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

Abstract

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.