Properties and Reactions Important for Structure Determination

For structure determination of the Rabdosia diterpenoids, spectroscopic investigations and chemical reactions are useful in general and X-ray analysis is used by necessity. 

Oxidation patterns of ring D in Rabdosia diterpenoids derived from ent-16-kaurene are classified into 5 types shown in Table VI. Each class exhibits characteristic spectral data, from which the structural features of ring D are easily deduced. Table VI shows typical UV, IR, and NMR data for a typical example belonging to each class. 

This type of diterpenoids contains C-20-non-oxygenated and C-20-oxygenated w kauranes. The latter are classified into three groups on the basis of their oxidation patterns. Table VII exemplifies representative diterpenoids of the four groups and their NMR data which are characteristic and therefore useful for structure determination. 

Longikaurin C-type diterpenoids (6 representatives) contain an oxygen function at C-18 or C-19 in addition to the oridonin-type oxidation pattern. Whether C-18 or C-19 is oxidized can be ascertained by establishing the presence or absence of an NOE between C-18 and C-20 or between C-19 and C-20 51, 52). 

The conformations of rings A, B, and C in these ewt-kaurane-type diterpenoids were elucidated by X-ray analysis and use of NMR data. Thus, rings A, B, and C of shikoccidin (15) were shown to have chair conformations (73), while ring A of mebadonin (16) was shown to have a skew boat conformation because of the steric interaction between the 20- 

---