Organocatalytic Approach to α-Substituted syn-α,β-Diamino Acids

Significance

Amidinium salt 1 was found to be a highly active and enantioselective catalyst for the addition of α-alkyl α -nitroesters to N-Boc-imines. A bulky ester group turned out to be essential to obtain high diastereoselectivity due to a synergistic effect with the catalyst. Under mild conditions the products were obtained in moderate to good yields (59-88%) and good to excellent diastereo- and enantioselectivities (dr = 5:1 to >20:1, er = 97:3 to 99.5:0.5). The reason for the opposite and thus unexpected syn-diastereoselectivity of this reaction compared to related transformations (J. N. Johnston and co-workers J. Am. Chem. Soc. 2004, 126, 3418; J. Am. Chem. Soc. 2007, 129, 3466) is so far unknown.