Antioxidant activity of quinones extracted from tanshen (Salvia miltiorrhiza Bunge)

Six compounds have been solvent extracted from tanshen and separated by column and thin-layer chromatography. The structures have been identified by spectroscopic methods as rosmariquinone, dehydrorosmariquinone, miltirone I, tanshinone IIA, cryptotanshinone, and dihydrotanshinone. The antioxidant activity of these quinones in lard at 100 degrees C was determined with a Rancimat. The following structural features are associated with increased activity: (1) additional conjugated double bonds in the A ring; (2) a dihydrofuran ring rather than a furan ring; (3) an isopropyl substituent ortho to a quinone carbonyl rather than a dihydrofuran ring. An addition product has been isolated from the reaction between 9,10-phenanthrenedione and methyl oleate, and it is proposed that the quinones act as primary antioxidants by addition of the lipid radical to the quinone to form a stabilized radical which interrupts the autoxidation chain reaction