Spectral aliasing in 2D-NMR. A straightforward method to considerably increase the resolution of signal clusters and facilitate identification of two cubebin epimers in Drimys winteri

Drimys winteri J.R. et G. Forster (Winteraceae) is a well-known plant in South America used as an insecticide and in the treatment of various inflammatory diseases. Phytochemical studies have shown the presence of diverse secondary metabolites such as terpenoids, sesquiterpenes, flavonoids and lignans [1]. In the latter, epimers of cubebin are difficult to distinguish in complex mixtures. In typical 2D NMR experiments, the width of signals in the carbon dimension of HSQC, HMBC, etc. is much larger than what is observed in a 1D carbon spectrum. This makes it difficult to resolve signals that are only a few Hertz apart, rendering assignment and structure determination difficult or impossible. The brut-force method of increasing the number of time increments is impractical as it imposes too long acquisition times especially when many spectra are required or when series of analyses have to be made.

Recording spectra with a 10-ppm carbon window [2] is a powerful method to improve the spectral resolution by a factor of 20–25 without increasing experimental time and approaches the resolution of 1D carbon spectra. Many applications have been developed among which diffusion measurements of complex mixtures [3] and kinetic measurements [4]. We illustrate the power of the Computer-Optimized Spectral Aliasing (COSA) by recording 10-ppm HSQC and HMBC spectra of a mixture of two epimers of cubebin extracted from the leaves of D. winteri.

References: [1] Muñoz, O. et al. (1999) Plantas medicinales de uso en Chile. Editorial Universitaria. Santiago, Chile.

[2] Vitorge, B. et al. (2009) Chem. Commun. 950–952.

[3] Vitorge, B. et al. (2006) Anal. Chem. 78:5601–5606.

[4] Gasparini, G. et al. (2008) Chem. Commun. 26:3034–3036.