The detection of ultra-long-range
(4JCH and higher) heteronuclear
connectivities can complement
the conventional use of HMBC/HSQMBC data in structure elucidation
NMR studies of proton-deficient natural products, where two-bond and
three-bond correlations are usually observed. The performance of the
selHSQMBC experiment with respect to its broadband HSQMBC counterpart
is evaluated. Despite its frequency-selectivity nature, selHSQMBC
efficiently prevents any unwanted signal phase and intensity modulations
due to passive proton–proton coupling constants typically involved
in HSQMBC. As a result, selHSQMBC offers a significant sensitivity
enhancement and provides pure in-phase multiplets, improving the detection
levels for short- and long-range cross-peaks corresponding to small
heteronuclear coupling values. This is particularly relevant for experiments
optimized to small nJCH values (2–3 Hz), referred to as LR-selHSQMBC,
where key cross-peaks that are not visible in the equivalent broadband
LR-HSQMBC spectrum can become observable in optimum conditions.