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13C NMR and Natural products

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.

Projsumof2dstrychnine_mar252008_3 Projsumof2dseries_mar252008_4

TIP: Be especially careful with quaternary and overlapping carbons as they may not be evident on an HMBC.