1. Synthesis of Analogues of Nosokophic Acid
Timothy J. Tetrault, Deisy Peña-Romero, Marc A. Boudreau
University of New Hampshire, Department of Chemistry, Durham, New Hampshire 03824
Introduction
Lipid Chain Variation
Analogue Synthesis
As infectious pathogens become more and more resistant to modern antibiotics, the need for new antibacterial agents increases every day. β-lactam antibiotics are no longer of practical use in combatting methicillin-resistant Staphylococcus aureus (MRSA)1, causing the medicinal field to rely on last resort drugs such as vancomycin and linezolid to fight MRSA.2
Discovery of Nosokophic Acid
Nosokophic acid, an intermediate in the biosynthesis of phosphoglycolipid natural products, does not exhibit antibacterial activity,3 but is able to potentiate the activity of imipenem against MRSA, reducing its minimum inhibitory concentration (MIC) by 512-fold. Also, nosokophic acid does not potentiate the activity of antibiotics such as vancomycin or tetracycline, suggesting its mechanism of action may be specific to β-lactams.4 Thus, this natural product is an important new agent that restores the efficacy of β-lactams against MRSA.
The project was initiated by synthesizing analogues of nosokophic acid with modifications at the lipid moiety. The first target analogue to be synthesized incorporated a hexdecatetraene chain, rather than the farnesyl chain of nosokophic acid. This longer chain lipid needed to be obtained as its alcohol (L5) to be incorportated into the overall synthesis of the phosphoglycolipid.5
Synthesis of Galactosamide Moiety
The galactosamide fragment (S7) of nosokophic acid can be synthesized in 12 steps, of which eight steps have been completed.8
At the time of the isolation of L4, the synthesis of longer lipid chain analogues was aborted. This was decided upon due to similar compounds tending to increase in toxicity as lipid chains are increased. Therefore, the project was readjusted to synthesize analogues of nosokophic acid with a lipid chain shorter than farnesyl.
Total Synthesis
The project changed to synthesize an analog of nosokophic acid with a lipid chain shorter than farnesyl, in which a prenyl chain (R) was chosen. Since prenol is commercially available, the total synthesis could be started immediately.9
Synthesis of Starting Material
The starting material (I) needed to be synthesized prior to the total synthesis of the analogues. The reaction conditions needed to be changed a few times in order to isolate the desired product. The first conditions were to heat to 60 oC and continue with the work up. The next conditions tried were heating to reflux with a
Potential Analogues
There are three moieties of nosokophic acid that can be varied to develop a robust structure-activity relationship (SAR). Modifications can be done at the amide position of the galactosamide (R1), the glycerate group (R2), and the lipid chain (R3). The variation at the lipid chain can be done to probe the effects of changing double bond geometry, the length of the chain, and the degree of unsaturation on activity.
Dean-Stark apparatus, which resulted in decomposition of the substrate. Since the challenge was the equilibrium of water present, conditions were employed to drive the reaction by water removal. Triethyl orthoformate and lowering the pressure were employed to remove water and facilitate product formation.6-7
Once the synthesis is completed, more analogues with a change at the lipid moiety will be synthesized, focusing on analogues with a shorter lipid chain than the farnesyl chain of nosokophic acid.
References
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Future Work
Once a sufficient amount of analogues are isolated with variation at the lipid moiety, their ability to potentiate imipenem against MRSA will be investigated via MIC studies, as well as their toxicities. This will be repeated by conducting variations at the glycerate moiety and the galactosamide moiety. At the conclusion of this, a robust SAR will be gathered, allowing for mix and matching of promising substituent modifications. If there are promising compounds, they will be evaluated biologically to determine their mechanism of action at a cellular level, as well as pharmokinetic studies.
Acknowledgements
I would like to thank Dr. Marc A. Boudreau, Deisy Peña-Romero, the Boudreau Group, Pat Wilkinson, the University of New Hampshire Department of Chemistry, and my father.