Neil J. Hales

Pyrrolamide DNA Gyrase Inhibitors: Fragment-Based Nuclear Magnetic Resonance Screening To Identify Antibacterial Agents

ABSTRACT DNA gyrase is an essential enzyme in bacteria, and its inhibition results in the disruption of DNA synthesis and, subsequently, cell death. The pyrrolamides are a novel class of antibacterial agents targeting DNA gyrase. These compounds were identified by a fragment-based lead generation (FBLG) approach using nuclear magnetic resonance (NMR) screening to identify low-molecular-weight compounds that bind to the ATP pocket of DNA gyrase. A pyrrole hit with a binding constant of 1 mM formed the basis of the design and synthesis of a focused library of compounds that resulted in the rapid identification of a lead compound that inhibited DNA gyrase with a 50% inhibitory concentration (IC50) of 3 μM. The potency of the lead compound was further optimized by utilizing iterative X-ray crystallography to yield DNA gyrase inhibitors that also displayed antibacterial activity. Spontaneous mutants were isolated in Staphylococcus aureus by plating on agar plates containing pyrrolamide 4 at the MIC. The resistant variants displayed 4- to 8-fold-increased MIC values relative to the parent strain. DNA sequencing revealed two independent point mutations in the pyrrolamide binding region of the gyrB genes from these variants, supporting the hypothesis that the mode of action of these compounds was inhibition of DNA gyrase. Efficacy of a representative pyrrolamide was demonstrated against Streptococcus pneumoniae in a mouse lung infection model. These data demonstrate that the pyrrolamides are a novel class of DNA gyrase inhibitors with the potential to deliver future antibacterial agents targeting multiple clinical indications.

Small-molecule androgen receptor downregulators as an approach to treatment of advanced prostate cancer

Chemical starting points were investigated for downregulation of the androgen receptor as an approach to treatment of advanced prostate cancer. Although prototypic steroidal downregulators such as 6a designed for intramuscular administration showed insufficient cellular potency, a medicinal chemistry program derived from a novel androgen receptor ligand 8a led to 6-[4-(4-cyanobenzyl)piperazin-1-yl]-3-(trifluoromethyl)[1,2,4]triazolo[4,3-b]pyridazine (10b), for which high plasma levels following oral administration in a preclinical model compensate for moderate cellular potency.

The Azomethine Ylid Strategy in β-Lactam Synthesis. Application to Selenapenams

Abstract Using azomethine ylid reactivity available from the β-lactam-based oxazolidinone 1 , selenoketones 6a – e react as 1,3-dipolarophiles to give racemic selenapenams 7a – e in a single step. The cycloaddition sequence proceeds with complete control of regiochemistry and the thermodynamically more stable C (3)/ C (5) relationship is observed. The selenothiocarboxylate 9a and the selenocarboxylate 9b also function as effective dipolarophiles, but attempts to convert the resulting cycloadducts 10a and 10b to the corresponding selenapenems were unsuccessful. Other selenium-containing dipolarophiles failed to give characterizable cycloadducts.

CHROMIUM CARBONYL COMPLEXES AS NOVEL TRACELESS LINKERS

Abstract Substrates containing aromatic rings have been attached to ‘polymer supported triphenylphosphine’ using a chromium carbonyl linker, chemically manipulated, and released from the polymer to demonstrate the use of π-arene chromium complexes as traceless linkers.

New stable catalysts of the Pauson–Khand annelation

Abstract A range of phosphine- and phosphite-substituted carbonylcobalt(0) complexes have been synthesised and screened as catalysts of the Pauson–Khand annelation. The readily prepared stable solid, heptacarbonyl(triphenylphosphine)dicobalt(0) 1 , performed well (78% yield) in a benchmark annelation reaction.

The application of polymer-bound carbonylcobalt(0) species in linker chemistry and catalysis

Carbonylcobalt(0) species have been used as linkers between alkynes and a polymer support for the first time. The alkynes may be loaded indirectly onto a phosphine functionalised polymer via their hexacarbonyldicobalt(0) complex, or directly onto a cobalt coated polymer. The alkynes have been released either as alkynes, thus providing a traceless method of immobilising alkynes, or by reaction with an alkene to generate a cyclopentenone via the Pauson-Khand reaction. The cobalt coated polymers produced during this study were shown to catalyse the Pauson-Khand reaction.

2-Amino ketene S,S-acetals as α-amino acid homoenolate equivalents. Synthesis of 3-substituted prolines and molecular structure of 2-(N-pivaloylpyrrolidin-2-ylidene)-1,3-dithiane

Allylic deprotonation of the heterocyclic 2-amino ketene S,S-acetal 8a, followed by regioselective γ-alkylation reaction of the resulting organolithium 10(a proline homoenolate equivalent) with electrophiles, leads to adduct 11. Controlled hydrolytic cleavage of 11 gives a series of 3-substituted prolines, including the conformationally-constrained aspartate and glutamate derivatives, 14e and 14f respectively. The bicyclic thiolactam 18 has been prepared in an attempt to provide an asymmetric variant of organolithium 10 but efforts to generate the requisite ketene N,S-acetal 19 were unsuccessful. Extension of the ketene S,S-acetal chemistry to other ring sizes has been examined within the context of substituted azetidine-2-carboxylates. Condensation of the protected amino ester 20 with AIMe3–HS(CH2)3SH was complicated, however, by the reactivity of the four-membered ring and led to the ring-opened adduct 24, with none of the required ketene S,S-acetal 22 being observed.

Polymer-supported cobalt carbonyl complexes as novelsolid-phase catalysts of the Pauson–Khand reaction

Cobalt carbonyl complexes immobilised onto a ‘polymer-bound triphenylphosphine’ solid support are effective and practical catalysts of the Pauson–Khand reaction.

The synthesis and assay of radiolabelled benzene derivatives

Abstract Several new syntheses of 1and 4-[14C]anisole have been investigated and routes from [14C]cyanide and from 2-[14C]acetone are described which are better than those previously reported. The key step in one of the syntheses was catalytic dehydrogenation of 1-[14C]cyclohexanone to 1-[14C]phenol. Degradation of 5,6,7,8-tetrachloro-3,4-dihydro-1,4-etheno-[14C]naphthalen-2(1H)-one (10) prepared from [14C]anisole and tetrachlorobenzyne has shown that the key step had proceeded without scrambling of the label.

Generation of α-amino acid homoenolate equivalents. Synthesis of 3-substituted prolines.

Abstract Deprotonation of the N-protected aminoketene-S,S-acetal (6) and reaction of allylic anion (7) with electrophiles leads to adducts (8) which have been converted to 3-substituted prolines (11). Conformationally constrained variants (11d) and (11e) of aspartic and glutamic acid have been prepared.