Peculiarities of the 4,5-dinitrofluorescein esters synthesis: formation of reduced species

The problem of synthesising esters of fluorescein dyes has long been solved for the mother compound and its derivatives. The choice of an optimal method depends on the substituents in the xanthenes moiety and in the phthalic acid residue. To obtain alkyl esters of 4,5-dinitrofluorescein, we used thionyl chloride in ethanol. However, several other products appeared, which were identified as reduced species resulting from hydride transfer. A similar picture was observed when using sulphuric acid instead of thionyl chloride. The formation of reduced species was demonstrated by hydrogen-1 and carbon-13 nuclear magnetic resonance spectroscopy and X-ray analysis. In methanol, on the other hand, the esterification of the carboxylic group proceeds easily and with good yield. In the course of the current study, four new compounds of the fluorescein series were synthesised and characterised. The results show that nitro substitution in xanthenes favours dye reduction.