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Title: Synthesis of homochiral thiazolium catalysts
Author: Gerhard, A. U.
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 1997
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This thesis describes the synthesis of six bicyclic homochiral thiazolium salts for the catalysis of certain carbon-carbon bond formation reactions in an asymmetric manner. The synthesis of chiral thiazolium salts has been described in the literature previously. However, these catalysts gave either high chemical yields and low enantiomeric excess or low chemical yields and only moderate asymmetric induction in the carbon-carbon bond formation reaction. The synthesis of the rigid bicyclic thiazolium salts was attempted by the well-known α-haloketone route. This failed to yield the corresponding thiazolium salt because attempts to synthesise the α-haloketones of the bicyclo[2.2.1]heptane series gave only Wagner-Meerwein rearranged β-haloalcohols. The synthesis of the α-bromoketone of the bicyclo[2.2.2]octane series, however, was successful. Unfortunately, it did not react with thiocarbamates or thioformamide to give the desired thiazolium salts, probably due to steric congestion. Alternatively, thiazoles can be formed via the α-aminoketone route. The thiazoles were prepared in a three step synthesis, starting from bicyclic[2.2.2]octane silyl enol ethers in up to 35% yields. The silyl enol ethers were then converted into the α-ketooximes followed by reduction, formylation and thiocyclisation to give the desired thiazoles. The enantiomers of the thiazoles were separated by fractional recrystallisation of their diastereomeric salts with (R)- or (S)-camphorsulfonic acid. The homochiral thiazoles were then converted into the corresponding methyl and benzyl thiazolium salts. The catalysts were tested in the benzoin condensation to give benzoin, and in the dimerisation of butyraldehyde to produce butyroin. Benzoin was formed in up to 100% chemical yield and up to 25% e.e. For the first time, asymmetric induction was observed in the dimerisation of aliphatic aldehydes catalysed by thiazolium salts. Butyroin was formed in up to 70% chemical yield by only 8% e.e.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available