Non-equivalence of gem-dimethyl protons in some bi- and tricyclic compounds. Stereochemical assignments in the khusenic, isokhusenic, and chamigrene series

The non-equivalence of gem-dimethyl protons in two novel series of tricyclic sesquiterpenes, khusenic (1) and isokhusenic (2), is discussed in terms of anisotropy effects and compared with that found in the analogous monoterpenic compounds, fenchone (8), pinocamphone (9), camphene (10a), methylenenorbornane (10b), camphenilone (11), and dimethylnorcampholide (12). The stereochemistry of the gem-dimethyl groups has been assigned in the above compounds. The chemical shift data and the stereochemistry of the above compounds has been used to make stereochemical assignments in the novel chamigrene series 13, 15, 17, and 19.