4- Aminoimidazo pyridines, formation

Similarly, a novel one-pot microwave-assisted reaction has been described by Hulme and co-workers that enables the selective formation of 3-iminoaryl-imidazo[l,2-a]pyridines 19 and imidazo[l,2-a]pyridyn-3-ylamino-2-acetonitriles 20, in good yields. Reactions were performed in methanol by mixing a suitable a-aminopyridine, an aldehyde and trimethylsilylcyanide (TMSCN) with polymer-bound scandium triflate, to afford either product 19 or 20 (Scheme 16). Initially, the 3-iminoaryl-imidazo[l,2-a]pyridine 19, was observed as a minor side product (0-10%) during the pseudo-Ugi reaction affording the 3-aminoimidazo[l,2-a]pyridine 18. However, by increasing the aldehyde input to 2.2 equiv the reaction could be directed to the formation of 19 in moderate yields. Interestingly, low yields of... 

Refluxing ethyl 2-(3-aminopyridin-4-yl)hydrazinecarboxylate (419) in 36% methanolic hydrogen chloride for 8 hours gave l-aminoimidazo[4,5-c]pyridin-2(3i/)one hydrochloride (423), rather than the expected pyridotriazinone (420), thought to be an intermediate in the formation of (423) via (421) and (422) (76JHC601). 

The formation of pyridine IV-imines by an add-catalyzed rearrangement of some diazepinones and related compounds has been extensively investigated by Moore and co-workers (Eq. 9).115 The formation of pyridine N-imines from l/f-l,2-diazepines is also known (Eq. 10).68 69,116,117 Diels-Alder reactions of pyrazoles with dimethyl acetylenedicarboxylate, in the presence of BF3, have been reported to give IV-aminopyridinium salts (Eq. II).118 l,4-Dihydropyrido[l,2-a]-as-triazinium salts and their pyrimido derivatives undergo ring contraction in boiling aqueous acid, yielding 1-aminoimidazo[l,2-a]pyridinium and pyrimidinium salts, respectively (Eq. 

A catalyst-free protocol has been described by Adib for the synthesis in good to excellent yields of 3-aminoimidazo[l,2-o]pyridines and 5-aminoimidazo[2,l-6] [l,3]thiazoles via three-component reactions between 2-aminopyridines or 2-ami-nothiazoles, aldehydes and isocyanides in water (Scheme 1.10) [14]. Presumably, this process involves the initial formation of an imine, which then reacts with the isonitrile in a formal [4+2] cycloaddition. 

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