Zn-Catalyzed Asymmetric Allylation for the Synthesis of Optically Active Allylglycine Derivatives. Regio- and Stereoselective Formal α-Addition of Allylboronates to Hydrazono Esters

摘要

Enantioselective addition to iminoester and equivalents provides an efficient route to optically active α-amino acids. While catalytic asymmetric Mannich-type reactions of iminoester derivatives have been reported, less progress has been made for the corresponding allylations. We now report catalytic asymmetric allylation of hydrazono esters with allylboronates; the remarkable regio- and stereoselective formal α-addition of allylboronates is also described. Recently we reported catalytic asymmetric allylation of hydrazono esters with allyltrimethoxysilane. While the reactions proceeded smoothly in aqueous media (water/THF = 1/9) in the presence of ZnF_2-chiral diamine to afford the corresponding allylated products in high yields with good selectivities, they suffered from the requirement to employ an excess (3 equiv) of allyltrimethoxysilane, a relatively low reactivity, and a narrow substrate scope. To address these issues we decided to investigate allylboronates as allylating agents instead of allyltrimethoxysilane.