Synthesis of 1,4,7,8,9,10-Hexahydro-9-methyl-6-nitropyrido(3,4-f) quinoxaline-2,3-dione and Related Quinoxalinediones: Characterization of α-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic Acid (AMPA) and N-Methyl-D-aspartate (NMDA) Rec