Practical and theoretical aspects of flavanone–chalcone isomerisations

Isomerisation equilibria of the flavanone/2′-hydroxychalcone, 7-hydroxyflavanone/2′,4′-dihydroxychalcone and 4′,5,7-trihydroxyflavanone (naringenin)/2′,4′,6′,4-tetrahydroxychalcone systems have been studied in dilute and saturated solutions – the latter in equilibrium with the solid precipitated form. Seemingly inconsistent results found in the literature are rationalized in terms of proton transfer reactions and solubility effects. The results indicate that at least two different isomerisation mechanisms operate, one in the acidic and weakly alkaline region, the second in strongly alkaline solutions. Practical conclusions may ease the preparation of pure chalcones or flavanones in which contamination with the other isomer is minimized.