Anti-tumor-promoting activity of derivatives of abieslactone, a natural triterpenoid isolated from several Abies genus - Takayasu,J.; Tanaka,R.; Matsunaga,S.; Ueyama,H.; Tokuda,H.; Hasegawa,T.; Nishino,A.; Nishino,H.; Iwashima,A. (Cancer Lett., 1990)