Imidazo pyridines, tautomerism

The parent system (99) has been obtained by lithium aluminum hydride reduction of perhydro-imidazo[l,5-a]pyridine-l,3-dione (from ethyl 2-piperidine carboxylate and cyanic acid) and by ring closure of 2-aminomethylpiperidine with formaldehyde. 3-Substituted derivatives have also been described. The 3-phenyl- and 3-/-butyl derivatives (but not the propyl and benzyl derivatives) show ring-chain tautomerism (70JHC355). 

Disuccinimido carbonate (DSC) is a useful reagent for the insertion under mild conditions of a carbonyl group in diamines [3019]. A cyclization which leads to imidazo[4,5-c]pyridin-2-one is described in Section III. Carbonyldi imidazole-TEA is another reagent for this type of cyclization [3925]. Potassium ethylxan-thate (potassium ethyidithiocarbonate) is an alternative to carbon disulphide in the synthesis of the tautomeric 2-benzimidazolethiones it was first used many decades ago and usually gives good yields [3259]. The synthesis of an imidazo[4,5 c]pyridin-2-one is described in Section 111.2 [2327]. 

Amino-imidazo[l,2-fl]pyridines exist as amino tautomers, but are even more unstable than amino-indolizines. 2- and 5-oxo compounds are in the carbonyl tautomeric form and react with phosphoryl chloride yielding chloro compounds. ... 

---