Sulfur heterocycles, reviews

Thionyl chloride (SOCl2) is the most versatile of all sulfur transfer reagents in heterocyclic synthesis. The sulfur-oxygen bond renders the sulfur atom more electrophilic the possibility of ready removal by elimination of the oxygen atom allows easy aromatization of many of the initially formed sulfur heterocycles. Two general reviews of the chemistry of thionyl chloride are available.53 Both cover the literature up to about 1970. 

Aromatic Sulfur Heterocycles , W. Carruthers, in MTP International Review of Science, Organic Chemistry Series Two , ed. D. H. Hey, Butterworth, London, 1975, vol. 4, pp. 251-278. 

The scarcity of reviews1 and recent interest in thiochromanones and related compounds calls for a comprehensive summary of this area of heterocyclic chemistry. The apparent similarity between these systems and the naturally occurring chromanones, chromones (flavones), chromenes, etc., is responsible for the continued importance of these sulfur heterocycles. Chemical Abstracts (through November, 1973) has been employed as the principal reference source and nomenclature guide for this review. 

The photoelimination of nitrogen from azo compounds, diazo compounds, and azides is a topic that has been thoroughly reviewed elsewhere. These transformations are found in oxygen and sulfur heterocycles, but as they are not unique to these systems, they will not be discussed in this article. Examples can be found in a review on the photochemistry of nitrogen-containing heterocycles.1... 

Sulfur Heterocyclics. Sulfur containing compounds (thiols, thiophenes, thiazoles,. .. etc.) play a major role in the flavor of raw and processed foods. These compounds have characteristic flavor notes and the flavor thresholds are mostly low. Several reviews (ill, 112, 113) demonstrate the important role of sulfur compounds in food flavors. Organoleptic properties of these compounds may be pleasant, strong nut-like odor of U-methyl-5-vinylthiazole which is present in cocoa (llU) objectionable pyridine-like odor of thiazole (115) quinoline-like odor of benzothia-zole (ll6) strong tomato leaf-like odor of isobutylthiazole (117) and bread crust flavor of acetyl-2-thiazoline (ll8). A mixture of oxazoles, thiazoles, thiazolines, imidazoles, trithiolanes and... 

The reaction has been the subject of reviews,316,45 from which it will be seen that the reaction can be applied to synthesis of five- and six-membered oxygen-heterocycles, sulfur-heterocycles, bicyclic compounds, and compounds containing an additional heteroatom in the ring. 

A limited number of accounts of the literature have appeared Brandsma [36], Kwart [5], Paquer [37], Anisimov [38], our contribution in the context of thiocarbonyl compounds in synthesis [39] and a recent review by Majumdar [40] with an emphasis on the synthesis of sulfur heterocycles. 

A number of sulfurated heterocycles can be prepared using this transposition. The principal synthetic routes have been already surveyed in previous reviews [38,40). 

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