(3)-Alkyl-isoquinoline cpds. prepn. - by reacting benzylamines with glyoxal acetals and cyclising with polyphosphoric acid

摘要

In a new process benzylamines are reacted with glyoxal monoacetals of formula R1-CO-CH(OR2)2, (I), (where R1 and R2 are 1-4C alkyl) and the product is cyclised by treatment with polyphosphoric acid. 3-Alkyl-isoquinolines such as 3-methylisoquinoline are useful as intermediates for pharmaceuticals. Reaction of benzylamines with (I) gives the corresponding Schiff's bases in good yields, and these can then be converted into 3-alkylisoquinolines in good yield. In an example, benzylamine and methylglyoxal dimethyl acetal are refluxed in toluene (water separated) to give 2-benzylimino-propionaldehyde dimethyl acetal. Part of this is treated with polyphosphoric acid over 2 hrs. at 60-80 degrees C and stirred 2 hrs. at 80 degrees C to give 3-methylisoquinoline.