The Synthesis of 6‐Amidino‐2‐oxopurine Revisited: New Evidence for the Reaction Mechanism

Treatment of N ‐aryl‐ or N ‐alkyl‐5‐amino‐4‐(cyanoformimidoyl)‐1 H ‐imidazoles 1 with benzoyl or ethoxycarbonyl isocyanates resulted in the formation of 5‐amino‐4‐[ N ‐benzoyl‐ or N ‐(ethoxycarbonyl)carbamoylcyanoformimidoyl]‐1 H ‐imidazoles 2 . In the presence of catalytic amounts of DBU (1,8‐diazabicyclo[5.4.0]undec‐7‐ene), these compounds cyclized to give the 5′‐amino‐5‐imino‐4,4′‐bi‐1 H ‐imidazol‐2(5 H )‐ones 3 . Compounds 3 efficiently rearrange to yield 6‐amidino‐2‐oxopurines 5 in ethanol or DMF solution. The formation of purines 5 both from imidazoles 2 and from bi‐imidazoles 3 was followed by 1 H NMR, allowing a deeper understanding of the reaction mechanism. The rearrangements are acid‐catalysed (trifluoroacetic acid), but the use of one equivalent of acid produced different products, identified as the bi‐imidazoles 8 .(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)