# A Supramolecular Extension of Mosher’s Method: Absolute Configuration Assignment of N-Amino Acid Derivatives via Bis-Thiourea Chiral Solvating Agent

## TL;DR

This paper introduces a new NMR method using a chiral solvating agent to determine the absolute configuration of amino acid derivatives without chemical modification.

## Contribution

The study introduces a non-derivatizing, supramolecular extension of Mosher’s method using a bis-thiourea CSA for stereochemical analysis.

## Key findings

- The BTDA CSA enables reliable determination of absolute configuration in DNB amino acid derivatives via NMR.
- Chemical shift differences correlate with stereochemistry and are supported by ROESY and binding constant measurements.
- The method works effectively with pure enantiomers and offers operational simplicity and broad applicability.

## Abstract

The bis-thiourea chiral solvating agent (CSA) BTDA enables the NMR-based determination of absolute configuration in N-3,5-dinitrobenzoyl (DNB) amino acid derivatives without requiring covalent derivatization. A reliable trend of the sense of nonequivalence and absolute configuration is found in both 1H and 13C NMR spectra. A dual-enantiomer approach, using (R,R)- and (S,S)-BTDA, generates diastereomeric complexes with the enantiopure substrate, and distinct spatial arrangements are reflected in consistent and interpretable Δδ values. The observed chemical shift differences correlate reliably with the stereochemistry of the chiral center and are further supported by ROESY (Rotating-frame Overhauser Enhancement SpectroscopY) experiments and binding constants’ measurements, confirming the formation of stereoselective non-covalent complexes. This methodology extends the logic of Mosher’s analysis to solvating agents and remains effective even in samples containing single pure enantiomers of the amino acid derivative. The BTDA-based dual-CSA system thus represents a robust, non-derivatizing strategy for stereochemical assignment by NMR, combining operational simplicity with broad applicability to DNB derivatives of amino acids with free carboxyl function.