Tautomeric Equilibria in 3‐Amino‐1‐(2‐aminoimidazol‐4‐yl)prop‐1‐ene, a Central Building Block of Marine Alkaloids

The tautomers I – IV of the marine metabolite 3‐amino‐1‐(2‐aminoimidazol‐4‐yl)prop‐1‐ene ( 1 ) were previously suggested to have rather similar stabilities.1,2 Through a series of DFT and ab initio calculations, their relative stabilities were investigated in both the gas phase and in water, and also compared to their Z‐isomers V – VIII . The tautomers I and III have almost identical stability in the gas phase and in water. The Z‐isomer VII is more stable than other tautomers in the gas phase and quite competitive with I and III in water. The tautomers II and IV are much less stable and unlikely to coexist in equilibrium with I and III . The calculated p K a of 1 ‐H + of +10.9 suggests that 1 is fully protonated even under mildly acidic conditions. Protonation decreases the stability difference between the most stable tautomer III and the less stable tautomer IV .(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)