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.
This journal is © The Royal Society of Chemistry.