RegioselectivePreparation of 3,4-Dihydro-4-methyl-2H,6H-(1,3)thiazino( 2,3-b)quinazolin-6-ones and 2,3-Dihydro-3-methyl-5H-thiazolo(2,3-b) quinazolin-5-ones and Analogous Polycyclic Pyrimidinone Derivatives from Condensed 2-Alkenylthiopyrimidine-4(3H)-ones.-Acid-mediated cyclization of the 2-allylthiopyrimidinones (III) yields the thiazole derivatives (IV) whereas the corresponding crotylthiopyrimidinones (V) give the thiazine derivatives regioselectively. -(WIPPICH, P.; HENDREICH, C.; GUETSCHOW, M.; LEISTNER, S.; Synthesis (1996) 6, 741-747; Inst. Pharm.,