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.