Studies on heteropentalenes. V. Cycloadditions of 5-aryl-3-methylimidazo-(5,1-b)thiazoles with acetylenic esters leading to 5-aryl-3-methyl-thiazolo(2,3-c)benzimidazoles and related heterocycles.
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Cycloaddition of 5-aryl-3-methylimidazo[5,1-<I>b</I>]thiazoles with dialkyl acetylenedicarboxylate in an aprotic nonpolar solvent gives a number of products including epimeric thiazolo[2,3-<I>c</I>]benzimidazoles (<B>3</B> and <B>4</B>) [1:2-cycloadducts], epimeric 5,10b-ethenothiazolo[3′,2′:3,4]imidazo[1,5-<I>a</I>]-pyridines (<B>5</B> and <B>6</B>) [1:3-cycloadducts], and thiazolo[3′,2′:3,4]imidazo[1,2-<I>a</I>]pyridines [1:3-adducts]. At higher temperature, formation of the 1:2-cycloadducts is favored over the 1:3-adducts, whereas the latter predominates at room temperature. In an aprotic polar medium, the 1:3-cycloadducts (<B>5</B> and <B>6</B>) and tetraalkyl 6-arylpyridine-2,3,4,5-tetracarboxylates are substantially produced. Epimerization of <B>3</B> and <B>4</B> has been found to compete well with fragmentation to 4<I>H</I>-1,4-benzothiazine.
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- Studies on heteropentalenes. V. Cycloadditions of 5-aryl-3-methylimidazo-(5,1-b)thiazoles with acetylenic esters leading to 5-aryl-3-methyl-thiazolo(2,3-c)benzimidazoles and related heterocycles.