Cytotoxic neo-clerodane diterpenoid alkaloids from Scutellaria barbata

Six new neo-clerodane diterpenoid alkaloids, named scutehenanines A-D (1, 4, 5, 6), 6-O-acetylscutehenanine A (2), and 6-O-(2-carbonyl-3-methylbutanoyl)scutehenanine A (3), were isolated from the whole plant of Scutellaria barbata. Their structures were established on the basis of detailed physical data analyses. In vitro, the six new isolated compounds showed cytotoxic activities against three human cancer lines (HONE-1 nasopharyngeal, KB oral epidermoid carcinoma, and HT29 colorectal carcinoma cells) and gave IC50 values in the range 2.8-6.4 microM.