Fused Systems containing Thiazole

Fused Systems containing Thiazole.—A large number of polyheterocyclic systems have been prepared, but in most cases the methods of synthesis, and the properties of the products, are straightforward extensions of earlier work they will not, therefore, be further discussed. 

The thiazolo[3,2-a]pyridinium structure has, however, received much attention, perhaps because it is so readily accessible. For example, the cyclization of 2-(pyridylthio)acetic acids (78) affords thiazolo[3,2-a]pyridinium-3-olates (79).  

Reagents i, BuLi ii, R COCHjR iii, PjO, MeSOjH iv, FSOjMe v, NaBH vi, AgNO HjO 

The nucleus C-2 of the thiazole ring in these compounds is appreciably nucleophilic, and it will react readily with electrophiles, including diazonium salts and aldehydes. 

In much the same way, 3-hydroxypyridine-2-thione (80) reacts with 1,1,2,2-tetrabromoethane to afford the 8-olate (81) the remaining bromine atom of (81) allows selective deuteriation on C-2 and C-3 of the thiazole ring. Other halogen compounds react with (80) in an analogous fashion quinoline-2-thione (82) can also be used.  

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Numerous structures containing the thiocarbonyl ylide dipole are conceivable. Incorporation of the thiocarbonyl ylide dipole into a bicyclic heterocyclic system is possible by the conversion of the cyclic thione (203) into the ring-fused mesoionic system (204). The thiocarbonyl ylide dipole (205) undergoes cycloaddition with both alkenic and alkynic electron-poor dipolarophiles in refluxing benzene or xylene so that, after extrusion of hydrogen sulfide or sulfur, respectively, from the initial 1 1 cycloadducts (206) and (207), a ring-fused pyridinone is formed. The method has been used for the annelation of pyridinones to the imidazole, 1,2,4-triazole, thiazole and 1,3,4-thiadiazole systems... 

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