The Complete Stereostructure of Capsidiol. X-Ray Analysis and 13C Nuclear Magnetic Resonance of Eremophilane Derivatives Having trans-Vicinal Methyl Groups

Abstract

To define the stereochemistry of capsidiol (1), an antifungal sesquiterpene isolated from sweet peppers, a detailed 13C n.m.r. investigation was carried out. From the results for 1 and a series of its derivatives (2–6) compared with those for several well-defined model compounds (7–21) it has been established that the vicinal methyl groups are trans, in contrast to all other previously described eremophilanes. This was confirmed by an X-ray structure analysis of 1 which also established the stereochemistry of the isopropenyl group as well as the detailed conformations of the two rings.The 13C spectra for the series 1–21 have been completely analyzed and these results are discussed in terms of the observed substituent effects. Further evidence of deshielding syn-axial δ effects is presented.