The present invention relates to a method for preparing an iodine-doped TiO.sub.2 nano-catalyst and use of the catalyst in heterogeneously catalyzing configuration transformation of trans-carotenoids. The iodine-doped TiO.sub.2 nano-catalyst is prepared by a sol-gel process using a titanate ester as a precursor and an iodine-containing compound as a dopant in the presence of a diluent, inhibitor and complexing agent. The catalyst exhibits high activity for isomerization of the trans-carotenoids into their cis-isomers within a short catalytic time. The catalyst can be easily prepared and is highly efficient, economical, recyclable and environmentally friendly.

A method of producing alpha-tocotrienol quinone or a stereoisomer thereof, the method comprising selective opening of alpha-tocotrienol chroman to alpha-tocotrienol quinone in the presence of non-alpha tocotrienol chromans by oxidizing alpha-to-cotrienol with a metal salt oxidizing agent, wherein the stoichiometric ratio of metal salt oxidizing agent/alpha-tocotrienol is at least 4:1 and wherein said metal oxidizing agent is added in sequential additions, in order to reduce oxidation of any amounts of non-alpha tocotrienol chromans that might have been present in the starting alpha-tocotrienol chroman material. This process uses conditions favoring oxidation rates of the alpha tocotrienol chroman vs. the non-alpha tocotrienol chromans.