Part I. Synthesis of a Stable, Fluorinated Thromboxane A₂ Analogue and its Application as a Hapten for Monoclonal Antibody Generation. Part II. Difunctionalisation of C–C σ-bonds Enabled by Strained Boronate Complexes.

Abstract

Part I: Synthesis of a Stable, Fluorinated Thromboxane A2 Analogue and its
Application as a Hapten for Monoclonal Antibody Generation.
Cardiovascular disease (CVD) is the leading cause of death worldwide, accounting for
more deaths per annum than cancer, diabetes, and respiratory diseases combined. One
contributing factor to CVD is the overexpression of thromboxane A2 (TxA2)—a member
of the prostanoid family of natural products that assists with blood clot formation.
Pharmacological inhibition of TxA2 has so far failed to meet expectations due to issues
related to selectivity, toxicity, or efficacy. To overcome these problems, a TxA2-specific
monoclonal antibody has been proposed; however, the short half-life of TxA2 (t½ = 32 s,
pH 7.4) renders this impossible. In this respect, 10-fluorothromboxane A2 (F-TxA2) has
been developed as a stable TxA2 mimic using a 15 step process from a key building block
that the Aggarwal group has used routinely in the synthesis of several prostaglandins.
Unfortunately, this route to F-TxA2, and other thromboxanes of interest, is particularly
long and involves significant synthetic challenges. Alongside this, F-TxA2 itself is not
immunogenic and must be attached to a large carrier protein such as keyhole limpet
hemocyanin (KLH) in a structural design known as an antigen, which is required to elicit
an immune response. This thesis details the development of both an improved strategy
towards the total synthesis of thromboxanes, with a focus on thromboxane B2 (TxB2) a
natural metabolite of TxA2, as well as the synthesis of a tetra-valent F-TxA2-containing
antigen. In the former, a key oxidative ring-expansion provided rapid access to the core
of the thromboxanes in the form of a hemi-acylal intermediate. However, the sensitive
nature of this species and its downstream intermediates towards epimerisation ultimately
required various route re-designs. This was able to be overcome with a diastereoselective
dynamic kinetic resolution process to provide stereocontrolled access to a unique acylal
that is currently being evaluated for its potential in the improved total synthesis of TxB2
and eventually F-TxA2. In the latter work, F-TxA2 isolated from the initial total synthesis
was rendered immunogenic by coupling to a tri-lysine peptide and conjugating to KLH.
This strategy allowed for the preparation of a tetra-valent F-TxA2-containing antigen that
has since been administered to murine hosts for the potential generation of TxA2-specific
monoclonal antibody that could find use as a novel therapeutic for the treatment of CVD.

Part II: Difunctionalisation of C–C -Bonds Enabled by Strained
Boronate Complexes.
Cyclobutanes are becoming increasingly prevalent within drug discovery programmes;
however, access to this type of scaffold is generally limited by the lack of synthetic
methods that permit its construction. In this respect, the synthesis of cyclobutanes,
particularly those of defined geometry, is often the limiting step in medicinal chemistry.
This section discloses the development of a modular approach to the diastereoselective
synthesis of 1,1,3-trisubstituted borylcyclobutanes using bicyclobutyl boronate
complexes. These strained intermediates were prepared through the reaction of
bicyclobutyl lithium with boronic esters and found to be highly reactive with a broad
range of electrophiles, facilitating the introduction of both a migrating group and an
electrophile across the central C–C -bond in a formal difunctionalisation process.
The unique reactivity of these highly strained boronate complexes was studied using
in situ IR spectroscopy and showcased in a reaction with benzaldehyde, which was found
to be complete after only 5 min at −78 °C. Furthermore, unique electrophiles such as
carbon dioxide, which has not previously been shown to react with other boronate
complexes, was amenable to the process. Density functional theory (DFT) calculations
were also undertaken to provide a model for the stereochemical outcome of the process,
and finally, the boronic ester handle was further derivatised into a range of useful
functional groups to provide a third point of diversification. This methodology may find
use within drug discovery programmes to rapidly build-up functionalised cyclobutanes
for use in screening libraries.

Date of Award	24 Jun 2021
Original language	English
Awarding Institution	University of Bristol
Supervisor	Chris L Willis (Supervisor) & Varinder K Aggarwal (Supervisor)