A sticky situation for targeted cancer therapy

In this issue of Cell Chemical Biology, Andlovic et al.1 unravel how certain fusicoccin toxins synergize with interferon-α to kill cancer cells. The results suggest that fusicoccins potentiate apoptosis by "sticking together" 14-3-3 protein-protein interactions.

A Stereoselective Photoinduced Cycloisomerization Inspired by Ophiobolin A

A stereoselective synthetic entry point to the 5-8-5 carbocyclic core of the ophiobolins was developed. This strategy exploits the chiral tertiary alcohol of ophiobolin A to guide assmebly of the 5-8-5 scaffold in a single step via a photoinitiated cycloisomerization. Mechanistic insights into the origin of stereocontrol in this reaction are described, as are efforts to elaborate the resultant fused 5-8-5 ring system to the pharmacophore of ophiobolin A.

Programmed Polyene Cyclization Enabled by Chromophore Disruption

A new polyene cyclization strategy exploiting β-ionyl derivatives was developed. Photoinduced deconjugation of the extended π-system within these chromophores unveils a contrathermodynamic polyene that engages in a Heck bicyclization to afford [4.4.1]-propellanes. This cascade improves upon the limited regioselectivity achieved using existing biomimetic tactics and tolerates both electron-rich and electron-deficient (hetero)aryl groups. The utility of this approach was demonstrated with the diverted total synthesis of taxodione and salviasperanol, two isomeric abietane diterpenes that were previously inaccessible along the same synthetic pathway.

Pyridazine N-Oxides as Photoactivatable Surrogates for Reactive Oxygen Species

A method for the photoinduced evolution of atomic oxygen from pyridazine N-oxides was developed. This underexplored oxygen allotrope mediates arene C-H oxidation within complex, polyfunctional molecules. A water-soluble pyridazine N-oxide was also developed and shown to promote photoinduced DNA cleavage in aqueous solution. Taken together, these studies highlight the utility of pyridazine N-oxides as photoactivatable O(3P) precursors for applications in organic synthesis and chemical biology.

Site-Specific Alkene Hydromethylation via Protonolysis of Titanacyclobutanes

Methyl groups are ubiquitous in biologically active molecules. Thus, new tactics to introduce this alkyl fragment into polyfunctional structures are of significant interest. With this goal in mind, a direct method for the Markovnikov hydromethylation of alkenes is reported. This method exploits the degenerate metathesis reaction between the titanium methylidene unveiled from Cp₂ Ti(μ-Cl)(μ-CH₂ )AlMe₂ (Tebbe's reagent) and unactivated alkenes. Protonolysis of the resulting titanacyclobutanes in situ effects hydromethylation in a chemo-, regio-, and site-selective manner. The broad utility of this method is demonstrated across a series of mono- and di-substituted alkenes containing pendant alcohols, ethers, amides, carbamates, and basic amines.

Probing the 14-3-3 Isoform-Specificity Profile of Protein-Protein Interactions Stabilized by Fusicoccin A

Fusicoccin A (FC) is a fungal phytotoxin that stabilizes protein-protein interactions (PPIs) between 14-3-3 adapter proteins and their phosphoprotein interaction partners. Recently, FC has emerged as an important chemical probe of human 14-3-3 PPIs involved in cancer and neurobiology. These previous studies have established the structural requirements for FC-induced stabilization of 14-3-3·client phosphoprotein complexes; however, the effect of 14-3-3 isoforms on FC activity remains underexplored. This is a relevant question for the continued development of FC variants because there are seven isoforms of 14-3-3 in humans. Despite their sequence and structural similarities, a growing body of experimental evidence supports both tissue-specific expression of 14-3-3 isoforms and isoform-specific functions in vivo. Herein, we interrogate the isoform-specificity profile of FC in vitro using recombinant 14-3-3 isoforms and a library of fluorescein-labeled hexaphosphopeptides mimicking the C-terminal recognition domains of client proteins that are characterized targets of FC in vivo. Our results reveal modest isoform preferences for individual client phospholigands and demonstrate that FC differentially stabilizes PPIs involving 14-3-3σ. Together, these data support the feasibility of developing FC variants with enhanced isoform selectivity.

