New [c,d]-Fused Purinediones: 2-Substituted 9-Methyl-4,5-dihydro-6H,8H-pyrimido-[1,2,3-cd]purine-8,10(9H)-diones - imo, Ondrej; Rybár, Alfonz; Alföldi, Juraj (Synthesis, 1995)