Synthesis
Step 1: 5.2 g of bromoacetaldehyde diethyl acetal was dissolved in 10 mL of tetrahydrofuran and slowly added dropwise to a mixture containing 4.00 g of 4-methylbenzenethiol, 1.4 g of 60% sodium hydride (dispersed in mineral oil) and 35 mL of tetrahydrofuran. The reaction mixture was stirred at room temperature for 15 hours. Upon completion of the reaction, the reaction was quenched by the addition of 10 mL of saturated aqueous ammonium chloride, followed by three extractions with tert-butyl methyl ether. The organic phases were combined, washed sequentially with water and saturated brine, dried over anhydrous magnesium sulfate, and concentrated under reduced pressure to remove the solvent.
Step 2: The above concentrated residue was added to a mixture of 5 g of phosphorus pentoxide and 10 g of phosphoric acid that had been preheated to 175°C and stirred for 45 minutes, and stirring was continued for 5 minutes. At the end of the reaction, the mixture was quenched by pouring it into ice water and extracted three times with tert-butyl methyl ether. The organic phases were combined, washed sequentially with water and saturated brine, dried over anhydrous magnesium sulfate, and concentrated under reduced pressure to remove the solvent.
Step 3: The final residue was purified by silica gel column chromatography to give 2.77 g 5-methylbenzo[b]thiophene.
