0.80 g (2.1 mmol) of 4-benzoyloxy-5,8-dimethyl-1,1-dioxo-1,2,3,4-tetrahydro-1λ6-thiochromene-6-carboxylic acid in 30 ml of methylene chloride was admixed with 2 drops of N,N-dimethylformamide and 1.69 g (13.3 mmol) of oxalyl chloride, and the mixture was boiled under reflux for 3.5 h. The solvents were subsequently stripped off using a rotary evaporator, and the residue was taken up in 100 ml of methylene chloride and, at 0° C., admixed with 0.25 g (2.2 mmol) of cyclohexanedione and 0.44 g (7.4 mmol) of triethylamine. The mixture was stirred at room temperature for 4 h and subsequently concentrated using a rotary evaporator, and the residue was purified by chromatography (silica gel, ethyl acetate: hexane=1:1).
