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a-Arylation of Lithiated Allylic and Vinylic Ureas
[Thesis]. Manchester, UK: The University of Manchester; 2014.
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Abstract
Chiral amines are widespread in naturally occurring and synthetic bioactive molecules. However, it remains challenging to synthesise these molecules in an enantiopure form. Investigations within our group have shown that treatment of vinylic ureas with organolithium reagents results in an umpolung addition of the organolithium, followed by N to C aryl migration of the lithiated intermediate. This thesis details investigations into developing an enantioselective version of this reaction through the use of the chiral ligands (−)-sparteine and the (+)-sparteine surrogate. The enantioenriched ureas are easily transformed into the corresponding a-tertiary amines. Use of the different chiral ligands allows access to either enantiomer of the rearranged urea. Further work has centred on the application of this methodology towards cyclic substrates; with a view to synthesising a-tertiary cyclic diaryl amines. Studies have shown that treatment of vinylic ureas bearing cyclic substituents with a number of different organolithium reagents successfully produce the desired rearranged ureas, with complete diastereoselectivity. Subsequent solvolysis of these ureas yields hindered cyclic a-tertiary amines in good yields. During these investigations it has been shown that allylic ureas undergo a rearrangement to the least sterically hindered side of the cyclic urea. This allows for fine tuning of the migration reaction depending on where the double bond is positioned in the molecule. A range of vinylic and allylic ureas, including substrates with interesting pharmaceutical properties, have been investigated. Their synthesis and reactivities upon lithiation are discussed within this thesis.