Stereochemical Assignment of Diastereomeric Imidazolidinone‐Ring‐Containing Bicyclic Sugar‐Peptide Adducts: NMR Spectroscopy and Molecular Calculations

Abstract

A combination of NMR spectroscopy and molecular simulations is used to determine trans/cis configurational features of two diastereomeric bicyclic imidazolidinone compounds obtained by intramolecular cyclization of the monosaccharide ester of leucine-enkephalin related to D-glucose. The stereochemical assignment of the five-membered imidazolidinone ring system was performed by consideration of longrange proton-proton coupling observed between protons at C2 and C4 in the COSY spectrum of the major isomer only. A long-range 4 J H,H coupling across the nitrogen atoms is larger in the trans isomer than in the cis. In addition, comparison of these results with the NMR spectroscopic data obtai-