Long‐range 1 H‐ 15 N correlation at natural abundance using gradient‐enhanced inverse‐detection

Inverse‐detected heteronuclear shift correlation efficiency has been significantly augmented by the incorporation of pulsed field gradients (PFG). Phase‐cycling requirements for t 1 ‐noise suppression in gradient‐enhanced experiments are, for the most part, obviated, making it feasible to acquire data in one or a few transients/t 1 increment. The benefits which acerue for 1 H‐ 13 C correlation (using GHMQC, GHMBC, and variants of GHMQC‐TOCSY) are well documented. Less obvious is the increased facility with which long‐range 1 H‐ 15 N correlation spectra can be acquired. An IDR‐(Inverted Direct Response)‐GHMQC‐TOCSY was used to establish unequivocal proton resonance assignments for the alkaloid ajmaline. Long‐range 1 H‐ 15 N heteronuclear couplings to the two nitrogen atoms of ajmaline were then probed using a gradient‐enhanced 1 H‐ 15 N heteronuclear shift correlation experiment derived from HMQC. Long‐range 1 H‐ 15 N couplings in ajmaline are assigned for the first time.