Synthesis and Regioselective N- and O-Alkylation of 1H- or 3H-[1,2,3]Triazolo[4,5-d]pyrimidine-5,7(4H,6H)-diones (8-Azaxanthines) and Transformation of Their 3-Alkyl Derivatives into 1-Alkyl Isomers - Islam, Rafiqul; Nagamatsu, Tomohisa (Synthesis, 2006)