NASCA-HMBC, a new NMR methodology for the resolution of severely overlapping signals: application to the study of agathisflavone

Abstract

Introduction: Standard NMR 2D heteronuclear HMBC spectra have a low resolution in the indirect carbon dimension, making it very difficult to assign signals to individual carbons when their chemical shifts are < 0.3 ppm apart.

Objective: To establish spectral aliasing for HMBC experiments to improve the resolution in the carbon dimension without increasing the total experimental time and avoiding ambiguities in the observed chemical shifts.

Methodology: The NASCA-HMBC (Non-ambiguous Assignment by Superposition of Coupled Aliased HMBC) methodology combines a pair of HMBC spectra recorded with slightly different carbon windows to provide typically one order of magnitude increase in the resolution and unambiguous chemical shifts.

Results: The application of this methodology to a biflavonoid found in Ouratea gilgiana resulted in spectra with a sufficiently high resolution to make the assignment straightforward and report, for the first time, the full assignment of agathisflavone.

Conclusion: The methodology should find many applications in dimeric and oligomeric compounds such as peptides, carbohydrates, polyketides and other cases where signal clustering is expected.