March 31, 2008
by Arvin Moser, Team Manager, Application Scientists, ACD/Labs
Looking for the next big pharmaceutical drug among natural products is a growing endeavour. However, a common drawback to working with natural products is the limited amount of sample.
With a sample limitation, acquiring a 13C NMR spectrum within a reasonable time can be an issue. Rather than wasting instrument time on a 13C NMR, one can save time by extracting the sum (or projection) of slices across a 2D NMR.
Using strychnine as an example, the diagram below is a comparison of two datasets. The top trace (green) is the sum of all the F1 slices from an HMBC and the bottom spectrum (blue) is an acquired 13C NMR. From the top trace, one can gather a good approximation on the minimum number of carbons for strychnine without resorting to acquiring a 13C NMR.
TIP: Be especially careful with quaternary and overlapping carbons as they may not be evident on an HMBC.
Arvin,
It seems that the CDCl3 resonance shows up in the HMBC F1 sum you have presented. Why is this? There are neither any protons nor any long range proton carbon coupling in CDCl3. Is there a resonance under the CDCl3?
Glenn
Good eye Glenn. There is a resonance at 77.6 ppm for strychnine (CH-O). Although not clearly evident in the 13C NMR spectrum, there is a slight shoulder peak off the CDCl3 resonance.