Doyle, Amanda (2020) Studies in organic and medicinal chemistry: (i) Synthesis, antimicrobial evaluation, and photophysical studies of novel conjugated systems; (ii) Antimicrobial evaluation of pyrazolopyrimidinone heterocycles. PhD thesis, National University of Ireland Maynooth.
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Abstract
This thesis is divided into three distinct chapters. The first chapter highlights the synthesis
of a family of trans-cinnamaldehydes using a Wittig methodology, and their evaluation
as antibacterial agents against Gram-positive Staphylococcus aureus (S. aureus) and
Gram-negative Escherichia coli (E. coli) in vitro. Cinnamaldehydes are known
electrophiles, with some literature reports suggesting a Michael acceptor role as part of
their antibacterial mode of action. The electrophilicities of our family of transcinnamaldehydes
were calculated, and a relationship between electrophilicity and
observed antibacterial activity was considered. Here, the more electrophilic
cinnamaldehydes appeared to be more active in our antibacterial assays. An in vivo
toxicity assessment of a selection of these trans-cinnamaldehydes was undertaken using
Galleria mellonella larvae, and an initial mechanism of action study was performed using
protein and amino acid leakage as an indication of the effect the trans-cinnamaldehydes
might have on the integrity of the bacterial cell wall and/or membrane. The transcinnamaldehydes
studied were non-toxic and did show some protein and amino acid
leakage, although the results were such that the cell wall/membrane disruption may not
be the primary or only mechanism of action. Overall, the antibacterial activity of the
trans-cinnamaldehydes was modest to low. They were non-toxic and displayed both an
interesting relationship to electrophilicity and an ability to disrupt the cell
wall/membrane. However, they are not strong candidates for further development as
antibacterial agents and would require further structural modification to improve efficacy.
A second antimicrobial study was undertaken with the aim of identifying novel potent
antimicrobial agents. An initial screen of novel structures, which was available in the
Stephens group, identified a pyrazolo[1,5-a]pyrimidin-7-one family of compounds with
antibacterial potential. This family of pyrazolo[1,5-a]pyrimidin-7-ones were synthesized
by other group members and I carried out the antimicrobial activity study. One member
of this family, 2-butyl-5-(3,5-bis(trifluoromethyl))phenylpyrazolo[1,5-a]pyrimidin-
7(4H)-one, was identified as a leading hit compound against S. aureus in the preliminary
study. The family of thirty-five pyrazolo[1,5-a]pyrimidin-7-ones was divided into groups
based on their structure as part of a structure activity relationship (SAR) study, and each
member was tested in vitro against S. aureus (Gram-positive bacteria), E. coli (Gramnegative
bacteria), and Candida albicans (representative fungus). The pyrazolo[1,5-
a]pyrimidin-7-ones showed more potent antibacterial activity than antifungal activity.
The SAR studies resulted in the identification of a leading hit pyrazolo[1,5-a]pyrimidin-
7-one, 2-isopropyl-5-(3,5-bis(trifluoromethyl))phenylpyrazolo[1,5-a]pyrimidin-7(4H)-
one, which showed the highest activity against S. aureus, MIC50 of 1.2 μM. The activity
of 2-isopropyl-5-(3,5-bis(trifluoromethyl))phenylpyrazolo[1,5-a]pyrimidin-7(4H)-one
was superior to that of the commercial antibiotics tested against S. aureus, in that, a MIC50
of 21.8 μM, 11.03 μM, and 5.22 μM was obtained for ampicillin trihydrate, tetracycline,
and streptomycin sulfate, respectively. A selection of pyrazolo[1,5-a]pyrimidin-7-ones,
including our most active candidates, were then evaluated against more clinicallyrelevant
bacterial strains, namely methicillin-resistant S. aureus (MRSA, Gram-positive),
and Pseudomonas aeruginosa (Gram-negative). The most active compound was 2-(4-
trifluoromethylphenyl)-5-(3,5-bis(trifluoromethyl))phenylpyrazolo[1,5-a]pyrimidin-
7(4H)-one, with an MIC50 of 1.88 μM against MRSA, and compares favourably to the
MIC50 of 2.33 mM and 291.72 μM for the commercial antibiotics ampicillin trihydrate
and tetracycline, respectively. An in vivo toxicity assessment and an in vivo therapeutic
evaluation of the most active compounds was completed, using Galleria mellonella and
infected Galleria mellonella respectively. Here, the pyrazolo[1,5-a]pyrimidin-7-ones
evaluated were shown to be non-toxic and, in the therapeutic evaluation study, where
Galleria mellonella were infected with MRSA and subsequently treated with was 2-(4-
trifluoromethylphenyl)-5-(3,5-bis(trifluoromethyl))phenylpyrazolo[1,5-a]pyrimidin-
7(4H)-one, the survival rate improved by 25% for the treated Galleria mellonella over
the non-treated.
