Despite their importance in pharmaceuticals, many sterically hindered amino acids remain tough to build, particularly with high chiral purity. A team at Boehringer Ingelheim Pharmaceuticals has now surmounted this challenge with carbonyl anion chemistry (J. Am. Chem. Soc., DOI: 10.1021/ja402647m). Jonathan T. Reeves and colleagues explored carbamoyllithium species. These anions that were first generated in 1967 but have rarely been used in asymmetric reactions. When they combined these species with N-sulfinyl imines, which are well-known chiral-directing groups, they obtained α-amino amides, the essential subunit of peptides. The method worked with bulky substrates to produce chiral, highly hindered amino acid derivatives (shown). The team also performed the reaction twice in succession to obtain a dipeptide. “In terms of substrate scope and enantiomeric purity, it will be difficult, if not impossible, for asymmetric catalysis to compete with this method for preparing hindered and rare amino acids,” says Jeffrey W. Bode of ETH Zurich, an expert in peptide synthesis.