Abstract We have studied some properties and conversions of pyrimido [4, 5-b]-1, 4- benzothiazepines: reduction, oxidation, reactions with nucleophilic reagents (methanol, hydrazine, hydroxylamine, o-methylhydroxylamine, and thiosemicarbazide). We have synthesized derivatives of a novel heterocyclic system: pyrimido [5, 4-c] isoquinoline.