Synthesis of N-{4-[(2,4-Diamino-5-methyl-4,7-dihydro-3H- pyrrolo[2,3-d]pyrimidin-6-yl)thio]benzoyl}- l -glutamic Acid and N-{4-[(2-Amino-4-oxo-5-methyl-4,7-dihydro-3H-pyrrolo[2,3-d]pyrimidin- 6-yl)thio]benzoyl}- l -glutamic Acid as Dual Inhibitors of Dihydrofolate Reductase and Thymidylate Synthase and as Potential Antitumor Agents