A stacked pyrazolo[3,4-d]pyrimidine-based flexible molecule: the effect on stacking of a bulky isopropyl group in comparison with a methyl/ethyl group (original) (raw)

2003, Acta Crystallographica Section C-crystal Structure Communications

In the crystal structure of 1,1'-(1,3-propanediyl)bis(5methyl-6-methy lthio-4,5-dihydro-1 H-pyrazolo [3,4-d]pyrimidin-4-one), CITH20N802S2, the pairs of pyrazolo-[3A-d]pyrimidine rings of the molecules stack, due to intramolecular 7r-Tr interactions, between the heterocyclic rings. Comment Besides their role in DNA/RNA, non-covalent interactions between aromatic molecules are invoked to rationalize a variety of phenomena, such as the packing of aromatic molecules in crystals (Desiraju, 1995), molecular recognition (Hunter, 1994, and references therein) and asymmetric synthesis (Jones & Chapman, 1995), and also to explain the structure of many biologically relevant molecules (Saenger, 1984; Burley & Petsko, 1985; Hunter et al., 1991). Use of a 'propylene linker', first documented by Brown et al. (1968), for the promotion of intramolecular aromatic 7r-Tr interactions (APPI) is now increasing (Leonard, 1979; Newcomb & Gellman, 1994). However, bis-thymine (Frank & Paul, 1973) and bis-theophylline (Rosen & Hybl, 1971), both containing 'propylene linkers', remain the only examples for which X-ray structure determination has shown intramolecular 7r-Tr interactions, though with extensive hydrogen bonding. In spite of considerable research activity (Shetty et al., 1996, and references therein), APPI are not well understood and there is an urgent need for more simple and flexible models without too much bias for certain conformations. Recently, we have reported (Avasthi et al., 1995) the synthesis of 1,3-bis(4,6-dimethylthio-lH-pyrazolo[3,4d]pyrimidin-1-yl)propane, (1), the l H NMR spectrum of which showed an unusual high-field shift for one of the methylthio groups in comparison with the corresponding signals in the l H NMR spectra of 1-alkyl-4,6-dimethylthio-1H-pyrazolo[3,4-d]pyrimidines (Garg et al.,