Jeffrey L. Gustafson

Dynamic Kinetic Resolution of Biaryl Atropisomers via Peptide-Catalyzed Asymmetric Bromination

Selectively Spun Biaryl compounds, in which two phenyl rings are linked by a single bond, exhibit an interesting sort of chirality, termed atropisomerism. If bulky substituents block the mutual rotation of the rings about the linking bond, then two isomers can be isolated that differ only in the direction one ring has swiveled away from the plane of the other. This feature is useful in ligand design for asymmetric catalysis and also appears in a number of polycyclic natural products. However, selective synthesis of a single isomer is difficult. Gustafson et al. (p. 1251) now show that a simple tripeptide derivative acts as an efficient catalyst for this purpose, trapping a freely rotating precursor in one orientation through selective bromination; the large bromine substituents then inhibit further swiveling. A simple catalyst converts antibiotic-like molecules into near-single chiral form. Despite the widespread use of axially chiral, or atropisomeric, biaryl ligands in modern synthesis and the occurrence of numerous natural products exhibiting axial chirality, few catalytic methods have emerged for the direct asymmetric preparation of this compound class. Here, we present a tripeptide-derived small-molecule catalyst for the dynamic kinetic resolution of racemic biaryl substrates. The reaction proceeds via an atropisomer-selective electrophilic aromatic substitution reaction using simple bromination reagents. The result is an enantioselective synthesis that delivers chiral nonracemic biaryl compounds with excellent optical purity and good isolated chemical yields (in most cases a >95:5 enantiomer ratio and isolated yields of 65 to 87%). A mechanistic model is advanced that accounts for the basis of selectivity observed.

Catalytic in vivo protein knockdown by small-molecule PROTACs

The current predominant therapeutic paradigm is based on maximizing drug-receptor occupancy to achieve clinical benefit. This strategy, however, generally requires excessive drug concentrations to ensure sufficient occupancy, often leading to adverse side effects. Here, we describe major improvements to the proteolysis targeting chimeras (PROTACs) method, a chemical knockdown strategy in which a heterobifunctional molecule recruits a specific protein target to an E3 ubiquitin ligase, resulting in the targets ubiquitination and degradation. These compounds behave catalytically in their ability to induce the ubiquitination of super-stoichiometric quantities of proteins, providing efficacy that is not limited by equilibrium occupancy. We present two PROTACs that are capable of specifically reducing protein levels by >90% at nanomolar concentrations. In addition, mouse studies indicate that they provide broad tissue distribution and knockdown of the targeted protein in tumor xenografts. Together, these data demonstrate a protein knockdown system combining many of the favorable properties of small-molecule agents with the potent protein knockdown of RNAi and CRISPR.

Small-molecule inhibitors of the interaction between the E3 ligase VHL and HIF1α.

E3 ubiquitin ligases, such as the therapeutically relevant VHL, are challenging targets for traditional medicinal chemistry, as their modulation requires targeting protein-protein interactions. We report novel small-molecule inhibitors of the interaction between VHL and its molecular target HIF1α, a transcription factor involved in oxygen sensing.

A Case of Remote Asymmetric Induction in the Peptide-Catalyzed Desymmetrization of a Bis(phenol)

We report a catalytic approach to the synthesis of a key intermediate on the synthetic route to a pharmaceutical drug candidate in single enantiomer form. In particular, we illustrate the discovery process employed to arrive at a powerful, peptide-based asymmetric acylation catalyst. The substrate this catalyst modifies represents a remarkable case of desymmetrization, wherein the enantiotopic groups are separated by nearly a full nanometer, and the distance between the reactive site and the pro-stereogenic element is nearly 6 A. Differentiation of enantiotopic sites within molecules that are removed from the prochiral centers by long distances presents special challenges to the field of asymmetric catalysis. As the distance between enantiotopic sites increases within a substrate, so too may the requirements for size and complexity of the catalyst. The approach presented herein contrasts enzymatic catalysts and small-molecule catalysts for this challenge. Ultimately, we report here a synthetic, miniaturized enzyme mimic that catalyzes a desymmetrization reaction over a substantial distance. In addition, studies relevant to mechanism are presented, including (a) the delineation of structure-selectivity relationships through the use of substrate analogs, (b) NMR experiments documenting catalyst-substrate interactions, and (c) the use of isotopically labeled substrates to illustrate unequivocally an asymmetric catalyst-substrate binding event.

