Quaternary Chiral β^2,2-Amino Acids with Pyridinium and Imidazolium Substituents

Abstract

The reactions of cyclic sulfamidates as electrophiles with a variety of nitrogen-containing aromatic heterocycle nucleophiles, such as pyridines, N-alkylimidazoles and N-methylbenzimidazol, was explored. In all cases, although the nucleophilic substitution reactions occurred on quaternary centres, elimination products were not detected. The inversion of configuration at this quaternary centre was determined by X-ray diffraction analysis and the enantiomeric excess of the reactions was checked by chiral HPLC. This synthetic approach allowed us to obtain a new family of chiral charged β^2,2-amino acids, including a new bisamino acid that incorporates an imidazolium salt as a cross-linker. In this context, the treatment of these chiral imidazolium salts with Ag₂O opens the way to new chiral N-heterocyclic carbenes, which are important substrates in the fields of organometallic and organocatalytic chemistry. Additionally, we have done a thorough conformational analysis of these β-amino acid derivatives, both in the solid state and in solution. The most important conformational feature of these acyclic systems is the rigidity of the N-CH₂-C-N⁺ dihedral angle, which is essentially due to the gauche effect.