X-ray crystallographic and NOE studies on the conformation of periplanones and their analogues

The structure of periplanone-A was established by X-ray crystallographic analysis. In comparison of the X-ray and ¹H NMR data with those of periplanone-B, it was shown that periphanone-A and -B adopt essentially the same conformation. The epoxy epimer of periplanone-A has 10⁴-times lower biological activity than periplanone-A, and NMR analysis indicated that the molecule exists in a mixture of different conformera. The conformation of the regioisomer, in which the carbonyl and epoxy groups are transposed, was analysed by X-ray crystallography and ¹H NMR spectroscopy. It is highly biologically active, and the conformation of the germacranoid skeleton was shown to be almost identical with that of periplanone-A. Simplified analogues and germacrene-D had relatively lower activity. NOE Experiments on these compounds suggested the conformational resemblance of their germacranoid skeletons with those of periplanone-A and -B.