Limitations in the deduction of carbon NMR spectra from the f1 dimension of standard 2D heteronuclear experiments when applied to natural products

Abstract

The structure elucidation of a natural product requires a set of NMR spectra that includes both carbon observe experiments, such as 1D carbon and DEPT, and proton observe experiments, such as HSQC, HMBC, and COSY. Because NMR probes are optimized for either proton or carbon observe experiments, but not both, this often results in some experiments being acquired at a very suboptimal level of efficiency. An alternative is to deduce the carbon spectrum from the indirect, or f 1, dimension of the heteronuclear 2D experiments. This approach is sometimes being employed for the structure elucidation of newly isolated natural products in cases where the amount of material available precludes carbon observe experiments. However whether this approach is reliable in every case has not yet been established. This study applies the "indirect dimension" approach to a representative set of known natural products. The results are mixed. Analysis of E-HSQC spectra, in conjunction with COSY spectra, reliably defines the carbon spectra of the methyl, methylene, and methine carbons present. However, due to limits in resolution in the f 1 dimension of standard HMBC experiments, the presence and chemical shift positions of some quaternary carbons are fairly frequently obscured by those of other carbons. Thus it is often necessary to acquire a 1D carbon NMR spectrum to support the structure elucidation of a natural product.