Natural products structural chemistry

Clearly the development and dissemination of two-dimensional NMR techniques has had a profound impact in natural products structure elucidation. Some techniques, COSY and variants of the C-detected heteronuclear chemical shift correlation (variously referred to as HETCOR, HC-COSY, etc.) experiment, have been widely used by the natural products chemistry community. Inverse-detected heteronuclear shift correlation techniques are becoming recognized as a powerful adjunct to the COSY experiment and a replacement for their less sensitive and, in some cases, less versatile C-detected predecessor experiments (Martin and Crouch 1991). 

Diol cleavage played an important role in natural product structure analysis in the days before instrumental analysis. Understandably, the importance of periodic acid oxidations has diminished somewhat in modern organic chemistry. 

Thus, in the field of macrocyclic chemistry it is apparent that there can be a definite continuum from natural product structures to totally synthetic molecules that perform the same pharmacological tasks, aside from perhaps antibiotics, though that area may yet be conquered as shown by the success of the bicyclic, but not yet macrocyclic, quinolone-based antibacterial agents colloquially known as floxacins . ... 

This case history presents only a simple account of one of R.B. Woodward s adventures based on ingenious undentanding of structural features and experimental findings described in the literature. The hydrogenation of porphyrins is still one of the most active subjects in heterocyclic natural products chemistry, and the interested reader may find some modem developments in the publications of A. Eschenmoser (C.Angst, 1980 J.E. Johansen, 1980). 

Application of NMR spectroscopy to heterocyclic chemistry has developed very rapidly during the past 15 years, and the technique is now used almost as routinely as H NMR spectroscopy. There are four main areas of application of interest to the heterocyclic chemist (i) elucidation of structure, where the method can be particularly valuable for complex natural products such as alkaloids and carbohydrate antibiotics (ii) stereochemical studies, especially conformational analysis of saturated heterocyclic systems (iii) the correlation of various theoretical aspects of structure and electronic distribution with chemical shifts, coupling constants and other NMR derived parameters and (iv) the unravelling of biosynthetic pathways to natural products, where, in contrast to related studies with " C-labelled precursors, stepwise degradation of the secondary metabolite is usually unnecessary. 

The held of marine natural products chemistry, which encompasses the study of the chemical structures and biological activities of secondary metabolites produced by marine plants, animals, and microorganisms, began in earnest in the early 1960s. " This is in stark contrast to the study of terrestrial plant natural... 

Many of the most important naturally occurring minerals and ores of the metallic elements are sulfides (p. 648), and the recovery of metals from these ores is of major importance. Other metal sulfides, though they do not occur in nature, can be synthesized by a variety of preparative methods, and many have important physical or chemical properties which have led to their industrial production. Again, the solubility relations of metal sulfides in aqueous solution form the basis of the most widely used scheme of elementary qualitative analysis. These various more general considerations will be briefly discussed before the systematic structural chemistry of metal sulfides is summarized. 

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