Halogenated Hypoxanthine 2'‐Deoxyribonucleosides With Pyrazolo[3,4‐ d ]Pyrimidine Skeleton: Synthesis, Crystal Structures, Physical Properties, and Impact on Watson‐Crick and Purine‐Purine Base Pairs

Syntheses and base pairing properties of halogenated and nonhalogenated hypoxanthine nucleosides with 8‐aza‐7‐deazapurine skeleton (dI*) are reported. Modified hypoxanthine nucleosides were accessed from 6‐alkoxy precursors. Single‐crystal X‐ray analyses disclosed unit cells consisting of two conformers adopting high‐anti conformation around the glycosylic bond with N‐type C3’‐endo (3T2) or E‐type, O4’‐endo (0E) sugar puckering. Crystal packing is stabilized by inosine/inosine wobble base pairs as well as intermolecular and intramolecular hydrogen bonds. Hirshfeld surface analyses confirmed these findings. Halogenated and nonhalogenated dI* nucleosides were converted to phosphoramidites and applied to solid‐phase oligonucleotide synthesis. Single and consecutive incorporations were accomplished and Watson–Crick and purine–purine base pairing was investigated. Single incorporations opposite to canonical nucleosides afforded stable duplexes in all cases. T m values revealed the following order of stability dI*‐dC = dI*‐dA > dI*‐dT > dI*‐dG. Consecutive incorporations furnished more distinguished results. Double helices with halogenated dI*‐dC base pairs were the most stable ones, whereas hybridization for dI* opposite to dT was hindered. Halogenated dI*‐dG and dI*‐dA purine‐purine base pairs gave stable duplexes, whereas hybridization of nonhalogenated dI*‐dA pairs failed. From CD spectra, helical changes for duplexes with three dI*‐dG purine‐purine base pairs were detected. All other duplexes showed the typical shape of a B‐DNA.