Cinchona alkaloid based bifunctional urea catalyzed enantioselective aldol reactions between alpha azido ketones and alpha oxo esters

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2013
Okumuş, Seda
2-Azido-3-hydroxy-1,4-diones are valuable multifunctional synthons for many synthetic transformations. Selective manipulations of different functionalities of these compounds make them potential precursors for the synthesis of α-amino ketones, azido alcohols, 1,2-amino alcohols, phosphoranes and 1,2,3-triazoles. Chemoselective reduction of azide group leads to 1,2-amino alcohols which are great importance in pharmacological research on analgesics, anaesthetics etc. Due to the well-known stereoselectivity of receptorial centers in the cell, asymmetric synthesis of enantiopure drug intermediates are essential. In this project, we developed a new method for the asymmetric synthesis of ethyl 4-aryl-3-azido-2-hydroxy-2-methyl-4-oxobutanoates by chiral base promoted aldol addition of α-azido ketones to ethyl pyruvate. By the deprotonation of highly acidic alpha protons, α-azido ketones gain nucleophilic property and resulting carbanion readily reacts with electrophilic carbonyl moiety. Moreover, chiral bifunctional cinchona alkaloid-urea organocatalyst offers not only chiral induction in the product, but also simultaneous activation of nucleophile and electrophile. In the first part, after the synthesis of α-azido ketones according to the literature procedure, several parameters such as solvent, catalyst loading, substrate concentration and temperature were screened for the aldol addition. In the second part, derivatization of products was performed at optimized reaction conditions. Consequently, high diastereoselectivities up to 18: 1 and good enantioselectivities up to 81% enantiomeric excess were achieved in the desired products.