Target-Agnostic P-Glycoprotein Assessment Yields Strategies to Evade Efflux, Leading to a BRAF Inhibitor with Intracranial Efficacy.

The blood-brain barrier (BBB) presents a major hurdle in the development of central nervous system (CNS) active therapeutics, and expression of the P-glycoprotein (P-gp) efflux transporter at the blood-brain interface further impedes BBB penetrance of most small molecules. Designing efflux liabilities out of compounds can be laborious, and there is currently no generalizable approach to directly transform periphery-limited agents to ones active in the CNS. Here, we describe a target-agnostic, prospective assessment of P-gp efflux using diverse compounds. Our results demonstrate that reducing the molecular size or appending a carboxylic acid in many cases enables evasion of P-gp efflux in cell-based experiments and in mice. These strategies were then applied to transform a periphery-limited V600EBRAF inhibitor, dabrafenib, into versions that possess potent and selective anti-cancer activity but now also evade P-gp-mediated efflux. When compared to dabrafenib, the compound developed herein (everafenib) has superior BBB penetrance and superior efficacy in an intracranial mouse model of metastatic melanoma, suggesting it as a lead candidate for the treatment of melanoma metastases to the brain and gliomas with BRAF mutation. More generally, the results described herein suggest the actionability of the trends observed in these target-agnostic efflux studies and provide guidance for the conversion of non-BBB-penetrant drugs into versions that are BBB-penetrant and efficacious.

ThioCORMates: Tunable and Cost-Effective Carbon Monoxide-Releasing Molecules.

Carbon monoxide-releasing molecules (CORMs) can offer a safer alternative to CO delivery than the use of CO gas cylinders. Upon treatment with an amine base, S-aryl thioformates ("thioCORMates") release CO nearly quantitatively at room temperature, a gas which can then be harnessed for carbonylative cross-coupling, biological study, or inorganic synthesis. These bench-stable molecules are easily synthesized from the corresponding thiophenol and can be electronically tuned to release CO at different rates-from less than 1 min to greater than 1 h-offering a practical alternative to existing CORM technology. Finally, isotopically labeled 13 CO can be conveniently generated and used, with the thioCORMate synthesized from 13 C-formic acid.

Xenoprotein engineering via synthetic libraries.

Chemical methods have enabled the total synthesis of protein molecules of ever-increasing size and complexity. However, methods to engineer synthetic proteins comprising noncanonical amino acids have not kept pace, even though this capability would be a distinct advantage of the total synthesis approach to protein science. In this work, we report a platform for protein engineering based on the screening of synthetic one-bead one-compound protein libraries. Screening throughput approaching that of cell surface display was achieved by a combination of magnetic bead enrichment, flow cytometry analysis of on-bead screens, and high-throughput MS/MS-based sequencing of identified active compounds. Direct screening of a synthetic protein library by these methods resulted in the de novo discovery of mirror-image miniprotein-based binders to a ∼150-kDa protein target, a task that would be difficult or impossible by other means.

Recent advances in homogeneous nickel catalysis.

Tremendous advances have been made in nickel catalysis over the past decade. Several key properties of nickel, such as facile oxidative addition and ready access to multiple oxidation states, have allowed the development of a broad range of innovative reactions. In recent years, these properties have been increasingly understood and used to perform transformations long considered exceptionally challenging. Here we discuss some of the most recent and significant developments in homogeneous nickel catalysis, with an emphasis on both synthetic outcome and mechanism.

Nickel Catalysis: Synergy between Method Development and Total Synthesis.

