A carbon-shift analysis of labdanic diterpenes of the manool, agathic acid, and sciadin types is presented. The C-12 and C-20 configurations of sciadin have been determined and the structure of potamogetonin has been revised. 'Dedicated to Professor Edgar Lederer on the occasion of his seventy-fifth birthday. 2Carbon-13 Nuclear Magnetic Resonance Spectroscopy of Naturally Occurring Substances, LXXXI.

A methodology is described which allows the rationalization of the 13C chemical shifts of a variety of compounds including alkanes, alcohols and ketones. The geometries of 250 rigid compounds were computed by the MM2 force field. Multiple linear regression analysis was used to model the "C shifts from topological, geometric and energetic parameters. Among these parameters, the most important were those corresponding to interatomic distances, hybridization state of the quaternary carbons and energetic parameters. The final result is a rationalization of the 13C chemical shifts with an average RMS error of 1.25 ppm.

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