AbstractThe alkaloid thalactamine (N‐methyl‐5,6,7‐trimethoxy‐1(2H)isoquinolone) was synthesised in two steps from 4,5,6‐trimethoxyhomophthalic acid (1a). Heating la with DMF/POCI3at 100° furnished thalactamine‐4‐earhoxylic acid which was easily decarboxylated to give the alkaloid thalactamine. By the same two steps, the alkaloidN‐methyl‐6,7‐dimethoxy‐1(2H)‐isoquinolone is obtained from 4,5‐dimethoxyhomophthalic acid. Synthesis for la from 2‐bromogallic acid trimethyl ether was modified to give excellent yield. 5,6,7‐Trimethoxy and 6,7‐dimethoxyisocoumarin‐4‐carboxylic acid esters were synthesised from the homophthalic acids 1a and b by interacting them with DMF/phosphoryl chloride at 0°, to give corresponding 4‐(N,N‐dimethylaminoformylidene)isochroman‐1,3‐dione derivatives Vla and b and treating their alcoholic solutions with dry hydrogen chloride gas. The isocoumarins were converted intoN‐methyl‐1(2H)isoquinolonesby treating them with aqueous methylamine. The isochromandione Vla slowly changed into 3‐chloro‐4‐formyl‐5,6,7‐