The controversy about the 13C NMR assignment of the methyl groups in angelates and tiglates was settled by two-dimensional 'H-=C chemical shift correlations for angelic and tiglic acid.

Deuterium-induced NMR isotope shifts of 13C resonance frequencies, 'A(13C), have been measured for a variety of deuterium-labeled hydrocarbons. One-bond effects in ethane, ethylene, and acetylene, as well as in the corresponding phenyl compounds, where found to correlate linearly with carbon hybridization. Upon introduction of -substituents 'A(13C) increases. For methyl-and phenyl-substituted methanes the substituent effect is additive. Data for 1-monodeuterated «-alkanes give support for the steric origin of the well-known -effect on 13C chemical shifts. The torsional angle dependence of 3A(13C)-values shows maxima for dihedral angles of 0°and 180°and minima for 60°and 120°, respectively.

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Combined utilization of 'H-IH homonuclear and 'H-l'C heteronuclear N M R chemical shift correlations,two-dimensional J-resolved 'H NMR measurements and homonuclear 'H double resonance experiments allowed specific assignments for both 'H and -C NMR frequencies to be made for the tricyclic sesquiterpene cyperene. The results also provided information on the conformation of the six-membered ring of this natural product, which was recognized as a distorted chair.Cyperene (1) is a tricyclic monounsaturated hydrocarbon of pleasant odour originally isolated from Cyperus1 and, more recently, from Perezia.' Its structure is closely related to that of a -and y-patch~ulenes,'~~ constituents of the fine perfume of patchouli the only difference being the position of the double bond, which is located6 between carbons 4 and 5 in CYpatchoulene and as an exocyclic methylene group at C-4 in y-patchoulene. Two other related molecules, @-and 6 -p a t~h o u l e n e ,~~~ have the 4,7-methanoazulene skeleton"' instead of the 3a,7-methanoazulene framework present in cyperene (1) and in a -and y -patchoulenes. Owing to the close structural relationship of cyperene to the highly expensive and widely appreciated patchoulenes, and the fact that cyperene analogues have already been found as minor constituents in plant m a t e r i a l~, l "~~~-~~ it seemed desirable to undertake detailed NMR studies of the more easily accessible cyperene. These studies might be of further utility for the structural elucidation of other closely related natural products. RESULTS ~The detailed assignment of both the 'H and I3C NMR spectra of cyperene is difficult since, on the one hand, the twelve 'H methylene and methine signals are found between 1.1 and 2.7ppm and, on the other, in the 13C spectrum four methylene signals appear within a range of only 2ppm. Further, since three of these methylene signals are within a very narrow range of only 0.82 ppm, and sp3-sp3 one-bond carbon-carbon coupling constants are of the orderc6 of 35 Hz, the use * Authors to whom correspondence should be addressed.of double quantum coherence17 measurements is not recommended since, in such cases,l* strongly coupled AB systems with distorted intensities arise.Therefore, the 'H and 13C spectral parameters for cyperene were deduced from both heteronuclearl' (lH-13C) and homonuclear2' (lH-lH) chemical shift correlations, and several coupling constants were obtained from the homonuclear J-resolved'l 'H NMR spectrum and from one-dimensional 'H measurements with the aid of spin-spin decoupling experiments, which also provide information on the conformation of the six-membered ring of the natural product 1.Preliminary information about the number of directly attached protons to each sp3 carbon atom was obtained from two APT (attached proton test) experi-