Acid-catalyzed transformation of cassane diterpenoids fromCaesalpinia bonducto aromatic derivatives

Abstract

Transformations of cassane diterpenoids fromCaesalpinia bonducinto aromatic derivatives, either in CDCl3or in CHCl3irradiated with UV light or catalyzed by AlCl3, were described. Caesalmin C (2) was hydrolyzed with Na2CO3upon refluxing in MeOH to yield compound1. Dissolving compound1with CDCl3resulted in an unexpected aromatization process of a C ring to obtain1a, and aromatic derivatives 6-acetoxy-3-deacetoxycaesaldekarin e (2a), caesall A (3a), caesaldekarin e (5a), caesalpinin MC (5b), 2-acetoxycaesaldekarin e (6a) and new compound6bcould be obtained from corresponding cassane diterpenoids (2-8) under the same conditions. Furthermore, the photochemical reactions of cassane diterpenoids1-8occurring in CHCl3also yielded aromatic derivatives1a,2a,3a,5a,6a, new compounds2band3b, and 17-norcassane diterpenoids norcaesalpinin MC (2c) and caesalmin J (3c). In addition, cassane diterpenoids1-8, treated with AlCl3in CHCl3or CH2Cl2, gave the same results in CDCl3and with even shorter reaction time. The role of AlCl3in the aromatization of1has been explained by DFT calculations.