The 2′-de­oxy­ribo­furan­oside of 3-phenyl­tetra­hydropyrimido[4,5-c]pyridazin-7-one: a bicyclic nucleoside with sugar residues in N and S con­formations, and its mol­ecular recognition

The title compound 3-phenyltetra­hydro­pyrimido[4,5-c]pyridazine 2′-de­oxy­ribo­nucleoside [systematic name: 6-(2-deoxy-β-d-erythro-pentofuranosyl)-5,6,7,8-tetra­hydro-3-phenylpyrimido[4,5-c]pyridazin-7-one monohydrate, C(17)H(18)N(4)O(4)·H(2)O, 1] shows two con­formations in the crystalline state and the two con­formers (1a and 1b) adopt different sugar puckers. The sugar residue of 1a shows a C2′-endo S-type con­formation, while 1b displays a C3′-endo N-type sugar pucker. Both con­formers adopt similar anti con­formations around the N-glycosylic bonds, with χ = −97.5 (3)° for con­former 1a and χ = −103.8 (3)° for con­former 1b. The extended crystalline network is stabilized by several inter­molecular hydrogen bonds involving nucleoside and water mol­ecules. The nucleobases and phenyl substituents of the two con­formers (1a and 1b) are stacked and display a reverse alignment. A Hirshfeld surface analysis supports the hydrogen-bonding pattern, while curvedness surfaces visualize the stacking inter­actions of neighbouring mol­ecules. The recognition face of nucleoside 1 for base-pair formation mimics that of 2′-de­oxy­thymidine. Nucleoside 1 shows two pK (a) values: 1.8 for pro­ton­ation and 11.2 for deprotonation. DNA oligonucleotides containing nucleoside 1 were synthesized and hybridized with com­plementary DNA strands. Nucleoside 1 forms a stable base pair with dA which is as stable as the canonical dA–dT pair. The bidentate 1–dA base pair is strengthened by a third hydrogen bond provided by the dA analogue 3-bromo­pyrazolo­[3,4-d]pyrimidine-4,6-di­amine 2′-de­oxy­ribo­furan­oside (4). By this, duplex stability is increased and the sug­gested base-pairing patterns are supported.