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

Transformations of cassane diterpenoids from Caesalpinia bonduc into aromatic derivatives, either in CDCl3 or in CHCl3 irradiated with UV light or catalyzed by AlCl3, were described. Caesalmin C (2) was hydrolyzed with Na2CO3 upon refluxing in MeOH to yield compound 1. Dissolving compound 1 with CDCl3 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 CHCl3 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 AlCl3 in CHCl3 or CH2Cl2, gave the same results in CDCl3 and with even shorter reaction time. The role of AlCl3 in the aromatization of 1 has been explained by DFT calculations.