Synthesis of indole‐4‐carboxaldehydes and 4‐acetylindole from N‐alkyl‐5‐aminoisoquinolinium salts

N‐Alkyl‐5‐aminoisoquinolinium salts ( 8a‐d ) are converted into indole‐4‐carboxaldehydes ( 1a‐c ) on heating in a two phase alkyl acetate‐water system containing an excess of a 2:1 sodium bisulfite‐sodium sulfite mixture. 4‐Acetylindole 1e is prepared in the same way from 1‐methyl‐2‐cyanomethylisoquinolinium bromide 8f .