The structural assignment of a new natural product molecule is not only to establish
the 3D structure of a compound, but potentially to provide the basis for
research in a multitude of disciplines, ultimately generating new therapeutic
agents and/or new understanding of disease biology. The development of modern
spectroscopic techniques has transformed the structure assignment process,
which previously was essentially based on chemical degradation or derivatization
followed by partial or total synthesis. Notably, it was only in the specialization
era of the spectroscopic structural assignment of natural products that the
field of marine natural products chemistry took shape.
Today the processes of marine and terrestrial natural product isolation and
structural determination are frequently streamlined and expeditious due to the spectacular advances in chromatographic and spectroscopic technologies as
well as chemical synthesis.
The NMR spectroscopy is a powerful tool in structure elucidation because
the properties it displays can be related to the molecular structure. The chemical
environment of a particular nucleus is associated with the chemical shift (d,
ppm), and the area of a resonance, usually presented as its relative integral, is
related to the number of nuclei giving rise to the NMR signal. The interactions
between individual nuclei, mediated by electrons in a chemical bond, determine
the coupling constant (J, Hz). In this chapter we will present the techniques
commonly used, basic concepts, and how they are useful for chemists in the
structural elucidation of mainly bioactive marine natural products. Its complex
planar structure is determined by 1H and 13C NMR analysis strongly supported
by other 1D (DEPT) and 2D (COSY, TOCSY, HSQC/HMQC, HMBC) NMR
techniques. The stereochemistry is generally based on NOE experiments (NOE
difference, NOESY, and ROESY), 1H–1H and 1H–13C coupling constants, chiral
derivatizing agents, and also in empirical procedures comparing the chemical
shifts of unknown vicinal and proximal centers with libraries of configurationally
known stereomodels. However, the most reliable option to assign all the 3D
structure of a marine natural product still is their total synthesis.
The use of NMR hyphenated with other chromatographic and spectroscopic
techniques and microcoil probes and narrow diameter tube probes for the structural
elucidation of bioactive marine natural products, mainly associated with
the quantitative NMR determinations, will be also briefly described.
The chapter will finish with a description of the structural characterization of
several types of marine natural products using all the referred NMR techniques
followed by a small reference to the misassignments that still are very common.
