Enantioselective Allylations of Azlactones with Unsymmetrical Acyclic Allyl Esters

Abstract

A catalytic asymmetric synthesis of quaternary amino acids has been developed. The method derives from the asymmetric allylic alkylation (AAA) reaction with chiral palladium catalysts derived from π-allylpalladium chloride dimer and the bis-2-diphenylphosphinobenzamide of R,R-1,2-diaminocyclohexane and related ligands. Highly symmetrical allylating agents such as allyl acetate and 2-methallyl acetate give moderate to low ee. On the other hand, 1-monosubstituted and 1,1-disubstituted allyl systems give excellent results with ee's normally ≥90%. A most interesting dichotomy occurs in the facial selectivity with respect to the azlactone as it depends on the allylating agent as well as the ligand. For example, prenylation gives 99% ee derived from attack on the si face of the azlactone with a R,R-ligand, but cinnamylation gives a 90% ee of the product derived from attack on the re face with the same ligand. A model based upon the catalyst creating a chiral pocket is presented to explain these results. Using a trimethylsilyl-substituted allylating agent, excellent ee (97%) was obtained. Protodesilylation then provides the simple allylated amino acid with high ee. Oxidative cleavage of these allylated systems provides a practical asymmetric synthesis of α-alkylated aspartic acids where variation of the alkyl group derives from using variously substituted azlactones. The ability to modify the double bond provides further flexibility to generate unusual amino acids.