Spirocyclic Zwitterionic λ<sup>5</sup><i>Si</i>-Silicates with Two Bidentate Ligands Derived from α-Amino Acids or α-Hydroxycarboxylic Acids: Synthesis, Structure, and Stereodynamics
A series of zwitterionic lambda(5)Si-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-phenyl lactic acid, or (S)-mandelic acid were synthesized and structurally characterized (solution and solid-state NMR spectroscopy; single-crystal X-ray diffraction). The chiral lambda(5)Si-licates with ligands derived from optically active a.-amino acids or (alpha-hydroxycarboxylic acids were isolated as enantiomerically and diastereomerically pure compounds that undergo a (Delta)/(Delta)-epimerization in solution.