Treatment of isoquinolinecarboxamide 4a with triethyl orthoformate, chloroacetyl chloride or carbon disulfide afforded the pyrimidinone 7, oxopyrimidinethione 12 and chloromethylpyrimidinone 18, respectively. These products were used as versatile starting materials for synthesis of other heterocyclic compounds. The heterocyclic hydrazide 6 was also obtained.