C-Selective and Diastereoselective Alkyl Addition to β,γ-Alkynyl-α-imino Esters with Zinc(II)ate Complexes

Since umpolung -imino esters contain three electrophilic centers, regioselective alkyl addition with traditional organometallic reagents has been a serious problem in the practical synthesis of versatile chiral -amino acid derivatives. An unusual C-alkyl addition to -imino esters using a Grignard reagent (RMgX)-derived zinc(II)ate was developed. Zinc(II)ate complexes consist of a Lewis acidic [MgX](+) moiety, a nucleophilic [R3Zn](-) moiety, and 2[MgX2]. Therefore, the ionically separated [R3Zn](-) selectively attacks the imino carbon atom ,which is most strongly activated by chelation of [MgX](+). In particular, chiral ,-alkynyl--imino esters can strongly promote highly regio- and diastereoselective C-alkylation because of structural considerations, and the corresponding optically active -quaternary amino acid derivatives are obtained within 5minutes in high to excellent yields.