Nuclear Magnetic Resonance Experiments Applicable to the Elucidation and Characterization of Alkaloid Structures Part II: Advanced Techniques for the Identification of Adjacent Carbons Using H2BC, 1,1-ADEQUATE, and Variants

If a given structure is still enigmatic with the ensemble of data amassed using the experiments described in Chapters 3 and 9, the next level of sophistication is embodied in the experiments described in this chapter. Specifically, this chapter deals with techniques for unequivocally defining adjacent protonated carbons using the H2BC experiment and adjacent protonated and quaternary carbons using the 1,1-or 1,1-HD-ADEQUATE experiments. Although the LR-HSQMBC experiment has considerable facility for accessing, for example, 4JCH correlations, there may be occasions when that heteronuclear coupling is small for one reason or another. In those cases, the 1,n-and 1,n-HD-ADEQUATE experiments can be employed, which provide access to primarily 3JCC correlations that are equivalent to 4JCH correlations. The inverted 1JCC 1,n-ADEQUATE experiment, which inverts the 1JCC correlations that “leak” into 1,n-ADEQUATE spectra, is also discussed. Finally, several examples of complex alkaloid studies that have employed the various experiments described in this and the previous chapter round out the discussion of nuclear magnetic resonance methods that are applicable to alkaloid structure characterization. Of course, any of the experiments described in this or the previous chapter can be readily applied to any class of natural products of interest to an investigator. We have intentionally restricted this and the previous chapter to the assembly of the skeleton of the molecule.