HaloPROTACS: Use of Small Molecule PROTACs to Induce Degradation of HaloTag Fusion Proteins

Small molecule-induced protein degradation is an attractive strategy for the development of chemical probes. One method for inducing targeted protein degradation involves the use of PROTACs, heterobifunctional molecules that can recruit specific E3 ligases to a desired protein of interest. PROTACs have been successfully used to degrade numerous proteins in cells, but the peptidic E3 ligase ligands used in previous PROTACs have hindered their development into more mature chemical probes or therapeutics. We report the design of a novel class of PROTACs that incorporate small molecule VHL ligands to successfully degrade HaloTag7 fusion proteins. These HaloPROTACs will inspire the development of future PROTACs with more drug-like properties. Additionally, these HaloPROTACs are useful chemical genetic tools, due to their ability to chemically knock down widely used HaloTag7 fusion proteins in a general fashion.

Linear Free Energy Relationship Analysis of a Catalytic Desymmetrization Reaction of a Diarylmethane-Bis(phenol)

Linear free-energy relationships have been found for enantioselectivity and various steric parameters in an enantioselective desymmetrization of symmetrical bis(phenol) substrates. The potential origin of this observation and the role of different steric parameters are discussed.

A Practical Lewis Base Catalyzed Electrophilic Chlorination of Arenes and Heterocycles

A mild phosphine sulfide catalyzed electrophilic halogenation of arenes and heterocycles that utilizes inexpensive and readily available N-halosuccinimides is disclosed. This methodology is shown to efficiently chlorinate diverse aromatics, including simple arenes such as anthracene, and heterocycles such as indoles, pyrrolopyrimidines, and imidazoles. Arenes with Lewis acidic moieties also proved amenable, underscoring the mild nature of this chemistry. Lewis base catalysis was also found to improve several diverse aromatic brominations and iodinations.

Synthesis of Atropisomerically Defined, Highly Substituted Biaryl Scaffolds through Catalytic Enantioselective Bromination and Regioselective Cross-Coupling†

The challenge of atropisomer-selective synthesis is often manifest in drug discovery projects,[1] and also in the synthesis of materials with interesting optical properties.[2] With respect to the former, numerous small molecule ligands for proteins and enzyme inhibitors exist as conformational racemates, with low barriers to atropisomerization (Figure 1). For example, terphenyl compounds (e.g., 1, disruptors of protein-protein interactions,)[3] heteroarene- and heteroatom-substituted biphenyls (e.g., kinase inhibitors; 2),[4] and other biologically active scaffolds (e.g., biphenyl tetrazole 3)[5] all exhibit the possibility for stereochemically unique atropisomeric conformations. Nonetheless, binding of the small molecule to the biological target often occurs with enantiospecificity, as the inherent chirality of the receptor effects in situ dynamic kinetic resolution of the ligand, provided the barrier to atropisomerization is low enough. Thus, the preparation of single atropisomer scaffolds could lead to increases in potency for small molecules, through an increase in the effective concentration of the biologically active atropisomer, with the exclusion of a less active, or alternatively active form.

Identification of hydrophobic tags for the degradation of stabilized proteins.

New HyTs are a knockout: we previously reported that labeling HaloTag proteins with low molecular weight hydrophobic tags (HyTs) leads to targeted degradation of HaloTag fusion proteins. In this report, we employed a chemical approach to extend this hydrophobic tagging methodology to highly stabilized proteins by synthesizing and evaluating a library of HyTs, which led to the identification of HyT36.

Small‐Molecule‐Mediated Degradation of the Androgen Receptor through Hydrophobic Tagging

Androgen receptor (AR)-dependent transcription is a major driver of prostate tumor cell proliferation. Consequently, it is the target of several antitumor chemotherapeutic agents, including the AR antagonist MDV3100/enzalutamide. Recent studies have shown that a single AR mutation (F876L) converts MDV3100 action from an antagonist to an agonist. Here we describe the generation of a novel class of selective androgen receptor degraders (SARDs) to address this resistance mechanism. Molecules containing hydrophobic degrons linked to small-molecule AR ligands induce AR degradation, reduce expression of AR target genes and inhibit proliferation in androgen-dependent prostate cancer cell lines. These results suggest that selective AR degradation may be an effective therapeutic prostate tumor strategy in the context of AR mutations that confer resistance to second-generation AR antagonists.