Differentiation of HMBC two- and three-bond correlations: A method to simplify the structure determination of natural products

The structure elucidation of natural products, today, relies heavily on the application of proton-detected heteronuclear NMR experiments. Perhaps the most useful of these methods is the HMBC experiment, which provides correlations between protons and carbons over two and three bonds. The application of the HMBC method for the direct translation of H,C correlations to yield bonding information is limited, however, by the fact that it does not distinguish between (2)J(CH) and (3)J(CH) correlations. Reported here is an application of the recently described 1,1-ADEQUATE experiment that yields only two bond H,C connectivities in H-C-C moieties and therefore allows the differentiation of HMBC two- and three-bond correlations. The method is demonstrated on a 14 mg sample of a new marine natural product, 5,6-dihydro lamellarin H (1).