Biotransformation of ent-kaur-16-ene and ent-trachylobane 7β-acetoxy derivatives by the fungus Gibberella fujikuroi (Fusarium fujikuroi).
Candol A (7β-hydroxy-ent-kaur-16-ene) (6) is efficiently transformed by Gibberella fujikuroi into the gibberellin plant hormones. In this work, the biotransformation of its acetate by this fungus has led to the formation of 7β-acetoxy-ent-kaur-16-en-19-oic acid (3), whose corresponding alcohol is a short-lived intermediate in the biosynthesis of gibberellins and seco-ring ent-kaurenoids in this fungus. Further biotransformation of this compound led to the hydroxylation of the 3β-positions to give 7β-acetoxy-3β-hydroxy-ent-kaur-16-en-19-oic acid (14), followed by a 2β- or 18-hydroxylation of this metabolite. The incubation of epicandicandiol 7β-monoacetate (7β-acetoxy-18-hydroxy-ent-kaur-16-ene) (10) produces also the 19-hydroxylation to form the 18,19 diol (20), which is oxidized to give the corresponding C-18 or C-19 acids. These results indicated that the presence of a 7β-acetoxy group does not inhibit the fungal oxidation of C-19 in 7β-acetoxy-ent-kaur-16-ene, but avoids the ring B contraction that leads to the gibberellins and the 6β-hydroxylation necessary for the formation of seco-ring B ent-kaurenoids. The biotransformation of 7β-acetoxy-ent-trachylobane (trachinol acetate) (27) only led to the formation of 7β-acetoxy-18-hydroxy-ent-trachylobane (33).