Design and synthesis of pyrazolo[3,4-d]pyrimidine core based dissymmetrical 'Leonard linker' compounds: 1H NMR and crystallographic evidence for folded conformation due to arene interactions

Proton NMR analysis of two newly synthesized 'Leonard/trimethylene linker' dissymmetrical compounds 1-(4,6-dimethylsulfanyl-1H-pyrazolo[3,4-d]pyrimidin-1-yl)-3-(5-methyl-6-methylsulfanyl-4-oxo-1,5-dihydropyrazolo[3,4-d]pyrimidin-1-yl)propane (8) and 1-(4-methoxy-6-methylsulfanyl-1H-pyrazolo[3,4-d]pyrimidin-1-yl)-3-(6-methoxy-5-methyl-4-oxo-1,5-dihydropyrazolo[3,4-d]pyrimidin-1-yl)propane (10) show intramolecularly stacked conformation in solution. X-Ray crystallography of dissymmetrical 'Leonard/trimethylene linker' compound (8) based on pyrazolo[3,4-d]pyrimidine core, for the first time, shows unusual U-motif formed due to intramolecular π-π stacking interactions, which is similar to earlier related symmetrical compounds 1 & 2. Supramolecular structures of new compound (8) show additional CH…O, CH…N, CH…S and S…S interactions.