Synthesis of Poly-γ-S-2-methylbutyl-l-glutamate and Poly-γ-S-2-methylbutyl-d-glutamate and Their Use as Enantiodiscriminating Alignment Media in NMR Spectroscopy

Graphical Abstract

A perfect match? The synthesis of two diastereomeric polyglutamate derivatives (see scheme) and their application as enantiodiscriminating alignment media for small organic molecules in NMR spectroscopy is described.

Abstract

Lyotropic liquid crystalline (LLC) phases of polyglutamic acid derivatives, such as poly-γ-benzyl-l-glutamate, are suitable alignment media for organic structure elucidation by NMR spectroscopy. Their helical structure is responsible for enantiodiscrimination. As part of our ongoing investigations concerning the alignment mechanism(s) of these systems, we considered whether an additional chiral center in the side chain could improve enantiodiscrimination relative to the helical polymer with an achiral side chain. Therefore, the diastereoisomers poly-γ-S-2-methylbutyl-l-glutamate (PSMBLG) and poly-γ-S-2-methylbutyl-d-glutamate (PSMBDG) were synthesized. These two polymers were tested for their liquid crystallinity and suitability as alignment media. The spatial structure of the solutes (−)-curcumol and isopinocampheol (IPC) were validated by the residual dipolar coupling data obtained. Additionally, enantiodiscrimination of IPC was observed in the new alignment media and compared with the enantiodiscrimination of IPC in other homopolypeptides.