Organoboron compounds are viewed as a crucial intermediate for a wide variety of reactions. The most notorious reaction that exemplifies the capability of organoboron reagents is the Suzuki-Miyaura cross-coupling reaction, which is used to generate carbon-carbon bonds under mild conditions. Because of the versatility of organoboron reagents, methods to selectively install boron remains crucial. Boron containing compounds have recently garnered significant interest in the medicinal chemistry field. Boron's unique properties allows the development of potential new boron-based drugs targeting novel signaling pathways with great efficacy. This dissertation describes the use of a diboron reagent to install boron on the electron withdrawing allenoate scaffold as well as on disubstituted 1,3-diynes. Lastly, this dissertation will cover the preliminary anti-fungal activity of a novel oxaborole scaffold.
We investigated the borylation of the electron withdrawing allenoate scaffold. Reports in the literature were scarce and limited in the scope of the work or required the use of less available diboron reagents. We developed a method for the addition of the diboron reagent B2pin2 and a copper chloride catalyst at 60 °C to generate the (Z)-β-borylen-oate in an 18 – 81% yield. A diverse substrate scope was produced, with the reaction being very tolerable with both electron donating and electron withdrawing functional groups attached to the phenyl ring with yields ranging from 29 – 81%. To our delight as well, straight alkyl chains maintained the respective Z stereoselectivity while having yields range from 46 – 60%. During the reaction, activation of the diboron reagent using the copper catalyst in methanol then undergoes boryl-cupration that can subsequently be protonated to form the Z-product. The steric effect of the activated boron complex and the allenoate drives the stereoselectivity of the reaction.
To continue the borylation of unique scaffolds, we developed a selective cis phosphinoboration of 1,3-diynes. In this reaction, a catalytic amount of tributyl phosphine and a diphenyl(4,4,5,5-tetramethyl-1-3-2-dioxaborolan-2-yl)phosphane is used to generate the corresponding cis 1,2-phosphinoboronate in yields ranging from 18 – 75%. The reaction is performed in dichloromethane at 40 °C. Substrates bearing an electron donating group on the phenyl ring resulted in the need for a longer reaction time and decreased yields (18 – 39%), while substrates bearing an electron withdrawing groups resulted in increased yields (55 – 72%). The phosphinoboration reaction was also tolerable towards heterocycles (64%) and alkyl groups (34 – 53%). This reaction is able to attach both boron and phosphorous simultaneously without the use of a transition metal. Mild oxidation using iodine resulted in oxidation of the phosphorous. The resulting product interestingly generated a B-O-P heterocycle.
In medicinal chemistry, new boron containing scaffolds have shown promising preliminary anti-fungal activity. The oxaborole scaffold is widely seen as a privileged scaffold due to the unique ability of boron to behave as a pharmacophore. The five-membered ring in the oxaborole scaffold also enhances the Lewis acidity of the boron. Our group has previously identified a synthetic route in obtaining a novel 3-monosubstituted oxaborole scaffold. Herein, we have developed a small library of compounds that were tested against a variety of fungal strains. Assaying the library at 25 μg/mL identified multiple hits that allowed the development of a preliminary structure activity relationship profile. Compounds containing electron withdrawing groups on the phenyl ring demonstrated higher anti-fungal activity. This phenomenon is explained due to the change in Lewis acidity of the compound. Incorporating electron withdrawing groups increases the overall Lewis acidity of the oxaborole scaffold, and therefore allowing for stronger covalent interactions in the active site. Increasing the length of the scaffold resulted in a drastic loss in activity, suggesting a smaller scaffold is a necessity. Compounds containing a 4-fluoro, 4-chloro, 4-trifluoromethoxy, and 4-tetrafluoromethoxy were all compounds that consistently observed below 30% cell survival in the candida albicans, aspergillus niger, metarhizium anisopliiae, aspergillus flavus, penicillum chrysogenum, and saccharomyces cerevisiae fungal assays. To further explore the promising potential of the new scaffold, minimum inhibitory concentrations for our lead compounds will be conducted in the future. / Doctor of Philosophy / Boron-containing compounds are prevalent in a multitude of chemical reactions. Due to the versatility of organoborons in chemical transformations, the development for new chemical reactions that install boron is vital. Of great importance in the installation is the ability to perform the reactions under mild conditions and low cost under environmentally friendly fashion. Boron-containing drugs are also a unique scaffold due to the ability boron has in its ability to act as a drug. Boron is able to covalently bind to molecules in the active site, creating an "anchor" that can then therefore deliver the respective therapeutic effect. This dissertation discusses two reactions that install boron in a selective fashion on challenging substrates. The first chapter discusses the installation of boron on a challenging allene scaffold. The focus of the installation is to maintain selectivity of where the boron ends up on the resulting product which we were successful in. The following chapter discusses the installation of both boron and phosphorous in a one-step fashion. Previous methods would require more steps, harsher conditions, and lower overall yields while we can now circumnavigate these challenges in our new method. The third part of my dissertation will discuss the discovery of a novel boron-containing drug scaffold that has promising anti-fungal activity. Anti-fungal drugs are usually difficult to come by, allowing for our scaffold to shine in a challenging field. We have identified multiple hits in our preliminary assays and we can show that manipulating the potential reactivity of the boron can result in greater or lesser anti-fungal activity.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/111144 |
| Date | 06 July 2022 |
| Creators | Szwetkowski, Connor |
| Contributors | Chemistry, Santos, Webster L., Merola, Joseph S., Carlier, Paul R., Schulz, Michael |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Language | English |
| Detected Language | English |
| Type | Dissertation |
| Format | ETD, application/pdf |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
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