Isomerisation and stability of Z/E isomers of urocanic acid and derivatives.

Thermal and photochemical isomerisation of urocanic acid and its derivative s (O- and/or N-alkylated) have been studied. The data show the wavelength a nd the molar extinction coefficient-dependent photochemistry off urocanic a cid and its derivatives. Thermal isomerisation of these compounds is diffic ult and undergoes a "one-way" Z --> E reaction. Thus, E isomers are thermod ynamically more stable than the Zones. Under basic conditions, derivatives having an unsubstitued imidazole ring (NH function free) exist in their ani onic forms, which lowers the energy barrier without changing the isomerisat ion pathway. (C) Academie des sciences/Elsevier, Paris.