercially available (R)-(-)-carvone was developed. Our synthetic strategy is inspired by nature's carbon-carbon bond formation sequence, which facilitates the construction of a highly functionalized sesquiterpene lactone skeleton in five steps via an enantioselective ketone alkylation and a diastereoselective pinacol cyclization. We envision that this strategy will permit the construction of other members of the family, structural analogs and provide a practical synthetic route to these important bioactive agents. In addition, we anticipate that the prodrug Mipsagargin, which is currently in late-stage clinical trials for the treatment of cancer, will also be accessible via this strategy. Hence, the limited availability from natural sources, coupled with an estimated demand of one metric ton per annum for the prodrug, provides a compelling mandate to develop practical total syntheses of these agents.

Enantioselective Rhodium-Catalyzed Allylic Alkylation of Prochiral α,α-Disubstituted Aldehyde Enolates for the Construction of Acyclic Quaternary Stereogenic Centers.

A highly enantioselective rhodium-catalyzed allylic alkylation of prochiral α,α-disubstituted aldehyde enolates with allyl benzoate is described. This protocol provides a novel approach for the synthesis of acyclic quaternary carbon stereogenic centers and it represents the first example of the direct enantioselective alkylation of an aldehyde enolate per se. The versatility of the α-quaternary aldehyde products is demonstrated through their conversion to a variety of useful motifs applicable to target-directed synthesis. Finally, mechanistic studies indicate that high levels of asymmetric induction are achieved from a mixture of prochiral (E)- and (Z)-enolates, which provides an exciting development for this type of transformation.

Stereospecific Rhodium-Catalyzed Allylic Substitution with Alkenyl Cyanohydrin Pronucleophiles: Construction of Acyclic Quaternary Substituted α,ß-Unsaturated Ketones.

A highly regio- and stereospecific rhodium-catalyzed allylic alkylation of tertiary allylic carbonates with alkenyl cyanohydrin pronucleophiles is described. This protocol offers a fundamentally novel approach toward the synthesis of acyclic quaternary-substituted α,ß-unsaturated ketones and thereby provides a new cross-coupling strategy for target directed synthesis. A particularly attractive feature with this process is the ability to directly couple di-, tri- and tetrasubstituted alkenyl cyanohydrin pronucleophiles to prepare the corresponding α,ß-unsaturated ketone derivatives in a highly selective manner. Additionally, the chemoselective 1,4-reduction of the enone products provides rapid access to acyclic enantiomerically enriched α,α'-dialkyl-substituted ketones, which are challenging motifs to prepare using conventional enolate alkylation.

Diastereoselective construction of anti-4,5-disubstituted-1,3-dioxolanes via a bismuth-mediated two-component hemiacetal oxa-conjugate addition of γ-hydroxy-α,ß-unsaturated ketones with paraformaldehyde.

The bismuth-mediated two-component hemiacetal oxa-conjugate addition of γ-hydroxy-α,ß-unsaturated ketones with paraformaldehyde affords anti-4,5-disubstituted-1,3-dioxolanes in an efficient and stereoselective manner. The reaction provides a practical, inexpensive and atom-economical approach to these types of heterocycles, which represent useful intermediates for target-directed synthesis and precursors to syn-1,2-diols.