Analysis of Interactions Stabilized by Fusicoccin A Reveals an Expanded Suite of Potential 14-3-3 Binding Partners

Fusicoccin A (FC) is a diterpene glycoside that stabilizes protein-protein interactions (PPIs) between 14-3-3 adapter proteins and their phosphoprotein interaction partners. Recently, FC has gained attention for its pro-apoptotic and neuroprotective properties in cell culture. Although the exact molecular mechanism(s) is (are) unresolved, 14-3-3 PPIs are central to this activity. With the goal of refining the pharmacology of this chemotype, we conducted a systematic analysis of the structural features that govern FC-induced stabilization of 14-3-3 PPIs utilizing a C-terminal phosphorylation recognition motif. This study confirmed that a C-terminal amino acid with a small alkyl group is required for the interaction of FC at canonical C-terminal 14-3-3 PPI interfaces. Using bioinformatics, this structural insight was leveraged to assemble a database of 119 candidate 14-3-3 PPIs that can serve as targets for FC. This group includes a subset of proteins with experimentally determined C-terminal phosphosites that have not been explored as potential targets of FC.

Modular access to functionalized 5-8-5 fused ring systems via a photoinduced cycloisomerization reaction

A 5-8-5 carbocyclic ring system forms the core of over 30 distinct natural products. Several members of this family have gained attention for their diverse activity in cell culture. In these cases, biological function is mediated by the arrangement of substituents around a conserved 5-8-5 nucleus. Despite the potential applications of this privileged substructure in medicinal chemistry, modular strategies for its assembly are underdeveloped. Herein, we describe a cycloisomerization reaction that forms the 5-8-5 framework directly. This strategy uniquely allows access to gram quantities of this valuable scaffold in four steps.

Tailored fragments of roseophilin selectively antagonize Mcl-1 in vitro

We have discovered a fragment of the natural product roseophilin, a member of the prodiginine family, that antagonizes Mcl-1 functions in a liposome-based assay for mitochondrial membrane permeabilization. By tailoring this substance such that it can participate in salt bridging with the protein surface, we have prepared the first prodiginine inspired structure that shows direct, saturable binding to a recombinant Bcl-2 family member in vitro.

Substituted 2,2'-bipyrroles and pyrrolylfurans via intermediate isoxazolylpyrroles

We describe a new synthesis of the 3-chloro-(4'-methoxy)-2,2'-pyrrolylfuran segment (3) of (+)- roseophilin. The route exploits a isoxazoylpyrrole intermediate, wherein the isoxazole ring serves as a β-diketone equivalent and a directing group for palladium catalyzed chlorination of the attached pyrrole. Subsequent reduction of the N-O bond and acid promoted cyclization afords roseophilin segment 3b in five steps and 19% overall yield. This strategy was extended to the synthesis of 3-chloro-(4'-alkoxy)-2,2'-pyrrolylfurans (16a-c) and 4-alkoxy-2,2'-bipyrroles (20a-c), which are building blocks to synthesize bioactive prodiginine natural products and their congeners.

Modular access to complex prodiginines: total synthesis of (+)-roseophilin via its 2-azafulvene prototropisomer

Ansa-bridged prodiginines are bioactive pigments produced by bacteria. Certain of these structures are reported to be antagonists of protein-protein interactions involved in apoptosis. We describe a new entry to alkaloids of this type, demonstrated with a concise asymmetric synthesis of (+)-roseophilin (3). Our route constructs the pyrrolophane motif via phosphoryl transfer-terminated macroaldolization and passes through a previously unexplored prototropic form of the natural product.

Palladacyclic imidazoline-naphthalene complexes: synthesis and catalytic performance in Pd(II)-catalyzed enantioselective reactions of allylic trichloroacetimidates

A new family of air- and moisture-stable enantiopure C,N-palladacycles (PIN-acac complexes) were prepared in good overall yield in three steps from 2-iodo-1-naphthoic acid and enantiopure β-amino alcohols. Three of these PIN complexes were characterized by single-crystal X-ray analysis. As anticipated, the naphthalene and imidazoline rings of PIN-acac complexes 18a and 18b were canted significantly from planarity and projected the imidazoline substituents R(1) and R(2) on opposite faces of the palladium square plane. Fifteen PIN complexes were evaluated as catalysts for the rearrangement of prochiral (E)-allylic trichloroacetimidate 19 (eq 2) and the S(N)2' allylic substitution of acetic acid with prochiral (Z)-allylic trichloroacetimidate 23. Although these complexes were kinetically poor catalysts for the Overman rearrangement, they were good catalysts for the allylic substitution reaction, providing branched allylic esters in high yield. However, enantioselectivities were low to moderate and significantly less than that realized with palladacyclic complexes of the COP family. Computational studies support an anti-acetoxypalladation/syn-deoxypalladation mechanism analogous to that observed with COP catalysts. The computational study further suggests that optimizing steric influence in the vicinity of the carbon ligand of a chiral C,N-palladacycle, rather than near the nitrogen heterocycle, is the direction to pursue in future development of improved enantioselective catalysts of this motif.