Natural halogen compounds

Some very interesting natural halogen compounds have been discovered only recently. They are included here due to their unusual structural... 

Natural products represent an unparalleled source of molecular diversity for innovative drug discovery. In the pharmaceutical industry, there is an urgent need to identify novel lead structures for effective drug development in many therapeutic areas. It is likely that natural halogen compounds will continue to contribute leads to future research. 

Laus G (2001) Biological Activities of Natural Halogen Compounds. Stud Nat Prod Chem 25 757... 

Higa T, Sakemi S. Environmental studies on natural halogen compounds. J ChemEcol 1983 9 495-502. 

Laus, G. Biological activities of natural halogenated compounds, In Studies in Natural Products Chemistry, Atta-ur-Rahman, Ed. Elsevier Press 2001,25, 757-809. 

Improved techniques of analysis, better understanding of biological/chemical processes in nature, and systematic research revealed up to 1998 more than 2500 natural halogen compounds, 1800 of them containing chlorine. A further 30 to 40 compounds are detected annually. 

Laus, G. (2001) Biological activities of natural halogen compounds, in Studies in Natural Product Chemistry (ed. Atta-Ur-Rahman), Elsevier, Amsterdam, Oxford, pp. 757-809. 

Higa, T. and Sakemi, S. (1983) Environmental studies on natural halogen compounds. I. Estimation of biomass of the acorn worm Ptychodera jlava Eschscholtz (Hemichordata Enteropneusta) and excretion rate of metabolites at Kattore Bay, Kohama Island, Okinawa. J. Chem. Ecol., 9, 495-502. 

The interatomic distance values given are stated to hold for hexahalogen, rym-trihalogen and -dihalogen benzenes. We expect the distances in these different compounds to be different, however and investigation of these and other halogen compounds is under way in this Laboratory. De Laszlo has also reported values of C-Br and C-I distances in ethylenic and acetylenic compounds in a letter to Nature, 130, 474 (1936). 

Finally, the presence in human post-mortem brain tissue of the active metabolite of diazepam, desmethyldiazepam, raised some curiosity and frank alarm (Sangameswaran et al. 1986). At the time of its discovery in the brain it was thought that there was no enzyme system capable of producing such halogenated compounds and that its presence in the brain reflected dietary intake from an environment contaminated by overuse of its parent compound. However, its discovery in stored brain tissue which had been obtained before the synthesis of the benzodiazepines allayed these fears. It is now thought possible that some benzodiazepines, including desmethyldiazepam, occur naturally and that they are taken in as part of a normal diet (Table 19.5). 

If L = halogen, this type of reaction is referred to as dehydrohalogenation. Thus, when assessing the fate of halogenated compounds in natural waters, this process has to be considered in addition to nucleophilic substitution. The question then is what structural features and environmental conditions determine whether only one or both of these two competing types of reactions will be important. 

The nature of the base fuel also influenced the response to halogen compounds. For instance, although isooctane and ethylbenzene both oxidize and ignite within almost the same temperature range, ethyl iodide markedly promoted the oxidation and ignition of isooctane, but with ethylbenzene it only promoted oxidation, while inhibiting ignition. 

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