Carbon‐13 n.m.r. studies of purines and 8‐azapurines in basic aqueous medium

The 13C n.m.r. spectra of a variety of purines and 8‐azapurines have been studied in aqueous basic medium; the shifts were measured using dioxane as reference, and corrected to the TMS scale. The compounds studied include adenine, hypoxanthine, 6‐mercaptopurine, guanine, xanthine, 8‐azaadenine, 8‐azahypoxanthine, 8‐azaguanine, 8‐azaxanthine, and 2‐thio‐8‐azaxanthine. Resonances were assigned using noise‐decoupling, selective deuteration at the 8‐position of purines, and coupled spectra. The general effect of changing from a purine to its 8‐aza analog is to produce a downfield shift of C‐2, C‐5, and C‐6 and an upfield shift of C‐4.