Nickel(0) catalysts have proven to be powerful tools for multicomponent coupling reactions in our laboratories over the past 15 years. This interest was originally sparked by the ubiquity of allylic alcohol motifs in natural products, such as (-)-terpestacin, which we envisioned assembling by the coupling of two π components (alkyne and aldehyde) with concomitant reduction. Mechanistic investigations allowed us to elucidate several modes of controlling the regioselectivity and stereoselectivity in the oxidative cyclization, and these insights enabled us to leverage combinations of alkenes and phosphine ligands to direct regioselective outcomes. The initial success in developing the first intermolecular reductive alkyne-aldehyde coupling reaction launched a series of methodological investigations that rapidly expanded to include coupling reactions of alkynes with other electrophilic π components, such as imines and ketones, as well as electrophilic σ components, such as epoxides. Aziridines proved to be more challenging substrates for reductive coupling, but we were recently able to demonstrate that cross-coupling of aziridines and alkylzinc reagents is smoothly catalyzed by a zero-valent nickel/phenanthroline system. Moreover, the enantioselective alkyne-aldehyde coupling and the development of novel P-chiral ferrocenyl ligands enabled the total synthesis of (-)-terpestacin, amphidinolides T1 and T4, (-)-gloeosporone, and pumiliotoxins 209F and 251D. We subsequently determined that alkenes could be used in place of alkynes in several nickel-catalyzed reactions when a silyl triflate activating agent was added. We reason that such an additive functions largely to enhance the electrophilicity of the metal center by coordination to the electrophilic π component, such that less nucleophilic alkene π donors can undergo productive combination with nickel complexes. This activation manifold was further demonstrated to be effective for alkene-aldehyde couplings. In a related manner, electrophilic promoters were also successfully employed for allylic substitution reactions of allylic carbonates with simple alkenes and in the Mizoroki-Heck reaction of both benzyl and aryl electrophiles. In these instances, it is proposed that counterion exchange from a more strongly coordinating anion to the weakly or noncoordinating triflate counterion enables reaction at an electrophilic Ni(II) center rather than by coordination to one of the coupling components. Mechanistic insights also played an important role in the development of mixed N-heterocyclic carbene/phosphite ligand systems to overcome challenges in regioselective alkene-aldehyde coupling reactions. We hope that, taken together, the body of work summarized in this Account demonstrates the constructive interplay among total synthesis, methodological development, and mechanistic investigation that has driven our research program.

Highly Regioselective Indoline Synthesis under Nickel/Photoredox Dual Catalysis.

Nickel/photoredox catalysis is used to synthesize indolines in one step from iodoacetanilides and alkenes. Very high regioselectivity for 3-substituted indoline products is obtained for both aliphatic and styrenyl olefins. Mechanistic investigations indicate that oxidation to Ni(III) is necessary to perform the difficult C-N bond-forming reductive elimination, producing a Ni(I) complex, which in turn is reduced to Ni(0). This process serves to further demonstrate the utility of photoredox catalysts as controlled single electron transfer agents in multioxidation state nickel catalysis.

Nickel-catalyzed Mizoroki-Heck reaction of aryl sulfonates and chlorides with electronically unbiased terminal olefins: high selectivity for branched products.

Achieving high selectivity in the Heck reaction of electronically unbiased alkenes has been a longstanding challenge. Using a nickel-catalyzed cationic Heck reaction, we were able to achieve excellent selectivity for branched products (≥19:1 in all cases) over a wide range of aryl electrophiles and aliphatic olefins. A bidentate ligand with a suitable bite angle and steric profile was key to obtaining high branched/linear selectivity, whereas the appropriate base suppressed alkene isomerization of the product. Although aryl triflates are traditionally used to access the cationic Heck pathway, we have shown that, by using triethylsilyl trifluoromethanesulfonate, we can effect a counterion exchange of the catalytic nickel complex, such that cheaper and more stable aryl chlorides, mesylates, tosylates, and sulfamates can be used to yield the same branched products with high selectivity.

Preparation of N-alkyl 2-pyridones via a lithium iodide promoted O- to N-alkyl migration: scope and mechanism.

An efficient and inexpensive LiI-promoted O- to N-alkyl migration of 2-benzyloxy-, 2-allyloxy-, and 2-propargyloxypyridines and heterocycles is reported. The reaction produces the corresponding N-alkyl 2-pyridones and analogues under green, solvent-free conditions in good to excellent yields (30 examples, 20-97% yield). This method has been shown to be intermolecular and requires heat and lithium cation to occur.

Preparation of Structurally Diverse Compounds from the Natural Product Lycorine.

The synthesis of a 52-member compound collection from the natural product lycorine is reported, highlighted by divergent cross-coupling and substitution strategies and an unusual ring rearrangement induced by reaction with aryne intermediates.

Synthesis of a new class of ß-iodo N-alkenyl 2-pyridones.

A new method for the synthesis of ß-iodo N-alkenyl 2-pyridones from substituted 2-propargyloxypyridines has been discovered . These compounds present a unique complement of orthogonal functionality and structural characteristics that are unavailable via other routes. The ready access to these compounds renders them an important entry point for the preparation of more complex N-alkyl pyridone-containing targets.