Synthesis of 4-hydroxy-7-dimethylamino-3-pyrazolinylcoumarins and their polarity-sensitive properties

Four 4-hydroxy-3-pyrazolinylcoumarin derivatives were synthesized and their UV–vis spectra in various compositions of MeOH and CH2Cl2 were measured. Among the prepared compounds, only one was found to exist mainly in the enol form in nonpolar solvents and the keto form in protic solvents, whereas the others are exclusively present in the enol forms regardless of solvent polarity. This polarity-sensitive property of 3-pyrazolinylcoumarins can be controlled by the electronic nature of the substituent at the 7-position of coumarin, the 1-position (nitrogen atom) of pyrazoline as well as the para-position of the benzene moiety.