Asymmetric synthesis of β-lactams and pseudopeptides via stereoselective conjugate additions of lithium (α-methylbenzyl)allylamide to α,β-unsaturated iron acyl complexes
Lithium (α-methylbenzyl)allylamide 2 undergoes stereoselective conjugate additions to α,β-unsaturated iron acyl complexes 3a–c to afford β-amino iron acyl adducts 5a–c and 6a–c. These adducts may be deallylated smoothly using palladium(0) catalysis providing the corresponding homochiral secondary amines 7a–c which, upon oxidative decomplexation with bromine or N-bromosuccinimide, undergo direct β-lactam ring formation. The diastereoselectivity for the conjugate addition to 3a may be further improved by the use of magnesium amide 10. Oxidative decomplexation of one of the β-amino iron acyl adducts 5b in the presence of α-amino esters provides pseudopeptide fragments comprising an α-amino acid coupled to a β-amino acid.