Direct Identification of the Carbon Skeleton of Organic Compounds using Double Quantum Coherence ¹³C‐NMR Spectroscopy. The INADEQUATE Pulse Sequence

Abstract

Abstract NMR spectroscopy plays an important part in the determination of the structures of organic compounds. The parameters of importance here are the chemical shifts of the 1 H and 13 C nuclei and the spin‐spin interactions both between 1 H nuclei and between 1 H and 13 C nuclei. Couplings between 13 C nuclei were almost completely neglected until a few years ago, since they were extremely difficult to observe because of the low natural abundance of 13 C. However, it is these couplings which afford information directly on the carbon‐carbon connectivities in the molecule. It is now possible to use a special NMR pulse sequence to make these couplings more readily visible: the result of using this sequence is a 13 C‐NMR spectrum from which the carbon skeleton concerned can be directly read off. Two‐dimensional spectra in particular are very easy to evaluate. The pulse sequence involved, which bears the somewhat puzzling name INADEQUATE, produces double‐quantum coherences from which the NMR signals of the coupled carbon nuclei can be obtained. In this article the principle of double‐quantum coherence is described and a number of examples for the application of the INADEQUATE pulse sequences to problems in synthetic organic chemistry, biosynthesis and natural products chemistry are presented; in addition, the possibility of applying the INADEQUATE method to other nuclei is considered.