One-Dimensional HMQC-TOCSY

An experiment that actually preceded SIMBA, but which probably has somewhat lower utility, was the selective ID HMQC-TOCSY experiment described by the authors (Crouch et al. 1990c). The selective experiment differs from the HMQC-TOCSY pulse sequence only in the replacement of the final C 90° pulse by a selective pulse. Initially, we utilized a square pulse in this application but, obviously, the improvement in the SIMBA experiment by using a Gaussian 270° or an E-BURP-2 pulse would be realized in this experiment as well. To the best of our knowledge, there have been no applications of this experiment to alkaloids reported in the literature. 

One problem inherent to the HMBC experiment is attaining sufficient digital resolution when very wide Fi spectral widths must be employed to include quaternary aromatic or carbonyl carbons. The same problem occurs with aralkyl alkaloids. Frequently, there may be a number of aromatic carbons separated by a wide region containing few or no responses from a congested aliphatic region. 

Electing to restrict the Fi spectral width is inappropriate, since responses to carbon resonances outside of the window can fold and could be mistaken for responses to a given proton if the fold is unfortuitous. This concern is particularly valid when we consider that in some cases it is easier to obtain 

A number of Fi region-selected inverse-detected 2D NMR experiments have been reported. One of the earliest (Davis 1989) took a different approach from more recent reports. Recognizing the need for higher digital resolution in assigning peptide spectra, Davis approached Fi region selection 

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