Imidazo thiazolium-3-olate

Avalos and co-workers (220-228) extensively investigated the 1,3-dipolar cycloaddition chemistry of 2-aminothioisomiinchnones with both acetylenic and olefinic dipolarophiles. For example, sugar derivatives of the mesoionic imi-dazo[2,l-Z7]thiazolium-3-olate system react regioselectively with a variety of acetylenic dipolarophiles [DMAD, diethyl azodicarboxylate (DEAD), methyl propiolate, ethyl phenylpropiolate] to give the corresponding imidazo[l,2-a]pyr-idin-4-ones (e.g., 323) following sulfur extrusion from the not isolable cycloadducts (220). Similarly, these thioisomtinchnones react with diethyl azodicarboxylate and arylisocyanates in the expected fashion (221), and also with aryl aldehydes to form episulfides (222). 

Imidazo[2,l-h]thiazolium-3-olate 160 (Section VI,C Scheme 47) underwent rapid cycloaddition with acetylene mono- and dicarboxylic esters to give the imidazo[l,2-a]pyridin-3-yl acyclo C-nucleoside 208 through sulfur extrusion from the intermediate 207 (91MI7) (Scheme 62). 

The synthesis of imidazo[2,l-h]thiazolium-3-olate zwitterions such as (100) from sugar thiourea derivatives, and their cycloaddition reactions to form imidazo[l,2-fl]pyridin-4-ones such as (101) (Scheme 10) have been detailed. ... 

---