Synthesis, NMR and ultraviolet spectroscopy of [10] (N6,9)-6-aminopurinophane; calculation of the chemical shifts in the [n](N6,9)-6-aminopurinophane series
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abstract
[10](N6,9)-6-Aminopurinophane was prepared via a Mitsunobu reaction involving 6-chloropurine and 10-azido-1-decanol. Reduction of the azido moiety to an amine allowed for subsequent cyclization to the cyclophane. N6-Nonyladenine, 9-nonyladenine, and N6,9-dinonyladenine were also prepared using the established chemistry. Heating the [10](N6,9)-6-aminopurinophane to 90 °C allowed for a complete assignment of the proton and carbon spectra, while cooling the sample to −77 °C revealed additional isomers likely due to the anti–syn isomerization about the N6—C6 bond. A variable temperature proton NMR study revealed a ΔGc≠ of 59.4 ± 5.5 kJ mol−1 associated with the interchange between the two major conformers. The ultraviolet spectra of the [n](N6,9)-6-aminopurinophane series show a bathochromic shift and a hypochromic effect in the transitions as the methylene chain length decreases (i.e., n = 10 to 9 to 8). The aminopurinophanes are used to assess the relative merits of a semi-empirical model that allows for the calculation of the chemical shifts of the methylene bridge protons. Keywords: synthesis of heterocyclophanes, variable temperature NMR, cyclophane conformational analysis, ultraviolet spectroscopy, calculation of chemical shift.
