Journal of the American Chemical Society, Vol.138, No.8, 2664-2669, 2016
Direct Catalytic Chemoselective alpha-Amination of Acylpyrazoles: A Concise Route to Unnatural alpha-Amino Acid Derivatives
A direct copper-catalyzed highly chemoselective alpha-amination is described. Acylpyrazole proved to be a highly efficient enolate precursor of a carboxylic acid oxidation state substrate, while preactivation by a stoichiometric amount of strong base has been used in catalytic alpha-aminations. The simultaneous activation of both coupling partners, enolization and metal nitrenoid formation, was crucial for obtaining the product, and wide functional group compatibility highlighted the mildness of the present catalysis. The bidentate coordination mode was amenable to highly chemoselective activation over ketone and much more acidic nitroalkyl functionality. Deuterium exchange experiments clearly demonstrated that exclusive enolization of acylpyrazole was achieved without the formation of a nitronate. The present catalysis was applied to late-stage alpha-amination, allowing for concise access to highly versatile alpha-amino acid derivatives; The product could be transformed; into variety of useful building blocks.