Interpretation of HMQC-TOCSY Spectra

Unless a specially modified variant of the HMQC-TOCSY experiment is utilized (see below), interpretation of an HMQC-TOCSY spectrum requires prior knowledge of the location of the direct proton-carbon correlation responses. To illustrate the interpretation of HMQC-TOCSY data, it is useful to begin with a simple schematic illustration of the connectivity network associated with the proton four-spin system of a 1,2-disubstituted 

Using the approach just described to interpret HMQC-TOCSY spectra is fine, provided, of course, that the resonances in the proton spectrum are resolved. This approach cannot be employed when there are significant overlaps in the data correlations to the overlapped protons will lead to assignment and/or structural ambiguities. Worse still was the situation encountered with cryptospirolepine (1) (Tackie et al. 1992), in which the proton spectrum contained two overlapped AB portions of two ABXY spin systems. To illustrate the considerable utility of HMQC-TOCSY spectra, this problem will be examined in detail below. Before doing so, however, it is useful to consider an alternative means of extracting correlation information from the HMQC-TOCSY spectrum. 

In contrast, using the axis defined by the proton resonating at 8.324ppm, we observe, in addition to the direct response at 8.324/122.99ppm, a relayed 

There are a number of possible variants and modifications of the HMQC-TOCSY experiment. One obvious modification would be to use HSQC rather than HMQC to label protons with the chemical shift of the carbon to which they are directly bound. To the best of our knowledge, applications of 

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