The tubes were purged with nitrogen, and the content mixed for 1 h on a reciprocal shaker (KS-15, Edmund Bühler GmbH) at 250 rpm. After addition of 1.5mL water, the mixture was extracted three times with 3mL petroleum ether. The petroleum ether layers were evaporated
in a dry block heater. The residue was dissolved in 0.2mL of DCM and then diluted to 2.0mL with mobile phase without DCM.