Proton magnetic resonance spectra of 3,4-dehydroproline derivatives

Abstract

Slight changes in the chemical shifts of protons in 3,4-dehydroproline derivatives cause the appearance of their proton magnetic resonance spectra to change markedly, and this can be effected by taking one compound in two different solvents or by observing closely related derivatives in the same solvent. The explanation involves a previously undescribed type of deceptively simple coupling and the necessary conditions are discussed. A first-order analysis of the ABMXX' pattern for 3,4-dehydroprolinamide in deuterium oxide is given. A full analysis is made of the ABXX' and ABXY patterns for this amide in deuterium oxide and deuterochloroform respectively after deuterium exchange of the labile H2; a very large homoallylic coupling is required. Two conformations exist at 40º in solution for all N-benzyloxycarbonyl methyl esters of proline and its derivatives due to restricted rotation about the amide bond. Free rotation of the amide bond of corresponding N-acetyl and AT-benzoyl derivatives still occurs at -50º. The deceptively simple spectrum of N-benzyloxycarbonyl-2,5-dihydroxy-Δ³-pyrolne is discussed and that of its diacetate is analysed as an A₂X₂ system; a very small homoallylic coupling is required.