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

Transformations of cassane diterpenoids from Caesalpinia bonduc into aromatic derivatives, either in CDCl 3 or in CHCl 3 irradiated with UV light or catalyzed by AlCl 3 , were described. Caesalmin C (2) was hydrolyzed with Na 2 CO 3 upon refluxing in MeOH to yield compound 1. Dissolving compound 1 with CDCl 3 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 3 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 3 in CHCl 3 or CH 2 Cl 2 , gave the same results in CDCl 3 and with even shorter reaction time. The role of AlCl 3 in the aromatization of 1 has been explained by DFT calculations.