Taday, Freya Frances Hughes
(2021)
Transaminase-triggered cascades for the construction of complex alkaloids.
PhD thesis, University of Nottingham.
Abstract
The natural defence mechanisms of plants often involve the use of secondary metabolites known as alkaloids. Alkaloids have been shown to act as poisons and herbicides to discourage predators or supress competition from other plants. This class of metabolite has been found to have biologically useful functions, including acting as antibiotics, anticancer agents and many more. However, synthesis of these molecules often involves the use of harsh, unsustainable methodologies.
During the biosynthesis of alkaloids, there are multiple cascade reactions taking place, for example, in the biosynthesis of pelletierine, where a transaminase (TA)-catalysed reaction proceeds via a Mannich reaction. The introduction of biocatalysts into the chemical toolbox means that research can look at ways in which organisms perform a synthesis and mimic this in the laboratory. The overall aim of this project was to use a transaminase reaction to trigger a concurrent cascade and enable access to a range of alkaloid scaffolds, which could then serve as templates for drug-like molecules.
One aspect of this project employed a transaminase-triggered intramolecular aza-Michael reaction for the preparation of cyclic -enaminones in good yields (43 – 95 %), with excellent enantio and diastereoselectivity, starting from prochiral ketoynones (Scheme A). This methodology required only two equivalents of isopropylamine, reflecting the powerful thermodynamic driving force associated with the spontaneous aza-Michael reaction, which displaces the transaminase reaction equilibrium towards product formation.
To further demonstrate the potential of this methodology, the original cascade was combined with annulation chemistry, either in a concurrent cascade to provide a range of tetrahydroquinolines (THQ) in a range of yields (10 – 90 %) or in a two-step approach to afford a selection of fused-ring cyclic alkaloids (44 – 92 %).
The second aspect of this work focused on the development of the entirely novel concept of ‘amine borrowing’ via shuttle catalysis. Shuttle catalysis is a relatively new methodology and involves in situ catalytic shuttling of a functional moiety from one molecule to another. The approach is often employed to avoid the handling of toxic or unstable intermediates. The biocatalytic transaminase reaction lends itself to shuttle catalysis and proved a useful transformation for the development of the concept of amine borrowing (scheme B).
It was hypothesised that by using a transaminase to shuttle the amine moiety from the donor molecule to the acceptor molecule, two reactive species could be generated in situ. These species are then able to undergo an intermolecular reaction, leading to the ‘borrowed’ amine moiety ultimately being incorporated into the target compound and representing the first example of amine borrowing (Scheme B). Three reactions were chosen to investigate amine borrowing including the Mannich, Knorr Pyrrole synthesis and Pictet-Spengler reactions (Scheme C).
When delving into the transaminase-Mannich cascade, initial results revealed the Mannich reaction was able to be conducted under biotransformation conditions to afford the natural product pelletierine in a 74 % conversion. However, this observed conversion was not under amine borrowing conditions and to date, suitable substrates and conditions have yet to be established for this transaminase triggered Mannich reaction.
Amine borrowing conditions were successfully employed during the synthesis of pyrrole derivates with good conversions (41 – 99 %) and reasonable yields (32 – 80 %). These conversions result from the pyrrole formation effectively displacing the equilibrium of the transaminase reaction, even in the presence of just one equivalent of available amine donor. The highly reactive intermediate formed in situ highlights as to why it is advantageous to use an amine borrowing methodology.
To further demonstrate the potential of amine borrowing, the conditions were applied to a transaminase-Pictet-Spengler cascade. Multiple screening attempts were conducted to obtain the best conditions for a range of potential amine donors and carbonyls. Despite this, disappointing conversions (1 – 31 %) were only achieved. However, this project presents important proof-of-concept results for completely novel methodology, which has the potential to impact amine synthesis, in the development of high value chemicals.
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