The early C6 oxidation pathway

In this pathway (Fig. 3), campestanol is hydroxylated at C6 to give 6a-hydroxy-campestanol. This steroid is further oxidized to 6-oxocampestanol [41]. It is predicted that 

6-oxocampestanol is hydroxylated at C22 to give cathasterone because both compounds are endogenous in cultured cells of C. roseus. However, this conversion could not be observed [42], leaving the possibility that cathasterone may be synthesized by 

23-hydroxylation of unknown precursors other than 6-oxocampestanol. 

Teasterone is then converted to typhasterol via 3-dehydroteasterone [45 7] and this is also the case in the lily cells [48]. This reaction includes a (3 to a inversion of the 

3-hydroxyl by successive oxidation and reduction. Such epimerization of the 3-hydroxyl has also been observed in the metabolism of bile acids, ecdysteroids and cardenolide. 

---

Typhasterol is then hydroxylated at C2 to give castasterone having a pair of vicinal hydroxyls which is very frequently and abundantly found in plants. The pathway from teasterone to castasterone was also confirmed in the seedlings of C. roseus, tobacco and rice, indicating that the early C6 oxidation pathway is operative in intact plants [49]. 

---