The complete analysis of the "C NMR spectra of 30 cedrane derivatives with various functionalities on the 2,6, 6,8-tetramethyltricycl0[5.3.1*~~0~~~]undecane skeleton is reported. The assignment of the signals to the appropriate carbons bas been made using the various functionalities on the molecular framework and the chemical shift theory (a, f3, y effects). When necessary, off-resonance decoupling, I N E R and DEPT experiments have been used, as well as specific deuterium labelling. The last ambiguities were resolved with the aid of lanthanide shift reagents and SFORD experiments. The assignments for the reference compounds neoisocedranol oxide, cedrol and cedrene were made using the carbon-carbon connectivity 2D INADEQUATE pulse sequence. The examination of cedrane derivatives functionalized on C-9 (carbonyl, hydroxyl, acetate) and C-8 (methyl, axial or equatorial) showed that the "C chemical shifts of C-5, -6, -11 and -13 are sensitive probesfor the conformation of the six-membered ring (ring C), which is essentially a flattened chair, and for the stereochemistry of the C-8 and C-9 substituents. Comparison with 2-methylbicyclo[3.2.l]octan-3-ones shows that the Anti Reflex effect is operative in cedrane derivatives.
Cedrane oxides and their derivatives have also been examined and the corresponding "C data are reported. Using all this inforniation it was possible to carry out a complete analysis of lac acids, which have a functionalized tricycl0[5.3~", O1.sJundecane framework like the cedrane derivatives. The "C spectral analysis shows that, from the structural point of view, this tricyclic system behaves as a bicyclo[3.3.0] skeleton with an additional three-carbon bridge between C-1 and C-7, rather than as a bicyclo[3.2.l]octane with an additional cyclopentane fused on C-1 and C-5; when additional strain is introduced into this system, the five-membered ring A and the six-membered ring C undergo conformational deformations more easily than ring B.