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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 40� 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 -50�. 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.

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The wide range of deceptive simplicities observed in spectra of 3,4- dehydro-prolinamide in different solvents makes analysis extremely difficult. The power of an approach using ABXYM tables combined with iterative computation is demonstrated to obtain accurate parameters for spectra of saturated solutions in deutero-chloroform and deuterium oxide. A detailed study of the effects of small changes in parameters on the observed spectral patterns provides a satisfactory explanation for each case of deceptive simplicity and, in particular, gives reasons for the added simplicity of spectra measured at 100 Mc/s. A trans- homoallylic coupling constant (J2,5) of 5.8 c/s was observed and a synergistic four-bond coupling through the heteroatom is suggested to explain its magnitude. The stereochemistry of pyrrolines and similar heterocycles is discussed in the light of this large trans-coupling.