Terpenes fromOtacanthus coeruleusLindl.: Identification of β‐copaen‐4α‐ol and a new criterion for discriminating between isomeric copaene and ylangene structures

The structure of the new sesquiterpene alcohol β‐copaen‐4α‐ol, isolated as a major constituent of the volatile fraction ofOtacanthus coeruleusLindl., was assigned using NMR information. The otherwise difficult discrimination between copaene and ylangene structures can be made unambiguously by a13C‐NMR criterion. All13C‐chemical shifts of the carbon atoms in copaene‐ylangene pairs are nearly superimposable, except for those of C‐2 and C‐6 which show striking differences (11 ppm). These differences are rationalized by invoking sterically induced shifts caused by 1,4‐'flagpole'‐interactions between the hydrogen atoms on C‐6 and C‐9 (for copaenes) or on C‐2 and C‐9 (for ylangenes) in boatlike conformations. In the1H‐NMR spectra, the chemical shift of the ‘compressed’ hydrogen atoms H‐6 (in the copaene) and H‐2 (in the ylangene) shows a 0.5 ppm downfield shift in comparison with the chemical shift of the unhindered ones (H‐2 in the copaene and H‐6 in the ylangene).