In this connection, compounds of this type show elements of similarity with other tricyclic compounds possessing psychotropic activity. In further consideration of the similarity of these compounds with earlierdescribed derivatives of dihydropyrimidobenzothiazines[2] was an investigation of a series of recently-synthesized, pharmacologically significant 9-oxo- 6, 7, 8, 9-tetrahydropyrimido [4, 5-b](1, 4) benzothiazines containing ...