CONFORMATIONAL STUDY OF SHIROMODIOL AND RELATED EPOXYGERMACRANES - X-RAY, MOLECULAR MECHANICS AND NMR ANALYSES

The crystal structure of 8-dehydroshiromodiol (4) is reported and compared with that of the related compounds shiromodiol-8-O-angelate (2), its corresponding 6-ketone (3) and echinadiol (5). These similar compounds display different conformations in the solid state, while in solution each derivative is a mixture of conformers, as revealed by the temperature-dependent NMR spectra. Molecular-mechanics calculations on the four molecules were carried out: the computed minimum energy conformation always corresponds to that found by X-ray analysis, with good agreement being found between calculated and experimental values for all geometrical parameters. Molecular-mechanics calculations reveal that four conformers are possible for each of the four compounds; all these geometries are accessible for compounds 3 and 4, whereas only two conformers are possible for 2 and 5. Good agreement was found between the calculated and experimental energy differences between the two conformers of 2 and 5. The intramolecular hydrogen bond in 5 was analysed further using the AM1 semiempirical method and by ab initio calculations on a model fragment. © 1990.