4-Chloro-2-methylimidazo pyridine

Several 3-aryl-2-methylimidazo[4,5-b]pyridines (36 X = H, R = H, 4-F, 4-Et X = Cl, R = H, 4-F, 3-CF3, 2,4-Me2, 2,4-Ch, 2-Me-6-Et, 3-CF3-4-C1) with pesticidal activity have been prepared in moderate yields by the reaction of /V-(2-chloro-3-pyridyl)acetamides (37) with aromatic amines in the presence of P2O5 and Et3N-HCl at 150 C.140 With X = H the cyclized products were isolated directly, whereas with X = Cl treatment of the intermediate amidines (38) with K2CO3 in DMF was required to arrive at the final cyclized product. 

On being heated with 5" aqueous alkali, 3-bromo-2-phenyl-4-oxo-4//-pyrido[l,2-a]pyrimidine was transformed to 2-phenylimidazo[l,2-u]pyri-dine, whereas the 2-methyl analog in 7"0 aqueous potassium hydroxide yielded a mixture of the 3-unsubstituted- and 3-carboxy-2-methylimidazo-[l,2-u]pyridines. Under similar conditions, 3-bromo-2-chloro- and 3-bromo-4-oxo-4ff-pyrido[l,2-a]pyrimidines decomposed to 2-aminopyridine.294... 

Halogenation of the imidazo[4,5-6]pyridine 4-oxides has provided some interesting results. Treatment of compound (123 R = H) with phosphorus oxychloride gave, with loss of the /v-oxide group, a mixture of the 5-chloro (124) and 7-chloro (125) isomers in comparable amounts in an overall 74% yield (Equation 3) <82JHC513>. The reaction of 3-methylimidazo[4,5-6]pyridine 4-oxide (123 R = CH3) with phosphorus oxychloride also gave a mixture of products but the yields were different, i.e. 5-chloro (124) (20%) and 7-chloro (125) (41%). However, with l-methylimidazo[4,5-6]pyridine 4-oxide (126), the chlorine atom was introduced exclusively into the more sterically hindered 7 position to give the 7-isomer (127). None of the 5-isomer was detected in this reaction (Equation (4)). 

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