Structure elucidation of a proton-deficient natural product using LR-HSQMBC supported by DFT calculations

Abstract

A severely proton-deficient marine alkaloid (C28H21N7O2) was intractable to structural characterization using only conventional 1D and 2D NMR techniques including HMBC data, which generally provides primarily two- and three-bond correlations. A sensitive, complementary approach to obtain very long-range (≥ 4 J CH) heteronuclear correlations is afforded by the recently reported LR-HSQMBC experiment. An INEPT pulse sequence element inserted after t1 evolution, converts anti-phase long-range heteronuclear coupling information back into in-phase magnetization thereby alleviating cancelation of responses emanating from small (< 2 Hz) scalar couplings. The role of the LR-HSQMBC experiment in the determination of this natural product structure will be illustrated.