A highly efficient and versatile synthetic approach to the synthesis of annelated quinazoline derivatives viz. 3,4,9,10a-tetraazaphenanthrenes 5 - 7, thiazolidinylquinazoline 9, 2,4,9,10a-tetraazaphenanthrene 11, quinazolino[4,3-b]quinazolin-8-one 12 and imidazoquinazolines 14a,b, 15. Also, a variety of pyrazolylquinazolines 19 - 21, pyrimidinylquinazolines 22a,b were obtained via a sequence of heterocyclization reactions of 4-methyl-N-[4-(4-oxo-3,4-dihydroquinazolin-2-yl)phenyl]benzenesulfonamide (2) with different reagents. The new compounds were synthesized with the objective of study their antimicrobial activity