New ent-kaurane diterpenoids with anti-platelet aggregation activity from Annona squamosa

A phytochemical investigation on the stems of Annona squamosa led to the isolation of six new ent-kaurane diterpenoids, annomosin A (16beta-hydroxy-19-al-ent-kauran-17-yl 16beta-hydro-19-al-ent-kauran-17-oate) (1), annosquamosin C (16alpha-hydro-17-hydroxy-19-nor-ent-kauran-4alpha-ol) (2), annosquamosin D (16beta-acetoxy-17-hydroxy-19-nor-ent-kauran-4alpha-ol) (3), annosquamosin E (16beta-hydroxy-17-acetoxy-19-nor-ent-. kauran-4alpha-formate) (4), annosquamosin F (16beta-hydroxy-17-acetoxy-18-nor-ent-kauran-4beta-hydroperoxide) (5), and annosquamosin G (16beta,17-dihydroxy-18-nor-ent-kauran-4beta-hydroperoxide) (6), along with 14 known ent-kaurane diterpenoids. The structures of 1-6 were elucidated by spectroscopic data interpretation. Compound 1 is the first dimeric ent-kaurane derivative to have been reported from a plant in the family Annonaceae. Certain useful NMR data were generalized to determine the stereochemistry of C-16 among the ent-kaurane diterpenoids investigated. ent-Kaur-16-en-19-oic acid (9) and 16alpha-hydro-19-al-ent-kauran-17-oic acid (17) showed complete inhibitory effects on rabbit platelet aggregation at 200 muM.