Formation of Bicyclic Heterocyclic Ring Systems

Recent studies have demonstrated the synthetic potential of 5-aminoim-idazoles (96) as intermediates for heterocyclic synthesis, particularly for the synthesis of bicyclic systems, including imidazo[4,5-6]pyridines 

The addition-elimination adducts (143)-(145) are also useful precursors to imidazo[4,5-b]pyridines. Thus, the malonate derived products (143) on treatment with hot ethanolic HC1 [92JCS(P1)2789] or hot ethanolic triethylamine (78H241) gave the imidazo[4,5-b]pyridones (157). The dinitrile derivatives (144) gave the ortho amino nitriles (158) by treatment with hot methanolic sodium hydroxide solution and the nitrile esters (145) 

The N-adducts derived by reaction of 1,5-diaminoimidazoles (96 R = NHPh, R2 = aryl, /Pr) with the /V-acylimidates (140) and (141) were cyclized smoothly in hot xylene solution to give the corresponding purines (168) and (169) in good yield (85MI1). 

The tri-O-acetyl derivative of 5-amino-l-(/3-D-ribofuranosyl)imidazole (173) has been found to undergo a facile rearrangement at pH 7 to give a mixture of the 5-acetamido derivatives (174) and (175) (90JA4891). The transacetylation was quite rapid (/l/2 = 15h), but the authors noted that it proceeded more slowly at both higher and lower pH. An earlier report 

Treatment of an ethyl acetate solution of the 5-aminoimidazole (96 R1 = Me, R2 = S02Me) with p-nitrobenzaldehyde in the presence of trifluo-roacetic acid gave a crystalline product, which was assigned the tricyclic structure (176) on the basis of spectral data [82IJC(B) 1087]. 

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