Sulfur systems, heterocyclic

This review chapter focuses on the syntheses and reactions of these 5-membered heterocyclic ring systems containing nitrogen and sulfur (or selenium) (reported during 2006). The importance of these 71-rich heterocycles in medicinal chemistry and natural products is also covered. 

Cimetidine contains an imidazole ring comparable to histamine, a sulfur atom (thioether group) in the side-chain, and a terminal functional group based upon a guanidine (see Section 4.5.4). Ranitidine bears considerable similarity to cimetidine, but there are some important differences. The heterocycle is now furan rather than imidazole, and the guanidine has been modified to an amidine (see Section 4.5.4). A newer drug, nizatidine, is a variant on ranitidine with a thiazole heterocyclic ring system. 

In common with the sulfur systems, the most important organophosphorus-selenium heterocycle is the four-membered ring R(Se)P(p-Se)2P(Se)R. In the solid state, the R groups adopt a trans arrangement with respect to the P2Sc2 ring (R = Ph ). Solid-state P and Se NMR spectroscopy has been... 

The bathochromic response brought about by the incorporation of the heterocyclic ring system can be attributed to greater stabilization of the appropriate dipolar resonance canonical as in (104), but is probably associated with the diene character of the heterocycle. PPP calculations have suggested that the role of the sulfur atom is unimportant. 

Reviews which cover heterocyclization of unsaturated sulfur compounds have been published recently.16 Cyclizations of sulfur systems can also be effected by radical chain mechanisms or by reactions in which the sulfur acts as an electrophile. Examples of nucleophilic sulfur functional groups used in heterocyclization reactions are shown in Figure 3. ... 

Photocyclization arising by intramolecular elimination of HC1, HBr, or HI has been extensively used in the synthesis of nitrogen-containing heterocycles, but much less so in oxygen and sulfur systems. The mechanism of these transformations is not always clear some proceed by way of an initial carbon-halogen bond homolysis, whereas others involve cyclization and... 

In keeping with the known data, the sulfur in polynuclear aromatic systems is difficult to remove in the absence of hydrogen. The heterocyclic rings tend to remain intact and are incorporated into the coke. The presence of hydrogen facilitates the removal of heterocyclic sulfur. Indeed, there is also the possibility the hydrogen prior to cracking or hydrodesulfurization may render sulfur removal and hydrocarbon production even more favorable. 

Another less common heterocyclic sulfur volatile is 1 - 7/]-pyrrolo-[2,1 c]-1,4-thi-azine, identified in bread crust by Baltes and Song 249 and previously obtained by Guntert et al.269 from a ribose-cysteine system. 

This relationship is undoubtedly responsible for much of the intensive theoretical and experimental investigation currently being carried out on these systems. The purely theoretical aspects of the relationship were recently discussed in detail by Zahradnik in a review on the Electronic Structure of Heterocyclic Sulfur Compounds, published in Volume 5 of this series.1... 

There are three heterocyclic ring systems that can be included imder this heading. These rings include oxygen, nitrogen, or sulfur hetero atoms while the most studied ones are those with oxygen. 

Hetero Diels-Alder cycloaddition reactions are of great importance in organic chemistry for the synthesis of six-membered heterocyclic ring systems. " In the past heterodienophiles and -dienes with one or more oxygen, nitrogen or sulfur atoms have been examined in thermal and Lewis acid-catalyzed reactions. " ... 

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