Fortschritte der Chemie organischer Naturstoffe / Progress in the Chemistry of Organic Natural Products

The structure of eunicillin (89), a carbobicyclic diterpenoid isolated from the Mediterranean gorgonian Eunicella stricta, was reported in 1968 (1). At that time chlorine-containing diterpenoids had been dis- covered in the gorgonian Briareum asbestinum (2, 3), but it was not until 1977 that the structure of the first briaran, briarein A (211), was resolved by X-ray analysis (4). The first trinervitane diterpenoid (26), which was isolated from Trinervitermes termites, was reported in 1976 (5). OAc OAc ACO'**** OH 0 89 211 26 The discovery of these compounds marked the appearance of a large and growing group of diterpenoids which are commonly viewed as being formed from cembrane precursors by secondary carbon-carbon bond closures. It should be emphasized, however, that in the absence of biosynthetic evidence the question of which groups of diterpenoids that should be classified as cyclized cembranoids remains ambiguous. In the present article we have included five tobacco diterpenoids (1-5) which. possess prerequisite structural features and co occur with appropriate cembrane precursors in tobacco. Although structurally reminiscent of cyclized cembranoids, verticillanes, taxanes and cleomeolide are not dealt with, since these diterpenoids of plant origin are not believed to arise via preformed cembranoids (6-11).