Virtual Screening of a Chemically Diverse "Superscaffold" Library Enables Ligand Discovery for a Key GPCR Target

The advent of ultra-large libraries of drug-like compounds has significantly broadened the possibilities in structure-based virtual screening, accelerating the discovery and optimization of high-quality lead chemotypes for diverse clinical targets. Compared to traditional high-throughput screening, which is constrained to libraries of approximately one million compounds, the ultra-large virtual screening approach offers substantial advantages in both cost and time efficiency. By expanding the chemical space with compounds synthesized from easily accessible and reproducible reactions and utilizing a large, diverse set of building blocks, we can enhance both the diversity and quality of the discovered lead chemotypes. In this study, we explore new chemical spaces using reactions of sulfur(VI) fluorides to create a combinatorial library consisting of several hundred million compounds. We screened this virtual library for cannabinoid type II receptor (CB2) antagonists using the high-resolution structure in conjunction with a rationally designed antagonist, AM10257. The top-predicted compounds were then synthesized and tested in vitro for CB2 binding and functional antagonism, achieving an experimentally validated hit rate of 55%. Our findings demonstrate the effectiveness of reliable reactions, such as sulfur fluoride exchange, in diversifying ultra-large chemical spaces and facilitate the discovery of new lead compounds for important biological targets.

Cross-Electrophile Coupling of Benzyl Halides and Disulfides Catalyzed by Iron

Cross-electrophile couplings are influential reactions that typically require a terminal reductant or photoredox conditions. We discovered an iron-catalyzed reaction that couples benzyl halides with disulfides to yield thioether products in the absence of a terminal reductant and under photoredox conditions. The disclosed platform proceeds without sulfur-induced catalyst poisoning or the use of an exogenous base, supporting a broad scope and circumventing undesired elimination pathways. We applied the developed chemistry in a new mode of disulfide bioconjugation, drug synthesis, gram-scale synthesis, and product derivatization. Lastly, we performed mechanistic experiments to better understand the stereoablative reaction between two electrophiles. Disulfides and benzylic thioethers are imperative for biological and pharmaceutical applications but remain severely understudied in comparison to their ethereal and amino counterparts. Hence, we expect this platform of iron catalysis and the downstream applications to be of interest to the greater scientific community.

Highly Enantioselective, Hydrogen-Bond-Donor Catalyzed Additions to Oxetanes

A precisely designed chiral squaramide derivative is shown to promote the highly enantioselective addition of trimethylsilyl bromide (TMSBr) to a broad variety of 3-substituted and 3,3-disubstituted oxetanes. The reaction provides direct and general access to synthetically valuable 1,3-bromohydrin building blocks from easily accessed achiral precursors. The products are readily elaborated both by nucleophilic substitution and through transition-metal-catalyzed cross-coupling reactions. The enantioselective catalytic oxetane ring opening was employed as part of a three-step, gram-scale synthesis of pretomanid, a recently approved medication for the treatment of multidrug-resistant tuberculosis. Heavy-atom kinetic isotope effect (KIE) studies are consistent with enantiodetermining delivery of bromide from the H-bond-donor (HBD) catalyst to the activated oxetane. While the nucleophilicity of the bromide ion is expected to be attenuated by association to the HBD, overall rate acceleration is achieved by enhancement of Lewis acidity of the TMSBr reagent through anion abstraction.

Canvass: A Crowd-Sourced, Natural-Product Screening Library for Exploring Biological Space

Natural products and their derivatives continue to be wellsprings of nascent therapeutic potential. However, many laboratories have limited resources for biological evaluation, leaving their previously isolated or synthesized compounds largely or completely untested. To address this issue, the Canvass library of natural products was assembled, in collaboration with academic and industry researchers, for quantitative high-throughput screening (qHTS) across a diverse set of cell-based and biochemical assays. Characterization of the library in terms of physicochemical properties, structural diversity, and similarity to compounds in publicly available libraries indicates that the Canvass library contains many structural elements in common with approved drugs. The assay data generated were analyzed using a variety of quality control metrics, and the resultant assay profiles were explored using statistical methods, such as clustering and compound promiscuity analyses. Individual compounds were then sorted by structural class and activity profiles. Differential behavior based on these classifications, as well as noteworthy activities, are outlined herein. One such highlight is the activity of (-)-2(S)-cathafoline, which was found to stabilize calcium levels in the endoplasmic reticulum. The workflow described here illustrates a pilot effort to broadly survey the biological potential of natural products by utilizing the power of automation and high-throughput screening.

