Long‐range heteronuclear coupling constants in 2,6‐disubstituted purine derivatives

Four‐ and five‐bond heteronuclear J ‐couplings between the hydrogen H‐8 and carbons C‐6 and C‐2 in a series of 7‐ and 9‐benzyl substituted purine derivaties with variuous substituents in positions 2 and 6 were studied by coupled 13 C NMR and H,C‐HMBC experiments and by DFT calculations. We have found that for some of the derivatives, the five‐bond coupling H8‐C2 is higher than the four‐bond H8‐C6 coupling, which is also evidenced by a stronger crosspeak in the HMBC. This finding contradicts the generally accepted opinion that only strong three‐bond crosspeaks and one weak four‐bond H8‐C6 crosspeak can be observed in the HMBC spectra of purine derivatives. The misinterpretation of HMBC spectra may lead to an incorrect determination of the purine derivatives' structure. Copyright © 2012 John Wiley & Sons, Ltd.