CONFORMATIONAL STUDIES BY DYNAMIC NMR .45. STEREODYNAMICS AND CONFORMATION OF FURAN-3-ALDEHYDE AND OF ITS CORRESPONDING FUROYL SIGMA-RADICAL

NOE measurements, combined with MM-2 calculations, predict that the proportion of the Z conformer in the title compound is high enough (14% at room temperature) as to give line broadening effects due to E,Z exchange. Line shape analysis of the dynamic C-13 NMR signals at variable temperature yielded a free energy of activation of 8.3 Kcal/mol. One of the lines of the spectrum of the Z conformer was also directly observed (0.8% at -140-degrees-C, corresponding to 11% at room temperature if the entropy difference is assumed zero). ESR spectra and INDO calculations show that also the sigma-radical (obtained by H-abstraction from the HCO moiety of the title compound) essentially adopts the E conformation.