Sulfur-containing heterocyclic systems

There is a wealth of NMR data available on sulfur-containing heterocyclic systems. There are however, still relatively few papers dedicated to discussion of their NMR spectra. The majority of the spectroscopic data discussed in this work has been selected from papers concerned with either the synthesis or reactivity of the six-membered sulfur heterocycles. Chemical shifts are reported in ppm for spectra recorded in CDCI3 solution unless otherwise stated. Early NMR spectroscopic data have featured in the previous editions of Comprehensive Heterocyclic Chemistry and NMR Spectra of Simple Heterocycles by Batterham is still of fundamental importance for rapid access to H NMR data on the parent sulfur heterocycles . 

This section deals with the most interesting reactions of thienothiophenes giving rise to various sulfur-containing heterocyclic systems. 

Formation of a bond between two heteroatoms is also of considerable utility in the synthesis of sulfur-containing polyheteroatom systems. Reaction of 1,5-dinucleophiles with electrophilic sulfur reagents such as thionyl chloride or sulfur dichloride leads directly to the heterocycles (e.g. equation 55). A fuller discussion of these types of reaction is given in Chapter 2.28. 

Catalytic hydrodesulfurization (HDS) is a very important industrial process that involves removal of sulfur from crude oils by high-temperature ( 400°C) treatment with hydrogen over Co- or Ni-promoted Mo or W catalysts supported on alumina. In an attempt to determine the mechanism of this process, many transition metal complexes of thiophene, a sulfur-containing heterocycle that is particularly difficult to desulfurize, have been prepared and their reactivities studied in order to compare their behavior with those of the free thiophenes that give H2S and C4 hydrocarbons under HDS conditions (88ACR387). Thiophene can conceivably bind to the catalyst surface by either cr-donation via a sulfur electron pair or through a variety of -coordination modes involving the aromatic system... 

The investigation of a series of model meat systems has demonstrated the important role of volatile sulfur-containing heterocyclic components substituted with sulfur in the 3-position. One of these 3-substituted sulfur compounds, 2-methy1-3-methy1thio-furan was identified recently in the volatiles from cooked beef aroma (5J and from a heated yeast extract composition (6J and is considered a meaty character impact compound. 

In the case of 4-thiazolidinone 1, the Et3N 3HF/MeCN system proved rather effective and convenient for anodic fluorination, which is a highly regioselective reaction giving monofluorinated derivative 2. These are the first examples of effective anodic fluorination of sulfur-containing heterocyclic compounds (92JOC3755, 90JOC6074). Subsequent oxidation with peradds affects the sulfur atom to form 3 (Scheme 3). 

Sulfur-Containing Heterocyclic Compounds with Antioxidative Activity Formed in Maillard Reaction Model Systems... 

Antioxidative Activity of Thiazoiidine and 1,3-Ditiiiolane. Saturated cyclic sulfides represent another common class of sulfur-containing heterocyclic compounds formed in model systems and cooked foods. We have selected thiazoiidine and 1,3-dithiolane as model compounds for which to evaluate the antioxidative activity of this class of compounds and the results are shown in Figure 6. 1,3-Dithiolane appears to exhibit slightly higher activity than that of thiazoiidine. Both, however, inhibit heptanal oxidation for a longer period than do the thiophene derivatives presumably due to their different structural characteristics and hence potentially different mechanisms of antioxidative action. 

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