Chemical Transformations of 7,9-Disubstituted Purines and Related Heterocycles. Selective Reduction of Imines and Immonium Salts

Ten 7,9-disubstituted purines and related heterocycles were reduced with borohydride to afford the corresponding 7,8-dihydro species, a type of heterocycle which has not previously been studied in any detail. The reduced species were found to reoxidize quantitatively at rates characteristic of and predictable for the individual heterocycles; the reoxidation phenomenon was investigated and shown to involve reaction with water or oxygen. In solutions containing mixtures of reduced and oxidized heterocycles, the two species were found to be in rapid equilibrium, undoubtedly via hydride transfer. This observation prompted the utilization of the 7,8-dihydro-7,9-disubstituted heterocycles for the novel and selective reduction of imines and immonium salts. Benzylideneaniline, benzylidenebenzylamine, and cyclohexylidenepyrrolidinium perchlorate, e.g., were converted to their respective amines in excellent yields. Over the range of conditions employed for these transformations, no significant reduction of aldehydes, ketones, or several other common organic functional groups was observed, so the heterocycles may prove useful as selective reducing agents. The reactivity of the disubstituted heterocycles with nucleophiles was also studied. Certain of the oxidized compounds, e.g., were found to undergo ring opening at high pH, as has been observed previously. Treatment of the reduced, ribosylated species with aniline at pH 4.5 resulted in deribosylation of the nucleosides. This transformation, which previously has been possible only after opening of the imidazole moiety at high pH, should be of considerable utility to biochemists for the depurination of 7-methylguanosine moieties in transfer and messenger RNA's.