CONFORMATIONAL STUDIES BY DYNAMIC NMR .52. STRUCTURE AND STEREOMUTATION OF CONFORMATIONAL ENANTIOMERS OF 1,8-DIACYLNAPHTHALENES

Contrary to the symmetrically substituted 1,8-dibenzoylnaphthalenes the corresponding 1,8-diacylnaphthalenes were found to exist solely in the anti (racemic) conformation. The structure was assessed by C-13 (magic angle spinning) NMR spectroscopy as well as by X-ray diffraction in the solid and by differential. NOE experiments in solution. The presence of prochiral substituents allowed to measure the degenerate enantiomerisation barrier by computer simulation of the H-1 NMR line shape as function of temperature. The barriers increased with the bulkiness of the alkyl substituents R bonded to the carbonyl moiety: they cover the range 6.7 kcal mol(-1) (R = CH(2)Me) to 9.0 kcal mol(-1) (R = CH(2)Ph). Molecular mechanics calculations suggest that the stereomutation is a stepwise rotational process with the R groups passing over the position 2 of the naphthalene ring.