Chemoenzymatic Preparation of the Novel Antifolate Thymidylate Synthase Inhibitor N-(4-{N-[(6S)-2-Methyl-4-oxo-3,4,7,8-tetrahydro-6H-cyclopenta [g]quinazolin-6-yl]-N-(prop-2-ynyl)amino}benzoyl)-L-glutamic Acid (Ia) and Its Glutamyl Cl