Studies on Heteropentalenes. V. Cycloadditions of 5-Aryl-3-methylimidazo[5,1-b]thiazoles with Acetylenic Esters Leading to 5-Aryl-3-methylthiazolo[2,3-c]benzimidazoles and Related Heterocycles

Abstract

Cycloaddition of 5-aryl-3-methylimidazo[5,1-b]thiazoles with dialkyl acetylenedicarboxylate in an aprotic nonpolar solvent gives a number of products including epimeric thiazolo[2,3-c]benzimidazoles (3 and 4) [1:2-cycloadducts], epimeric 5,10b-ethenothiazolo[3′,2′:3,4]imidazo[1,5-a]-pyridines (5 and 6) [1:3-cycloadducts], and thiazolo[3′,2′:3,4]imidazo[1,2-a]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 (5 and 6) and tetraalkyl 6-arylpyridine-2,3,4,5-tetracarboxylates are substantially produced. Epimerization of 3 and 4 has been found to compete well with fragmentation to 4H-1,4-benzothiazine.

Imidazo[5,1-b]thiazol-3-ones/thiazin-4-ones: Synthesis and Reactivity Investigation for Library Generation

“…Previously reported literature methods to the [5-5] fused ring system 2 are few, and all require harsh dehydrating conditions to construct the bicyclic architecture, whereas a route to the [5-6] ring system 3 has yet to be reported 3f, To the best of our knowledge, the chemistry of the thiazolone ring of 2 or the thiazinone ring of 3 has not been previously examined. This we anticipated would be of particular interest due to the five- and six-membered lactams contained within these structures.…”