An interesting backbone rearrangement and novel derivatives from the biotransformation of trachyloban-19-oic acid by Rhizopus stolonifer

A biotransformation experiment of trachyloban-19-oic acid (2) was carried out with Rhizopus stolonifer. After twenty days of reaction, four metabolites were extracted, isolated and characterized: two trachylobane type compounds, the 7beta ( 3) and the 17 (5) hydroxyl derivatives, and two rearranged ent-kaur-11-en-19-oic acids, the 16beta (4) and the 9alpha, 16beta (6) hydroxylated compounds. Products 4 and 5 were isolated as their methyl esters (4a and 5a) after esterification of a mixture containing their corresponding acids. The rearrangement of a trachylobane diterpene leading to ent-16beta-hydroxy-kaur-11-ene derivatives gives support to the biogenetic proposal based on the occurrence of this rare group of kaurenes only in plants containing trachylobane representatives. By the best of our knowledge, this is the first report on the isolation of compounds 5 and 6.