Acid-catalyzed transformation of cassane diterpenoids from to aromatic derivatives.

Transformations of cassane diterpenoids from into aromatic derivatives, either in CDCl or in CHCl irradiated with UV light or catalyzed by AlCl, were described. Caesalmin C (2) was hydrolyzed with NaCO upon refluxing in MeOH to yield compound 1. Dissolving compound 1 with CDCl resulted in an unexpected aromatization process of a C ring to obtain 1a, and aromatic derivatives 6-acetoxy-3-deacetoxycaesaldekarin e (2a), caesall A (3a), caesaldekarin e (5a), caesalpinin MC (5b), 2-acetoxycaesaldekarin e (6a) and new compound 6b could be obtained from corresponding cassane diterpenoids (2-8) under the same conditions. Furthermore, the photochemical reactions of cassane diterpenoids 1-8 occurring in CHCl also yielded aromatic derivatives 1a, 2a, 3a, 5a, 6a, new compounds 2b and 3b, and 17-norcassane diterpenoids norcaesalpinin MC (2c) and caesalmin J (3c). In addition, cassane diterpenoids 1-8, treated with AlCl in CHCl or CHCl, gave the same results in CDCl and with even shorter reaction time. The role of AlCl in the aromatization of 1 has been explained by DFT calculations.