Andrew G. Myers

The emergence of bacterial antimicrobial resistance threatens to undermine the utility of antibiotic therapy in medicine. This threat can be addressed, in part, by reinventing existing antibiotic classes using chemical synthesis. Here we present the discovery of BT-33, a fluorinated macrobicyclic oxepanoprolinamide antibiotic with broad-spectrum activity against multidrug-resistant bacterial pathogens. Structure–activity relationships within the macrobicyclic substructure reveal structural features that are essential to the enhanced potency of BT-33 as well as its increased metabolic stability relative to its predecessors clindamycin, iboxamycin and cresomycin. Using X-ray crystallography, we determine the structure of BT-33 in complex with the bacterial ribosome revealing that its fluorine atom makes an additional van der Waals contact with nucleobase G2505. Through variable-temperature ¹H NMR experiments, density functional theory calculations and vibrational circular dichroism spectroscopy, we compare macrobicyclic homologues of BT-33 and a C7 desmethyl analogue and find that the C7 methyl group of BT-33 rigidifies the macrocyclic ring in a conformation that is highly preorganized for ribosomal binding.

A new, more scalable route for the synthesis of the common oxepanoproline southern fragment of the antibiotic candidates iboxamycin, cresomycin, and BT-33 is presented. A key transformation in the route is a diastereoselective TiCl4-mediated conjugate addition of an allylsilane to an Evans N-acryloyloxazolidinone, followed by syn-aldol addition of the resultant titanium enolate to (R)-Garner’s aldehyde, which assembles all the stereocenters within the target molecule in a single operation.

Scalable syntheses of the northern macrobicyclic thiolincosamine fragments of two structurally complex antibiotic candidates, BT-33 and cresomycin, are presented. A key transformation in each route is the highly diastereoselective addition of a putative allenylzinc nucleophile to a common Ellman sulfinimine intermediate using a zinc-promoted Barbier-type propargylation protocol that is detailed herein. These transformations proceed with dynamic kinetic resolution and use just 1.2 equiv of each respective propargyl bromide precursor.

We report the discovery through chemical synthesis of BT-33, a fluorinated macrobicyclic oxepanoprolinamide antibiotic. BT-33 potently inhibits the growth of multidrug-resistant clinical isolates of Gram-positive and Gram-negative bacteria and has an extended half-life in vivo relative to its predecessors cresomycin and iboxamycin. We report structure-activity relationships within the macrobicyclic substructure, which reveal structural features that are essential to the enhanced potency of BT-33 as well as its increased metabolic stability. We determine the structure of BT-33 in complex with the bacterial ribosome by X-ray crystallography, analysis of which suggests that the newly introduced fluorine atom makes an additional Van der Waals contact with nucleobase G2505. Finally, we show that the C7-methyl group of BT-33 rigidifies the macrocyclic ring in a conformation that is highly preorganized for ribosomal binding by using variable-temperature 1H-NMR experiments, density-functional theory calculations, and vibrational circular dichroism spectroscopy to compare macrobicyclic homologs of BT-33 and a C7-desmethyl analog.

We report the design conception, chemical synthesis, and microbiological evaluation of the bridged macrobicyclic antibiotic cresomycin (CRM), which overcomes evolutionarily diverse forms of antimicrobial resistance that render modern antibiotics ineffective. CRM exhibits in vitro and in vivo efficacy against both Gram-positive and Gram-negative bacteria, including multidrug-resistant strains of Staphylococcus aureus , Escherichia coli , and Pseudomonas aeruginosa . We show that CRM is highly preorganized for ribosomal binding by determining its density functional theory–calculated, solution-state, solid-state, and (wild-type) ribosome-bound structures, which all align identically within the macrobicyclic subunits. Lastly, we report two additional x-ray crystal structures of CRM in complex with bacterial ribosomes separately modified by the ribosomal RNA methylases, chloramphenicol-florfenicol resistance (Cfr) and erythromycin-resistance ribosomal RNA methylase (Erm), revealing concessive adjustments by the target and antibiotic that permit CRM to maintain binding where other antibiotics fail.

The bacterial ribosome is an essential drug target as many clinically important antibiotics bind and inhibit its functional centers. The catalytic peptidyl transferase center (PTC) is targeted by the broadest array of inhibitors belonging to several chemical classes. One of the most abundant and clinically prevalent resistance mechanisms to PTC-acting drugs in Gram-positive bacteria is C8-methylation of the universally conserved A2503 nucleobase by Cfr methylase in 23S ribosomal RNA. Despite its clinical importance, a sufficient understanding of the molecular mechanisms underlying Cfr-mediated resistance is currently lacking. Here, we report a set of high-resolution structures of the Cfr-modified 70S ribosome containing aminoacyl- and peptidyl-transfer RNAs. These structures reveal an allosteric rearrangement of nucleotide A2062 upon Cfr-mediated methylation of A2503 that likely contributes to the reduced potency of some PTC inhibitors. Additionally, we provide the structural bases behind two distinct mechanisms of engaging the Cfr-methylated ribosome by the antibiotics iboxamycin and tylosin.

The mechanism proposed for the transformation of cyclopentanone to the dienoic acid 1, as published in this journal, is revealed to be in error. We show that carbon 11 derives not from dimethyl sulfoxide as proposed but from the dichloromethane present in the "quenching" solution. The intermediacy of an α-chloromethyl ketone and its subsequent fragmentation in the presence of a hydroxide ion is supported by experiments described herein and by extensive literature precedent.

The regioselectivity of a [3+2] dipolar cycloaddition reaction of a stabilized azomethine ylide with an electron-deficient dipolarophile was found to be counter to a report published in this journal.