[Preparation of 2,4-dithioxooctahydroquinazolines and thioxothiazolo- (or thioxothiazino-) (2,3-b)- and -(3,2-c) quinazolines substituted at position 3 (Transformation of 2,4-dithioxo-4H-1,2,5,6,7,8-hexahydro-3,1-benzothiazine with nucleophiles)].