Arynes and Cyclic Alkynes as Synthetic Building Blocks for Stereodefined Quaternary Centers

We report a facile method to synthesize stereodefined quaternary centers from reactions of arynes and related strained intermediates using β-ketoester-derived substrates. The conversion of β-ketoesters to chiral enamines is followed by reaction with in situ generated strained arynes or cyclic alkynes. Hydrolytic workup provides the arylated or alkenylated products in enantiomeric excesses as high as 96%. We also describe the one-pot conversion of a β-ketoester substrate to the corresponding enantioenriched α-arylated product. Computations show how chirality is transferred from the N-bound chiral auxiliary to the final products. These are the first theoretical studies of aryne trapping by chiral nucleophiles to set new stereocenters. Our approach provides a solution to the challenging problem of stereoselective β-ketoester arylation/alkenylation, with formation of a quaternary center.

Enantioselective Total Syntheses of Methanoquinolizidine-Containing Akuammiline Alkaloids and Related Studies

The akuammiline alkaloids are a structurally diverse class of bioactive natural products isolated from plants found in various parts of the world. A particularly challenging subset of akuammiline alkaloids are those that contain a methanoquinolizidine core. We describe a synthetic approach to these compounds that has enabled the first total syntheses of (+)-strictamine, (-)-2( S)-cathafoline, (+)-akuammiline, and (-)-Ψ-akuammigine. Our strategy relies on the development of the reductive interrupted Fischer indolization reaction to construct a common pentacyclic intermediate bearing five contiguous stereocenters, in addition to late-stage formation of the methanoquinolizidine framework using a deprotection-cyclization cascade. The total syntheses of (-)-Ψ-akuammigine and (+)-akuammiline mark the first preparations of akuammiline alkaloids containing both a methanoquinolizidine core and vicinal quaternary centers. Lastly, we describe the bioinspired reductive rearrangements of (+)-strictamine and (+)-akuammiline to ultimately provide (-)-10-demethoxyvincorine and a new analogue thereof.

Understanding and Interrupting the Fischer Azaindolization Reaction

Experimental and computational studies pertaining to the Fischer azaindolization reaction are reported. These studies explain why pyridylhydrazines are poorly reactive in Fischer indolization reactions, in addition to the origin of hydrazine substituent effects. Additionally, an interrupted variant of Fischer azaindolization methodology is disclosed, which provides a synthetic entryway into fused azaindoline scaffolds.

Pardon the Interruption: A Modification of Fischer's Venerable Reaction for the Synthesis of Heterocycles and Natural Products

This account provides an overview of our laboratory's studies of an unusual variant of the Fischer indolization reaction. We describe the discovery of the so-called 'interrupted Fischer indolization' and the development of the reaction from a methodological standpoint. In addition, our efforts to evaluate and apply this methodology in the context of akuammiline alkaloid total synthesis are discussed.

Enantioselective Total Syntheses of Akuammiline Alkaloids (+)-Strictamine, (-)-2(S)-Cathafoline, and (-)-Aspidophylline A

The akuammiline alkaloids are a family of natural products that have been widely studied for decades. Although notable synthetic achievements have been made recently, akuammilines that possess a methanoquinolizidine core have evaded synthetic efforts. We report an asymmetric approach to these alkaloids, which has culminated in the first total syntheses of (-)-2(S)-cathafoline and the long-standing target (+)-strictamine. Moreover, the first enantioselective total synthesis of aspidophylline A is described.

Computational predictions of substituted benzyne and indolyne regioselectivities

A computational study using DFT methods was performed for an array of mono and disubstituted benzynes and indolynes. The inherent distortion present in the geometry-optimized structures predicts the regioselectivity of aryne trapping by nucleophiles or cycloaddition partners. These studies will serve to enable the further use of unsymmetrical arynes in organic synthesis.