Catalytic enhancement effect of a chiral ligand on the asymmetric Mannich-Type reactions of menthyl alkanoates with aldimines

The lithium enolate of menthyl acetate underwent the Mannich-type reaction with benzaldehyde PMP-imine in toluene to give the corresponding adduct with 70:30 dr. The diastereoselectivity was dramatically improved to 97:3 dr by the addition of chiral 1.2-diphenyl-1,2-dimethoxyethane. The reaction of menthyl isobutyrate with imines was also influenced by a catalytic amount (5 mol%) of a chiral tridentate aminodiether ligand to give the corresponding beta-lactams in high enantioselectivities. Matching and mismatching phenomena were observed by the reaction of L- and D-menthyl isobutyrates.