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Terpenes from Otacanthus coeruleus Lindl.: Identification of β‐copaen‐4α‐ol and a new criterion for discriminating between isomeric copaene and ylangene structures
Author(s) -
De Buyck L. F.,
De Pooter H. L.,
Schamp N. M.,
De Bruyn Ré,
Zhang Weiguo,
Buděšínský M.,
Motl O.
Publication year - 1989
Publication title -
flavour and fragrance journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.393
H-Index - 70
eISSN - 1099-1026
pISSN - 0882-5734
DOI - 10.1002/ffj.2730040203
Subject(s) - chemistry , sesquiterpene , steric effects , terpene , chemical shift , stereochemistry , hydrogen , organic chemistry
The structure of the new sesquiterpene alcohol β‐copaen‐4α‐ol, isolated as a major constituent of the volatile fraction of Otacanthus coeruleus Lindl., was assigned using NMR information. The otherwise difficult discrimination between copaene and ylangene structures can be made unambiguously by a 13 C‐NMR criterion. All 13 C‐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 the 1 H‐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).