Design and synthesis of pyrazolo[3,4-d]pyrimidine and triazolo[4,5-d]pyrimidine based dissymmetrical ‘Leonard linker’ compounds: 1H NMR and crystallographic evidence for folded conformation due to arene interactions

Proton NMR analyses of several newly synthesized dissymmetrical ‘Leonard/trimethylene linker’ compounds 1/2/3-[(4,6-dimethylsulfanyl-1H-pyrazolo[3,4-d]pyrimidin-1-yl)propyl]-5,7-dimethyl/ethyl-sulfanyl-1H/2H/3H-triazolo[4,5-d]pyrimidines (6 and 8) show intramolecularly stacked conformation in solution. X-ray crystallography of one dissymmetrical ‘Leonard/trimethylene’ linker compound (6b) based on two different heterocycles namely pyrazolo[3,4-d]pyrimidine core and triazolo[4,5-d]pyrimidine, for the first time, shows unusual U-motif formed due to intramolecular π–π interactions, which is similar to earlier related pyrazolo[3,4-d]pyrimidine core based ten symmetrical (1 and 2) and one dissymmetrical compound (3). Supramolecular structures of the new compound (6b) show unusual chain motif due to weak intermolecular CH…N and CH…S interactions. More importantly, the new compound (6b) shows S…arene interaction not shown by any earlier compounds (1, 2 and 3).