An efficient total synthesis of ruprechstyril from Ruprechtia tangarana.

A short total synthesis of natural isocarbostyril ruprechstyril (3-n-pentyl-6-methoxy-8-hydroxy-1(2H)-isoquinolinone) isolated from Ruprechtia tangarana is reported. 6,8-Dimethoxy-3-pentylisocoumarin obtained by condensation of 3,5-dimethoxyhomophthalic anhydride with hexanoyl chloride was smoothly converted to O-methylruprechstyril by refluxing with methanamide. Regioselective demethylation of the latter using anhydrous aluminium chloride in dichloromethane furnished the ruprechstyril. Complete demethylation to give (6-desmethoxyruprechstyril) was achieved using same reagent in ethanethiol.