2-((4-methoxyphenyl)thio)benzoic acid (prepared in house, see Synthetic Page 846)
Sulfuric acid (Fisher Scientific)
Isopropyl alcohol (Fisher Scientific)

Into a conical flask was charged 2-((4-methoxyphenyl)thio)benzoic acid (2.60 g, 10 mmol) followed by concentrated sulphuric acid (40 mL) in one full portion*. The mixture was heated to 100°C** with constant stirring for 30 minutes. The hot solution was carefully poured over ice (200 g); with vigorous stirring a yellow solid precipitated from the reaction which was isolated by vacuum filtration at the pump and washed further with water (100 mL).  The crude solid was recrystallized in the minimum amount of hot isopropyl alcohol to yield 2-​methoxy-9H-​thioxanthen-​9-​one (0.61 g, 25%) as bright yellow needles.

*Upon addition of sulphuric acid an exothermic reaction occurs with effervescence.  This subsides with heat and stirring to produce a clear brown liquid.
**Temperature was established and maintained using a thermocouple which was placed into a metal heating block.  Stirring was accomplished using a standard stirring bar.

¹H NMR (400 MHz, Chloroform-d) δ 8.65 (d, J = 8.6 Hz, 1H), 8.10 (d, J = 2.9 Hz, 1H), 7.66 – 7.57 (m, 2H), 7.54 – 7.46 (m, 2H), 7.28 (dd, J = 8.9, 3.1 Hz, 1H), 3.96 (s, 3H).¹³C NMR (101 MHz, CDCl₃) δ 179.70, 158.39, 137.53, 132.03, 130.23, 129.88, 129.19, 128.58, 127.31, 126.10, 126.00, 122.78, 110.33, 55.71. LCMS – 242 [M+]