New methodology for the organocatalysed α-Amination reaction

Doctoral Thesis


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University of Cape Town

Research into organocatalyzed asymmetric reactions has been a rapidly growing and competitive field in recent times, wherein amino catalysis is widely used for the asymmetric functionalisation of carbonyl compounds. Since its simultaneous publication by List and Jørgensen, the organocatalysed α-amination reaction has become a key method for asymmetric heteroatom functionalisation of carbonyl compounds. Herein we report the first application of this methodology to acetals, with the ultimate goal of applying the methodology to the asymmetric desymmetrisation of bis-acetals as a novel contribution to this growing field. Following extensive optimisation, acidic reaction conditions for the reaction were established in which dibenzyl azodicarboxylate (DBAD) was used as the aminating agent and (S)-(-)-5-(2-pyrrolidinyl)-1H-tetrazole as the preferred organocatalyst. The desired aminated products were obtained in high yields and enantioselectivities. The reaction showed broad substrates cope in its application to ketals, dioxolanes and lactols. A hydrazide N-N bond cleavage methodology was also developed for the aminated products in oxazolidinone form. This methodology is based on Magnus' alkylation / E1CB strategy. The novel contribution here is using ditheyl bromoacetate as an alkylating agent and as a better elimination partner. A range of bis-acetals were synthesised via three synthetic routes using malonate-, sulfone and cyclopentene-based synthesis strategies. The acetal reaction was used for the desymmetrisation of two of these bisacetals as a proof of concept. This is a feat not achieved with the more reactive dicarbonyl analogue.

Includes bibliographical references