The first enantio- and chemoselective Brønsted-acid catalysed reduction of α-keto esters with catecholborane has been developed. The α-hydroxy esters were obtained under mild reaction conditions in virtually quantitative yields and excellent enantioselectivities. With slight modifications both enantiomers can be obtained without any loss of selectivity.
The chemo-and enantioselective reduction of a-imino esters with catecholborane has been developed employing 10 mol% of an enantiopure BINOL-based phosphoric acid as organocatalyst. Various differently substituted aromatic a-amino acid derivatives can be achieved in almost quantitative yields and very good to excellent enantioselectivities of up to 96% ee under mild reaction conditions.
The ester-substituted triscatechol ligand L-H 6 can be easily prepared in a triple imine condensation reaction. The required trisamine is prepared starting from diphenylamine and 2-bromobenzoic acid methylester. Upon coordination of titanium(IV) ions the tetrahedral M 4 L 4 coordination compound is obtained, which due to the symmetry breaking substituent exists as a mixture of isomers. However, the complex can be unambiguously characterized by ESI-MS.
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