Ozone → 2-(2-Oxoethyl)-4-thiochromanone

An ozone stream (3 g ozone/hour) was conducted while stirring for 15 minutes through a solution, cooled to -70°, of 3 g of (RS)-2-(2-buten-1-yl)-4-thiochromanone in 100 ml of anhydrous dichloro-methane and 30 ml of anhydrous methanol. Subsequently, the solution was flushed with oxygen for 5 minutes and with argon for 10 minutes. After the addition of 2 ml of dimethyl sulfide, the mixture was stirred at room temperature for 16 hours. The reaction mixture was subsequently evaporated in a vacuum. The residue was treated with 50 ml of dichloromethane and, after the addition of 5 ml of water and 5 ml of trifluoroacetic acid, stirred at room temperature for 1 hour. The mixture was subsequently poured into 50 ml of water and neutralized by the spatula-wise addition of sodium hydrogen carbonate while stirring. An additional 50 ml of water were added, the phases were separated and the aqueous phase was extracted twice with 80 ml of dichloromethane each time. The combined organic phases were dried over magnesium sulfate and concentrated in a vacuum. There were obtained 2.8 g (99%) of 2-(2-oxoethyl)-4-thiochromanone as a yellow oil which was used in the next reaction without further recrystallization.