6-Methylisoquinoline

Preparation

A benzene solution (50 ml, CAUTION) of equimolar amounts of p-tolualdehyde (8.4 g, 0.07 mol) and aminoacetaldehyde dimethyl acetal (7.35 g, 0.07 mol) is refluxed overnight into a Dean and Stark trap. The solution is evaporated in vacuo and then twice evaporated again with added benzene and the viscous oil dissolved in dry tetrahydrofuran. This solution is cooled to - 10 °C and 1 equivalent of ethyl chloroformate (7. 1 ml, 0.07 mol) is added with rapid stirring and the mixture stirred for a further 5 minutes. The cooling bath is removed and 1.2 equivalents of trimethyl phosphite ( 10.5 ml, 0.09 mol) is added with stirring. The solution is stirred at room temperature for 1 5 hours and then evaporated t o an oil. The oil is then re-evaporated twice with added toluene to remove traces of trimethyl phosphite. The oil is dissolved in dry dichloromethane, 6 equivalents of titanium(1v) chloride (50 ml, 0.45 mol) is added and the solution refluxed for 36 hours under a drying tube. The cooled solution is shaken with 1 equivalent of aqueous sodium hydroxide solution to neutrality, whereupon titanium(1v) oxide precipitates as a white solid. The filtered organic layer is extracted with 3 M hydrochloric acid, and the extract is washed with dichloromethane, basified strongly with aqueous alkali, and extracted with dichloromethane. This organic extract is dried over anhydrous sodium sulphate and evaporated to afford 6-methylisoquinoline (yield 7 1 %, m.p. 88 °C).

Synthesis Reference(s)

The Journal of Organic Chemistry, 48, p. 3344, 1983 DOI: 10.1021/jo00167a043