Analysis of Brucine at 100 MHz – Getting COSY with Correlations

Brucine is a structurally complex molecule, commonly found in the bark and seeds of the Strychnos nux-vomica tree, which is native to India and southeast Asia.¹ A toxic alkaloid, brucine is typically present alongside strychnine, a structurally almost identical molecule that is more toxic than brucine.² While this molecule contains many different functionalities and chemical environments, ¹H NMR is an exceptionally useful tool for structural elucidation and remains one of the most powerful analytical techniques available to chemists. Additionally, the resolution offered on our 100 MHz instrument provides the necessary resolution for identifying and assigning the different protons in this molecule using ¹H NMR (Figure 1).

Figure 1. ¹H (100 MHz) NMR of brucine in chloroform-d.

While the alkaloid region from 1-4 ppm contains many overlapping signals, all the key resonances can be extracted and assigned to specific protons in the molecule. Additionally, the methoxy (H14 and H15), alkene (H6), and aromatic (H8 and H9) protons give rise to very characteristic chemical shifts. The power of NMR also lies in the fact that there is a broad range of multinuclear and two-dimensional experiments that can be performed, many of which are included in our benchtop NMR instruments. One of these is the ¹H-¹H COSY experiment, a 2D technique providing information on spins which are coupled to each other through bonds. Often, chemists will use a mixture of techniques to extract as much information as possible using NMR. The COSY spectrum of brucine is shown in Figure 2, and the key correlations are highlighted.³

Figure 2. ¹H-¹H COSY (100 MHz) NMR of brucine in chloroform-d.

The COSY spectrum can be used to identify and assign the major ¹H-¹H trough-bond interactions, which include: H6-H11a/b, H6-H16b, H6-H22, H8-H9, H8-H15, H9-H14, H10-H19, H10-H20a, H10-H20b, H12-H19, H13-H23b, H16a-H16b, H18a-H18b, H18a-H21a/b, H18b-H21a/b, H19-H22, H20a-H20b, H22-H23b, and H23a-H23b. Importantly, since we know the structure for brucine, we can confirm that all these correlations are expected due to the coupling between these specific groupings of protons. It’s interesting to note that although the H13 resonance is hidden underneath the two dominant methoxy signals produced by H14 and H15, we can still confirm its presence due to the strong correlation between H13-H23b. We know that there is no interaction between these methoxy substituents and the H23 protons, as they are at completely different ends of this molecule. This further illustrates the power of 2D NMR spectroscopy, which can sometimes provide more clues about the molecule being analyzed as compared to just a ¹H experiment. If you have any questions about 2D experiments using benchtop NMR, or about how you could incorporate our instruments into your workflow, please don’t hesitate to reach out to us!

References

(1) Aniszewski, T. Alkaloids: Chemistry, Biology, Ecology, and Applications, 2nd ed.; Elsevier: Amsterdam, Netherlands, 2015.
(2) Liu, F.; Wang, X.; Han, X.; Tan, X.; Kang, W. Int. J. Biol. Macromol. 2015, 77, 92–98.
(3) Bernstein, M. A.; Hall, L. D. Can. J. Chem. 1985, 63, 483–490.