A high-throughput phenotypic screen based on a Citrobacter freundii AmpC reporter expressed in Escherichia coli was executed to discover novel inhibitors of bacterial cell wall synthesis, an attractive, well-validated target for antibiotic intervention. Here we describe the discovery and characterization of sulfonyl piperazine and pyrazole compounds, each with novel mechanisms of action. E. coli mutants resistant to these compounds display no cross-resistance to antibiotics of other classes. Resistance to the sulfonyl piperazine maps to LpxH, which catalyzes the fourth step in the synthesis of lipid A, the outer membrane anchor of lipopolysaccharide (LPS). To our knowledge, this compound is the first reported inhibitor of LpxH. Resistance to the pyrazole compound mapped to mutations in either LolC or LolE, components of the essential LolCDE transporter complex, which is required for trafficking of lipoproteins to the outer membrane. Biochemical experiments with E. coli spheroplasts showed that the pyrazole compound is capable of inhibiting the release of lipoproteins from the inner membrane. Both of these compounds have significant promise as chemical probes to further interrogate the potential of these novel cell wall components for antimicrobial therapy. IMPORTANCEThe prevalence of antibacterial resistance, particularly among Gram-negative organisms, signals a need for novel antibacterial agents. A phenotypic screen using AmpC as a sensor for compounds that inhibit processes involved in Gram-negative envelope biogenesis led to the identification of two novel inhibitors with unique mechanisms of action targeting Escherichia coli outer membrane biogenesis. One compound inhibits the transport system for lipoprotein transport to the outer membrane, while the other compound inhibits synthesis of lipopolysaccharide. These results indicate that it is still possible to uncover new compounds with intrinsic antibacterial activity that inhibit novel targets related to the cell envelope, suggesting that the Gram-negative cell envelope still has untapped potential for therapeutic intervention.

Actinophyllic acid is a biologically active indole alkaloid with a unique structural framework that incorporates five contiguous stereocenters. A total synthesis of (±)-actinophyllic acid has been completed that proceeds in only 10 steps from readily available, known compounds and with the isolation of nine intermediates. The synthesis features a novel cascade of reactions of N-stabilized carbocations with π-nucleophiles to create the tetracyclic core of actinophyllic acid in a single chemical operation. This pivotal cascade sequence generates substructures of the actinophyllic acid core that are not otherwise accessible, and one key intermediate was modified to furnish several novel compounds having potentially promising anticancer activity, one of which induces cell death in a wide range of cancer cell lines.

A concise total synthesis of the complex indole alkaloid (±)-actinophyllic acid was accomplished by a sequence of reactions requiring only 10 steps from readily-available, known starting materials. The approach featured a Lewis acid-catalyzed cascade of reactions involving stabilized carbocations that delivered the tetracyclic core of the natural product in a single chemical operation. Optimal conversion of this key intermediate into (±)-actinophyllic acid required judicious selection of a protecting group strategy.

Several novel multicomponent assembly processes have been developed for the rapid and efficient assembly of various heterocyclic scaffolds bearing a tetrahydroisoquinoline core, each of which allows for facile derivatization to access a diverse array of compounds. This work led to the serendipitous discovery of a new method for the synthesis of a fused quinazolone ring system, which was applied to a one-step total synthesis of the quinazolinocarboline alkaloid rutaecarpine.

A strategy for generating diverse collections of small molecules has been developed that features a multicomponent assembly process (MCAP) to efficiently construct a variety of intermediates possessing an aryl aminomethyl subunit. These key compounds are then transformed via selective ring-forming reactions into heterocyclic scaffolds, each of which possesses suitable functional handles for further derivatizations and palladium-catalyzed cross coupling reactions. The modular nature of this approach enables the facile construction of libraries of polycyclic compounds bearing a broad range of substituents and substitution patterns for biological evaluation. Screening of several compound libraries thus produced has revealed a large subset of compounds that exhibit a broad spectrum of medicinally-relevant activities.