Spirocyclic Zwitterionic λ⁵Si-Silicates with Two Bidentate Ligands Derived from α-Amino Acids or α-Hydroxycarboxylic Acids:  Synthesis, Structure, and Stereodynamics

A series of zwitterionic λ⁵Si-silicates with a (2,2,6,6-tetramethylpiperidinio)methyl group and two identical bidentate ligands derived from glycine, (S)-alanine, (S)-phenylalanine, glycolic acid, (S)-lactic acid, (S)-3-phenyllactic acid, or (S)-mandelic acid were synthesized and structurally characterized (solution and solid-state NMR spectroscopy; single-crystal X-ray diffraction). The chiral λ⁵Si-silicates with ligands derived from optically active α-amino acids or α-hydroxycarboxylic acids were isolated as enantiomerically and diastereomerically pure compounds that undergo a (Λ)/(Δ)-epimerization in solution.