Chapter two is dedicated to the development of a synthetic methodology that
allows access to electron deficient trienes. Important applications of trienes can be
broadly divided into (i) medicinal applications and (ii) synthetic applications. An initial
attempt to access (1E,3E,5E)-1,3-bis-phenylsulfonyl-6-phenyl-hexa-1,3,5-triene from
trans-cinnamaldehyde and (E)-1,3-bis-phenylsulfonylprop-1-ene, using traditional
Knoevenagel-condensation reaction conditions, was unsuccessful. We then undertook a
study to determine Knoevenagel-type condensation reaction conditions that would allow
access to the desired triene. The model reaction chosen was that between transcinnamaldehyde
and (E)-1,3-bis-phenylsulfonylprop-1-ene. The study explored reaction
variables such as solvent, base, equivalents of base, and temperature, with conversion to
triene estimated using quantitative NMR. This allowed us to establish the preferred
reaction conditions for our Knoevenagel-type condensation synthetic strategy which used
DCM as solvent, 20 ℃, and 15 equivalents of Al2O3 as base. The desired triene was
generated in 93% isolated yield and the methodology was successfully applied to the
synthesis of 20 trienes. All of these novel compounds underwent full structural
characterization using NMR spectroscopy, liquid chromatography-mass spectrometry
(LC-MS), high resolution-mass spectrometry (HR-MS), and infrared (IR) spectroscopy.
In terms of substrate scope, our synthetic strategy allowed for variation of the starting
cinnamaldehyde, and some variation of the starting propene (nitrile and sulfonyl electron
withdrawing groups). However, our studies to date suggest that employing esters as an
electron withdrawing group on the propene may not be well tolerated. Within our attempts
to employ the esters, an alternative biaryl product was generated when dimethyl
glutaconate was employed as the starting ester propene. Other ester variations resulted in
the generation of complex mixtures.
In chapter three we report a thermal electrocyclization of our electron poor trienes
in the synthesis of a family of phenylsulfonyl biaryls. An optimization study led to the
identification of preferred reaction conditions and an isolated yield of 88% for our model
reaction was obtained using (1E,3E,5E)-1,3-bis-phenylsulfonyl-6-phenyl-hexa-1,3,5-
triene as the substrate. This allowed us to access nine phenylsulfonyl biaryl products. All
novel compounds underwent full structural characterization using NMR spectroscopy,
LC-MS, HR-MS, and IR spectroscopy. A photophysical study was also performed, as the
structure of the biaryls suggested that they may possess some interesting fluorescent
properties via a possible twisted intramolecular charge transfer (TICT) process. The
dimethylamino derivative showed the greatest sensitivity to solvent choice. In terms of
the molar extinction coefficient (ε), values of between 17332 and 37964 M-1 cm-1 were
recorded in chloroform. The highest value was obtained for the choro derivative, and the
lowest value for the dimethylamino substituted derivative. The derivative with the highest
quantum yield of 0.92 was also the dimethylamino derivative, but this was only observed
in the aprotic solvent chloroform. In methanol, the quantum yield dropped dramatically
for the dimethylamino derivative due to a suspected hydrogen bonding effect. The
preliminary photophysical properties suggest that these biaryls are potentially useful and
versatile fluorescent small molecules.
Item Type: | Thesis (PhD) |
---|---|
Keywords: | organic and medicinal chemistry; Synthesis; antimicrobial evaluation; photophysical studies; novel conjugated systems; Antimicrobial evaluation; pyrazolopyrimidinone heterocycles; |
Academic Unit: | Faculty of Science and Engineering > Chemistry |
Item ID: | 16847 |
Depositing User: | IR eTheses |
Date Deposited: | 12 Jan 2023 11:44 |
URI: | https://mu.eprints-hosting.org/id/eprint/16847 |
Use Licence: | This item is available under a Creative Commons Attribution Non Commercial Share Alike Licence (CC BY-NC-SA). Details of this licence are available here |
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