Catalytic Approach for the Formation of Optically Active Allyl α-Amino Acids by Addition of Allylic Metal Compounds to α-Imino Esters

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Abstract
A new catalytic enantioselective approach for the formation of allyl α-amino acid derivatives by reaction of N-tosyl α-imino esters with allyl stannanes and silanes catalyzed by chiral copper(I) complexes has been developed. A series of different BINAP and phosphine−oxazoline (P,N) ligands have, in combination with various Lewis acids, been tested as chiral catalysts for allylation of N-tosyl α-imino esters. It has been found that both type of ligands, in combination with copper(I) salts, give highly valuable unsaturated α-amino acid derivatives. The reaction has been investigated for different allyl stannanes and silanes, and it has been found that tri-n-butyl allyl stannane gives the best results of the simple allyl compounds tested, leading to γ,δ-unsaturated α-amino acid derivatives in up to 94% yield and with up to 83% ee, which can be improved to be >95% ee by recrystallization. The reaction has also been investigated using different acyclic and cyclic allyl stannanes leading to various types of unsaturated α-amino acid derivatives in very high yield (up to 95%) and with up to 98% ee. The stereochemistry and absolute configurations of the allyl α-amino acid derivatives have been determined by X-ray analysis, and it is suggested that the reaction takes place as an ene-like reaction.